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分类: C/C++

2006-05-05 23:22:51

's C++ Style and Technique FAQ

Modified May 2, 2006

These are questions about C++ Style and Technique that people ask me often. If you have better questions or comments on the answers, feel free to email me (bs@research.att.com). Please remember that I can't spend all of my time improving my homepages.

For more general questions, see my .

For terminology and concepts, see my .

Please note that these are just a collection of questions and answers. They are not a substitute for a carefully selected sequence of examples and explanations as you would find in a good textbook. Nor do they offer detailed and precise specifications as you would find in a reference manual or the standard. See for questions related to the design of C++. See for questions about the use of C++ and its standard library.

Here is of some of this Q&A with annotations.

Index


Often, especially at the start of semesters, I get a lot of questions about how to write very simple programs. Typically, the problem to be solved is to read in a few numbers, do something with them, and write out an answer. Here is a sample program that does that:
	#include
	#include
	#include
	using namespace std;

	int main()
	{
		vector v;

		double d;
		while(cin>>d) v.push_back(d);	// read elements
		if (!cin.eof()) {		// check if input failed
			cerr << "format error\n";
			return 1;	// error return
		}

		cout << "read " << v.size() << " elements\n";

		reverse(v.begin(),v.end());
		cout << "elements in reverse order:\n";
		for (int i = 0; iHere are a few observations about this program: 
  • This is a Standard ISO C++ program using the standard library. Standard library facilities are declared in namespace std in headers without a .h suffix.
  • If you want to compile this on a Windows machine, you need to compile it as a "console application". Remember to give your source file the .cpp suffix or the compiler might think that it is C (not C++) source.
  • Yes, main() returns an int.
  • Reading into a standard vector guarantees that you don't overflow some arbitrary buffer. Reading into an array without making a "silly error" is beyond the ability of complete novices - by the time you get that right, you are no longer a complete novice. If you doubt this claim, I suggest you read my paper "Learning Standard C++ as a New Language", which you can download from .
  • The !cin.eof() is a test of the stream's format. Specifically, it tests whether the loop ended by finding end-of-file (if not, you didn't get input of the expected type/format). For more information, look up "stream state" in your C++ textbook.
  • A vector knows its size, so I don't have to count elements.
  • This program contains no explicit memory management, and it does not leak memory. A vector keeps track of the memory it use to store its elements. When a vector needs more memory for elements, is allocates more; when a vector goes out of scope, it frees that memory. Therefore, the user need not be concerned with the allocation and deallocation of memory for vector elements.
  • for reading in strings, see How do I read a string from input?.
  • The program ends reading input when it sees "end of file". If you run the program from the keybord on a Unix machine "end of file" is Ctrl-D. If you are on a Windows machine that because of a bug doesn't recognize an end-of-file character, you might prefer this slightly more complicated version of the program that terminates input with the word "end":
    	#include
    	#include
    	#include
    	#include
    	using namespace std;
    
    	int main()
    	{
    		vector v;
    
    		double d;
    		while(cin>>d) v.push_back(d);	// read elements
    		if (!cin.eof()) {		// check if input failed
    			cin.clear();		// clear error state
    			string s;
    			cin >> s;		// look for terminator string
    			if (s != "end") {
    				cerr << "format error\n";
    				return 1;	// error return
    			}
    		}
    
    		cout << "read " << v.size() << " elements\n";
    
    		reverse(v.begin(),v.end());
    		cout << "elements in reverse order:\n";
    		for (int i = 0; i
For more examples of how to use the standard library to do simple things simply, see the "Tour of the Standard Library" Chapter of (available for download).

The main point of a C++ coding standard is to provide a set of rules for using C++ for a particular purpose in a particular environment. It follows that there cannot be one coding standard for all uses and all users. For a given application (or company, application area, etc.), a good coding standard is better than no coding standard. On the other hand, I have seen many examples that demonstrate that a bad coding standard is worse than no coding standard.

Please chose your rules with care and with solid knowledge of your application area. Some of the worst coding standards (I won't mention names "to protect the guilty") were written by people without solid knowledge of C++ together with a relative ignorance of the application area (they were "experts" rather than developers) and a misguided conviction that more restrictions are necessarily better than fewer. The counter example to that last misconception is that some features exist to help programmers having to use even worse features. Anyway, please remember that safety, productivity, etc. is the sum of all parts of the design and development process - and not of individual language features, or even of whole languages.

With those caveats, I recommend thee things:

  • Look at Sutter and Alexandrescu: "C++ coding standards". Addison-Wesley, ISBN 0-321-11358-. It has good rules, but look upon it primarily as a set of meta-rules. That is, consider it a guide to what a good, more specific, set of coding rules should look like. If you are writing a coding standard, you ignore this book at your peril.
  • Look at . I consider it a pretty good set of rules for safety critical and performance critical code. If you do embedded systems programming, you should consider it. Caveat: I had a hand in the formulation of these rules, so you could consider me biased. On the other hand, please send me constructive comments about it. Such comments might lead to improvements - all good standards are regularly reviewed and updated based on experience and on changes in the work environment. If you don't build hard-real time systems or safety critical systems, you'll find these rules overly restrictive - because then those rules are not for you (at least not all of those rules).
  • Don't use C coding standards (even if slightly modified for C++) and don't use ten-year-old C++ coding standards (even if good for their time). C++ isn't (just) C and Standard C++ is not (just) pre-standard C++.

You may have a problem with your compiler. It may be old, you may have it installed wrongly, or your computer might be an antique. I can't help you with such problems.

However, it is more likely that the program that you are trying to compile is poorly designed, so that compiling it involves the compiler examining hundreds of header files and tens of thousands of lines of code. In principle, this can be avoided. If this problem is in your library vendor's design, there isn't much you can do (except changing to a better library/vendor), but you can structure your own code to minimize re-compilation after changes. Designs that do that are typically better, more maintainable, designs because they exhibit better separation of concerns.

Consider a classical example of an object-oriented program:

	class Shape {
	public:		// interface to users of Shapes
		virtual void draw() const;
		virtual void rotate(int degrees);
		// ...
	protected:	// common data (for implementers of Shapes)
		Point center;
		Color col;
		// ...
	};

	class Circle : public Shape {
	public:	
		void draw() const;
		void rotate(int) { }
		// ...
	protected:
		int radius;
		// ...
	};

	class Triangle : public Shape {
	public:	
		void draw() const;
		void rotate(int);
		// ...
	protected:
		Point a, b, c;
		// ...
	};	
The idea is that users manipulate shapes through Shape's public interface, and that implementers of derived classes (such as Circle and Triangle) share aspects of the implementation represented by the protected members.

There are three serious problems with this apparently simple idea:

  • It is not easy to define shared aspects of the implementation that are helpful to all derived classes. For that reason, the set of protected members is likely to need changes far more often than the public interface. For example, even though "center" is arguably a valid concept for all Shapes, it is a nuisance to have to maintain a point "center" for a Triangle - for triangles, it makes more sense to calculate the center if and only if someone expresses interest in it.
  • The protected members are likely to depend on "implementation" details that the users of Shapes would rather not have to depend on. For example, many (most?) code using a Shape will be logically independent of the definition of "Color", yet the presence of Color in the definition of Shape will probably require compilation of header files defining the operating system's notion of color.
  • When something in the protected part changes, users of Shape have to recompile - even though only implementers of derived classes have access to the protected members.

Thus, the presence of "information helpful to implementers" in the base class that also acts as the interface to users is the source of instability in the implementation, spurious recompilation of user code (when implementation information changes), and excess inclusion of header files into user code (because the "information helpful to implementers" needs those headers). This is sometimes known as the "brittle base class problem."

The obvious solution is to omit the "information helpful to implemeters" for classes that are used as interfaces to users. That is, to make interfaces, pure interfaces. That is, to represent interfaces as abstract classes:

	class Shape {
	public:		// interface to users of Shapes
		virtual void draw() const = 0;
		virtual void rotate(int degrees) = 0;
		virtual Point center() const = 0;
		// ...

		// no data
	};

	class Circle : public Shape {
	public:	
		void draw() const;
		void rotate(int) { }
		Point center() const { return cent; }
		// ...
	protected:
		Point cent;
		Color col;
		int radius;
		// ...
	};

	class Triangle : public Shape {
	public:	
		void draw() const;
		void rotate(int);
		Point center() const;
		// ...
	protected:
		Color col;
		Point a, b, c;
		// ...
	};	
The users are now insulated from changes to implementations of derived classes. I have seen this technique decrease build times by orders of magnitudes.

But what if there really is some information that is common to all derived classes (or simply to several derived classes)? Simply make that information a class and derive the implementation classes from that also:

	class Shape {
	public:		// interface to users of Shapes
		virtual void draw() const = 0;
		virtual void rotate(int degrees) = 0;
		virtual Point center() const = 0;
		// ...

		// no data
	};

	struct Common {
		Color col;
		// ...
	};
		
	class Circle : public Shape, protected Common {
	public:	
		void draw() const;
		void rotate(int) { }
		Point center() const { return cent; }
		// ...
	protected:
		Point cent;
		int radius;
	};

	class Triangle : public Shape, protected Common {
	public:	
		void draw() const;
		void rotate(int);
		Point center() const;
		// ...
	protected:
		Point a, b, c;
	};	


To ensure that the addresses of two different objects will be different. For the same reason, "new" always returns pointers to distinct objects. Consider:
	class Empty { };

	void f()
	{
		Empty a, b;
		if (&a == &b) cout << "impossible: report error to compiler supplier";

		Empty* p1 = new Empty;
		Empty* p2 = new Empty;
		if (p1 == p2) cout << "impossible: report error to compiler supplier";
	}	
There is an interesting rule that says that an empty base class need not be represented by a separate byte:
	struct X : Empty {
		int a;
		// ...
	};

	void f(X* p)
	{
		void* p1 = p;
		void* p2 = &p->a;
		if (p1 == p2) cout << "nice: good optimizer";
	}
This optimization is safe and can be most useful. It allows a programmer to use empty classes to represent very simple concepts without overhead. Some current compilers provide this "empty base class optimization".

You don't. If you don't want data in an interface, don't put it in the class that defines the interface. Put it in derived classes instead. See, Why do my compiles take so long?.

Sometimes, you do want to have representation data in a class. Consider class complex:

	template class complex {
	public:
		complex() : re(0), im(0) { }
		complex(Scalar r) : re(r), im(0) { }
		complex(Scalar r, Scalar i) : re(r), im(i) { }
		// ...

		complex& operator+=(const complex& a)
			{ re+=a.re; im+=a.im; return *this; }
		// ...
	private:
		Scalar re, im;
	};
This type is designed to be used much as a built-in type and the representation is needed in the declaration to make it possible to create genuinely local objects (i.e. objects that are allocated on the stack and not on a heap) and to ensure proper inlining of simple operations. Genuinely local objects and inlining is necessary to get the performance of complex close to what is provided in languages with a built-in complex type.


Because many classes are not designed to be used as base classes. For example, see class complex.

Also, objects of a class with a virtual function require space needed by the virtual function call mechanism - typically one word per object. This overhead can be significant, and can get in the way of layout compatibility with data from other languages (e.g. C and Fortran).

See for more design rationale.


Because many classes are not designed to be used as base classes. Virtual functions make sense only in classes meant to act as interfaces to objects of derived classes (typically allocated on a heap and accessed through pointers or references).

So when should I declare a destructor virtual? Whenever the class has at least one virtual function. Having virtual functions indicate that a class is meant to act as an interface to derived classes, and when it is, an object of a derived class may be destroyed through a pointer to the base. For example:

	class Base {
		// ...
		virtual ~Base();
	};

	class Derived : public Base {
		// ...
		~Derived();
	};

	void f()
	{
		Base* p = new Derived;
		delete p;	// virtual destructor used to ensure that ~Derived is called
	}
Had Base's destructor not been virtual, Derived's destructor would not have been called - with likely bad effects, such as resources owned by Derived not being freed.


A virtual call is a mechanism to get work done given partial information. In particular, "virtual" allows us to call a function knowing only an interfaces and not the exact type of the object. To create an object you need complete information. In particular, you need to know the exact type of what you want to create. Consequently, a "call to a constructor" cannot be virtual.

Techniques for using an indirection when you ask to create an object are often referred to as "Virtual constructors". For example, see TC++PL3 15.6.2.

For example, here is a technique for generating an object of an appropriate type using an abstract class:

	struct F {	// interface to object creation functions
		virtual A* make_an_A() const = 0;
		virtual B* make_a_B() const = 0;
	};

	void user(const F& fac)
	{
		A* p = fac.make_an_A();	// make an A of the appropriate type
		B* q = fac.make_a_B();	// make a B of the appropriate type
		// ...
	}

	struct FX : F {
		A* make_an_A() const { return new AX();	} // AX is derived from A
		B* make_a_B() const { return new BX();	} // BX is derived from B
	};

	struct FY : F {
		A* make_an_A() const { return new AY();	} // AY is derived from A
		B* make_a_B() const { return new BY();	} // BY is derived from B
	};

	int main()
	{
		FX x;
		FY y;
		user(x);	// this user makes AXs and BXs
		user(y);	// this user makes AYs and BYs

		user(FX());	// this user makes AXs and BXs
		user(FY());	// this user makes AYs and BYs
		// ...
	}
This is a variant of what is often called "the factory pattern". The point is that user() is completely isolated from knowledge of classes such as AX and AY.

A pure virtual function is a function that must be overridden in a derived class and need not be defined. A virtual function is declared to be "pure" using the curious "=0" syntax. For example:
	class Base {
	public:
		void f1();		// not virtual
		virtual void f2();	// virtual, not pure
		virtual void f3() = 0;	// pure virtual
	};

	Base b;	// error: pure virtual f3 not overridden
Here, Base is an abstract class (because it has a pure virtual function), so no objects of class Base can be directly created: Base is (explicitly) meant to be a base class. For example:
	class Derived : public Base {
		// no f1: fine
		// no f2: fine, we inherit Base::f2
		void f3();
	};

	Derived d;	// ok: Derived::f3 overrides Base::f3
Abstract classes are immensely useful for defining interfaces. In fact, a class with only pure virtual functions is often called an interface.

You can define a pure virtual function:

	Base::f3() { /* ... */ }
This is very occationally useful (to provide some simple common implementation detail for derived classes), but Base::f3() must still be overridden in some derived class.

If you don't override a pure virtual function in a derived class, that derived class becomes abstract:

	class D2 : public Base {
		// no f1: fine
		// no f2: fine, we inherit Base::f2
		// no f3: fine, but D2 is therefore still abstract
	};

	D2 d;	// error: pure virtual Base::f3 not overridden

That question (in many variations) are usually prompted by an example like this:
	#include
	using namespace std;

	class B {
	public:
		int f(int i) { cout << "f(int): "; return i+1; }
		// ...
	};

	class D : public B {
	public:
		double f(double d) { cout << "f(double): "; return d+1.3; }
		// ...
	};

	int main()
	{
		D* pd = new D;

		cout << pd->f(2) << '\n';
		cout << pd->f(2.3) << '\n';
	}
which will produce:
	f(double): 3.3
	f(double): 3.6
rather than the
	f(int): 3
	f(double): 3.6
that some people (wrongly) guessed.

In other words, there is no overload resolution between D and B. The compiler looks into the scope of D, finds the single function "double f(double)" and calls it. It never bothers with the (enclosing) scope of B. In C++, there is no overloading across scopes - derived class scopes are not an exception to this general rule. (See or for details).

But what if I want to create an overload set of all my f() functions from my base and derived class? That's easily done using a using-declaration:

	class D : public B {
	public:
		using B::f;	// make every f from B available
		double f(double d) { cout << "f(double): "; return d+1.3; }
		// ...
	};
Give that modification, the output will be
	f(int): 3
	f(double): 3.6
That is, overload resolution was applied to B's f() and D's f() to select the most appropriate f() to call.


Yes, but be careful. It may not do what you expect. In a constructor, the virtual call mechanism is disabled because overriding from derived classes hasn't yet happened. Objects are constructed from the base up, "base before derived".

Consider:

	#include
	#include
	using namespace std;

	class B {
	public:
		B(const string& ss) { cout << "B constructor\n"; f(ss); }
		virtual void f(const string&) { cout << "B::f\n";}
	};

	class D : public B {
	public:
		D(const string & ss) :B(ss) { cout << "D constructor\n";}
		void f(const string& ss) { cout << "D::f\n"; s = ss; }
	private:
		string s;
	};

	int main()
	{
		D d("Hello");
	}
the program compiles and produce
	B constructor
	B::f
	D constructor
Note not D::f. Consider what would happen if the rule were different so that D::f() was called from B::B(): Because the constructor D::D() hadn't yet been run, D::f() would try to assign its argument to an uninitialized string s. The result would most likely be an immediate crash.

Destruction is done "derived class before base class", so virtual functions behave as in constructors: Only the local definitions are used - and no calls are made to overriding functions to avoid touching the (now destroyed) derived class part of the object.

For more details see 13.2.4.2 or 15.4.3.

It has been suggested that this rule is an implementation artifact. It is not so. In fact, it would be noticeably easier to implement the unsafe rule of calling virtual functions from constructors exactly as from other functions. However, that would imply that no virtual function could be written to rely on invariants established by base classes. That would be a terrible mess.


No but if you need one you can write your own.

Consider placement new used to place objects in a set of arenas

        class Arena {
        public:
                void* allocate(size_t);
                void deallocate(void*);
                // ...
        };

        void* operator new(size_t sz, Arena& a)
        {
                return a.allocate(sz);
        }

        Arena a1(some arguments);
        Arena a2(some arguments);
Given that, we can write
        X* p1 = new(a1) X;
        Y* p2 = new(a1) Y;
        Z* p3 = new(a2) Z;
        // ...
But how can we later delete those objects correctly? The reason that there is no built-in "placement delete" to match placement new is that there is no general way of assuring that it would be used correctly. Nothing in the C++ type system allows us to deduce that p1 points to an object allocated in Arena a1. A pointer to any X allocated anywhere can be assigned to p1.

However, sometimes the programmer does know, and there is a way:

        template void destroy(T* p, Arena& a)
        {
                if (p) {
                        p->~T();		// explicit destructor call
                        a.deallocate(p);
                }
        }
Now, we can write:
        destroy(p1,a1);
        destroy(p2,a2);
        destroy(p3,a3);
If an Arena keeps track of what objects it holds, you can even write destroy() to defend itself against mistakes.

It is also possible to define a matching operator new() and operator delete() pairs for a class hierarchy 15.6. See also 10.4 and TC++PL(SE) 19.4.5.


Yes, but why do you want to? There are two common answers:

  • for efficiency: to avoid my function calls being virtual
  • for safety: to ensure that my class is not used as a base class (for example, to be sure that I can copy objects without fear of slicing)
In my experience, the efficiency reason is usually misplaced fear. In C++, virtual function calls are so fast that their real-world use for a class designed with virtual functions does not to produce measurable run-time overheads compared to alternative solutions using ordinary function calls. Note that the virtual function call mechanism is typically used only when calling through a pointer or a reference. When calling a function directly for a named object, the virtual function class overhead is easily optimized away.

If there is a genuine need for "capping" a class hierarchy to avoid virtual function calls, one might ask why those functions are virtual in the first place. I have seen examples where performance-critical functions had been made virtual for no good reason, just because "that's the way we usually do it".

The other variant of this problem, how to prevent derivation for logical reasons, has a solution. Unfortunately, that solution is not pretty. It relies on the fact that the most derived class in a hierarchy must construct a virtual base. For example:

	class Usable;

	class Usable_lock {
		friend class Usable;
	private:
		Usable_lock() {}
		Usable_lock(const Usable_lock&) {}
	};

	class Usable : public virtual Usable_lock {
		// ...
	public:
		Usable();
		Usable(char*);
		// ...
	};

	Usable a;

	class DD : public Usable { };

	DD dd;  // error: DD::DD() cannot access
        	// Usable_lock::Usable_lock(): private  member
(from sec 11.4.3).


The C++ standard library provides a set of useful, statically type-safe, and efficient containers. Examples are vector, list, and map:
	vector< int> vi(10);
	vector< Shape*> vs;
	list< string> lst;
	list< double> l2
	map< string,Record*> tbl;
	map< Key,vector< Record*> > t2;
These containers are described in all good C++ textbooks, and should be preferred over arrays and "home cooked" containers unless there is a good reason not to.

These containers are homogeneous; that is, they hold elements of the same type. If you want a container to hold elements of several different types, you must express that either as a union or (usually much better) as a container of pointers to a polymorphic type. The classical example is:

	vector< Shape*> vi;	// vector of pointers to Shapes
Here, vi can hold elements of any type derived from Shape. That is, vi is homogeneous in that all its elements are Shapes (to be precise, pointers to Shapes) and heterogeneous in the sense that vi can hold elements of a wide variety of Shapes, such as Circles, Triangles, etc.

So, in a sense all containers (in every language) are homogenous because to use them there must be a common interface to all elements for users to rely on. Languages that provide containers deemed heterogenous simply provide containers of elements that all provide a standard interface. For example, Java collections provide containers of (references to) Objects and you use the (common) Object interface to discover the real type of an element.

The C++ standard library provides homogeneous containers because those are the easiest to use in the vast majority of cases, gives the best compile-time error message, and imposes no unnecessary run-time overheads.

If you need a heterogeneous container in C++, define a common interface for all the elements and make a container of those. For example:

	class Io_obj { /* ... */ };	// the interface needed to take part in object I/O

	vector< Io_obj*> vio;		// if you want to manage the pointers directly
	vector< Handle< Io_obj> > v2;	// if you want a "smart pointer" to handle the objects
Don't drop to the lowest level of implementation detail unless you have to:
	vector< void* > memory;	// rarely needed
A good indication that you have "gone too low level" is that your code gets littered with casts.


No. It does not. "Friend" is an explicit mechanism for granting access, just like membership. You cannot (in a standard conforming program) grant yourself access to a class without modifying its source. For example:
	class X {
		int i;
	public:
		void m();		// grant X::m() access
		friend void f(X&);	// grant f(X&) access
		// ...
	};

	void X::m() { i++; /* X::m() can access X::i */ }

	void f(X& x) { x.i++; /* f(X&) can access X::i */ }
For a description on the C++ protection model, see sec 2.10 and sec 11.5, 15.3, and C.11.

C++ inherited pointers from C, so I couldn't remove them without causing serious compatibility problems. References are useful for several things, but the direct reason I introduced them in C++ was to support operator overloading. For example:
	void f1(const complex* x, const complex* y)	// without references
	{
		complex z = *x+*y;	// ugly
		// ...
	}

	void f2(const complex& x, const complex& y)	// with references
	{
		complex z = x+y;	// better
		// ...
	}	
More generally, if you want to have both the functionality of pointers and the functionality of references, you need either two different types (as in C++) or two different sets of operations on a single type. For example, with a single type you need both an operation to assign to the object referred to and an operation to assign to the reference/pointer. This can be done using separate operators (as in Simula). For example:
	Ref r :- new My_type;
	r := 7;			// assign to object
	r :- new My_type;	// assign to reference
Alternatively, you could rely on type checking (overloading).For example:
	Ref r = new My_type;
	r = 7;			// assign to object
	r = new My_type;	// assign to reference


Because "this" was introduced into C++ (really into C with Classes) before references were added. Also, I chose "this" to follow Simula usage, rather than the (later) Smalltalk use of "self".


In terms of time and space, an array is just about the optimal construct for accessing a sequence of objects in memory. It is, however, also a very low level data structure with a vast potential for misuse and errors and in essentially all cases there are better alternatives. By "better" I mean easier to write, easier to read, less error prone, and as fast.

The two fundamental problems with arrays are that

  • an array doesn't know its own size
  • the name of an array converts to a pointer to its first element at the slightest provocation
Consider some examples:
	void f(int a[], int s)
	{
		// do something with a; the size of a is s
		for (int i = 0; iThe second call will scribble all over memory that doesn't belong to arr2. Naturally, a programmer usually get the size right, but it's extra work and ever so often someone makes the mistake. I prefer the simpler and cleaner version using the standard library vector: 
	void f(vector& v)
	{
		// do something with v
		for (int i = 0; i v1(20);
	vector v2(10);

	void g()
	{
		f(v1);
		f(v2);
	}

Since an array doesn't know its size, there can be no array assignment:

	void f(int a[], int b[], int size)
	{
		a = b;	// not array assignment
		memcpy(a,b,size);	// a = b
		// ...
	}
Again, I prefer vector:
	void g(vector& a, vector& b, int size)
	{
		a = b;	
		// ...
	}
Another advantage of vector here is that memcpy() is not going to do the right thing for elements with copy constructors, such as strings.
	void f(string a[], string b[], int size)
	{
		a = b;	// not array assignment
		memcpy(a,b,size);	// disaster
		// ...
	}

	void g(vector& a, vector& b, int size)
	{
		a = b;	
		// ...
	}

An array is of a fixed size determined at compile time:

	const int S = 10;

	void f(int s)
	{
		int a1[s];	// error
		int a2[S];	// ok

		// if I want to extend a2, I'll have to chage to an array
		// allocated on free store using malloc() and use ralloc()
		// ...
	}
To contrast:
	const int S = 10;

	void g(int s)
	{
		vector v1(s);	// ok
		vector v2(S);	// ok
		v2.resize(v2.size()*2);
		// ...
	}
C99 allows variable array bounds for local arrays, but those VLAs have their own problems.

The way that array names "decay" into pointers is fundamental to their use in C and C++. However, array decay interact very badly with inheritance. Consider:

	class Base { void fct(); /* ... */ };
	class Derived { /* ... */ };

	void f(Base* p, int sz)
	{
		for (int i=0; iIn the last call, the Derived[] is treated as a Base[] and the subscripting no longer works correctly when sizeof(Derived)!=sizeof(Base) -- as will be the case in most cases of interest. If we used vectors instead, the error would be caught at compile time: 
	void f(vector& v)
	{
		for (int i=0; i ab(20);
	vector ad(20);

	void g()
	{
		f(ab);
		f(ad);	// error: cannot convert a vector to a vector
	}
I find that an astonishing number of novice programming errors in C and C++ relate to (mis)uses of arrays.


There didn't (and doesn't) seem to be a sufficient need.


In C++, the definition of NULL is 0, so there is only an aesthetic difference. I prefer to avoid macros, so I use 0. Another problem with NULL is that people sometimes mistakenly believe that it is different from 0 and/or not an integer. In pre-standard code, NULL was/is sometimes defined to something unsuitable and therefore had/has to be avoided. That's less common these days.

If you have to name the null pointer, call it nullptr; that's what it's going to be called in C++0x. Then, "nullptr" will be a keyword.


Like C, C++ doesn't define layouts, just semantic constraints that must be met. Therefore different implementations do things differently. Unfortunately, the best explanation I know of is in a book that is otherwise outdated and doesn't describe any current C++ implementation: (usually called the ARM). It has diagrams of key layout examples. There is a very brief explanation in Chapter 2 of .

Basically, C++ constructs objects simply by concatenating sub objects. Thus

        struct A { int a,b; };
is represented by two ints next to each other, and
        struct B : A { int c; };
is represented by an A followed by an int; that is, by three ints next to each other.

Virtual functions are typically implemented by adding a pointer (the vptr) to each object of a class with virtual functions. This pointer points to the appropriate table of functions (the vtbl). Each class has its own vtbl shared by all objects of that class.


It's undefined. Basically, in C and C++, if you read a variable twice in an expression where you also write it, the result is undefined. Don't do that. Another example is:
	v[i] = i++;
Related example:
	f(v[i],i++);
Here, the result is undefined because the order of evaluation of function arguments are undefined.

Having the order of evaluation undefined is claimed to yield better performing code. Compilers could warn about such examples, which are typically subtle bugs (or potential subtle bugs). I'm disappointed that after decades, most compilers still don't warn, leaving that job to specialized, separate, and underused tools.


Well, you can, and it's quite easy and general.

Consider:

        template
        void draw_all(Container& c)
        {
                for_each(c.begin(),c.end(),mem_fun(&Shape::draw));
        }
If there is a type error, it will be in the resolution of the fairly complicated for_each() call. For example, if the element type of the container is an int, then we get some kind of obscure error related to the for_each() call (because we can't invoke Shape::draw() for an int).

To catch such errors early, I can write:

        template
        void draw_all(Container& c)
        {
                Shape* p = c.front(); // accept only containers of Shape*s

                for_each(c.begin(),c.end(),mem_fun(&Shape::draw));
        }
The initialization of the spurious variable "p" will trigger a comprehensible error message from most current compilers. Tricks like this are common in all languages and have to be developed for all novel constructs. In production code, I'd probably write something like:
	template
        void draw_all(Container& c)
        {
                typedef typename Container::value_type T;
                Can_copy(); // accept containers of only Shape*s

                for_each(c.begin(),c.end(),mem_fun(&Shape::draw));
        }
This makes it clear that I'm making an assertion. The Can_copy template can be defined like this:
	template struct Can_copy {
		static void constraints(T1 a, T2 b) { T2 c = a; b = a; }
		Can_copy() { void(*p)(T1,T2) = constraints; }
	};
Can_copy checks (at compile time) that a T1 can be assigned to a T2. Can_copy checks that T is a Shape* or a pointer to a class publicly derived from Shape* or a type with a user-defined conversion to Shape*. Note that the definition is close to minimal:
  • one line to name the constraints to be checked and the types for which to check them
  • one line to list the specific constraints checked (the constraints() function)
  • one line to provide a way to trigger the check (the constructor)

Note also that the definition has the desirable properties that

  • You can express constraints without declaring or copying variables, thus the writer of a constraint doesn't have to make assumptions about how a type is initialized, whether objects can be copied, destroyed, etc. (unless, of course, those are the properties being tested by the constraint)
  • No code is generated for a constraint using current compilers
  • No macros are needed to define or use constraints
  • Current compilers give acceptable error messages for a failed constraint, including the word "constraints" (to give the reader a clue), the name of the constraints, and the specific error that caused the failure (e.g. "cannot initialize Shape* by double*")

So why is something like Can_copy() - or something even more elegant - not in the language? contains an analysis of the difficulties involved in expressing general constraints for C++. Since then, many ideas have emerged for making these constraints classes easier to write and still trigger good error messages. For example, I believe the use of a pointer to function the way I do in Can_copy originates with Alex Stepanov and Jeremy Siek. I don't think that Can_copy() is quite ready for standardization - it needs more use. Also, different forms of constraints are in use in the C++ community; there is not yet a consensus on exactly what form of constraints templates is the most effective over a wide range of uses.

However, the idea is very general, more general than language facilities that have been proposed and provided specifically for constraints checking. After all, when we write a template we have the full expressive power of C++ available. Consider:

	template struct Derived_from {
		static void constraints(T* p) { B* pb = p; }
		Derived_from() { void(*p)(T*) = constraints; }
	};

	template struct Can_copy {
		static void constraints(T1 a, T2 b) { T2 c = a; b = a; }
		Can_copy() { void(*p)(T1,T2) = constraints; }
	};

	template struct Can_compare {
		static void constraints(T1 a, T2 b) { a==b; a!=b; a struct Can_multiply {
		static void constraints(T1 a, T2 b, T3 c) { c = a*b; }
		Can_multiply() { void(*p)(T1,T2,T3) = constraints; }
	};

	struct B { };
	struct D : B { };
	struct DD : D { };
	struct X { };

	int main()
	{
		Derived_from();
		Derived_from();
		Derived_from();
		Derived_from();
		Derived_from();

		Can_compare();
		Can_compare();
		Can_multiply();
		Can_multiply();
		Can_multiply();
	
		Can_copy();
		Can_copy();
		Can_copy();
	}

	// the classical "elements must derived from Mybase*" constraint:

	template class Container : Derived_from {
		// ...
	};
Actually, Derived_from doesn't check derivation, but conversion, but that's often a better constaint. Finding good names for constraints can be hard.


To a novice,
	qsort(array,asize,sizeof(elem),elem_compare);
looks pretty weird, and is harder to understand than
	sort(vec.begin(),vec.end());
To an expert, the fact that sort() tends to be faster than qsort() for the same elements and the same comparison criteria is often significant. Also, sort() is generic, so that it can be used for any reasonable combination of container type, element type, and comparison criterion. For example:
	struct Record {
		string name;
		// ...
	};

	struct name_compare {	// compare Records using "name" as the key
		bool operator()(const Record& a, const Record& b) const
			{ return a.name& vs)
	{
		sort(vs.begin(), vs.end(), name_compare());
		// ...
	}	

In addition, most people appreciate that sort() is type safe, that no casts are required to use it, and that they don't have to write a compare() function for standard types.

For a more detailed explanation, see my paper "Learning C++ as a New language", which you can download from my .

The primary reason that sort() tends to outperform qsort() is that the comparison inlines better.


An object that in some way behaves like a function, of course. Typically, that would mean an object of a class that defines the application operator - operator().

A function object is a more general concept than a function because a function object can have state that persist across several calls (like a static local variable) and can be initialized and examined from outside the object (unlike a static local variable). For example:

	class Sum {
		int val;
	public:
		Sum(int i) :val(i) { }
		operator int() const { return val; }		// extract value

		int operator()(int i) { return val+=i; }	// application
	};

	void f(vector v)
	{
		Sum s = 0;	// initial value 0
		s = for_each(v.begin(), v.end(), s);	// gather the sum of all elements
		cout << "the sum is " << s << "\n";
		
		// or even:
		cout << "the sum is " << for_each(v.begin(), v.end(), Sum(0)) << "\n";
	}
Note that a function object with an inline application operator inlines beautifully because there are no pointers involved that might confuse optimizers. To contrast: current optimizers are rarely (never?) able to inline a call through a pointer to function.

Function objects are extensively used to provide flexibility in the standard library.


By writing code that doesn't have any. Clearly, if your code has new operations, delete operations, and pointer arithmetic all over the place, you are going to mess up somewhere and get leaks, stray pointers, etc. This is true independently of how conscientious you are with your allocations: eventually the complexity of the code will overcome the time and effort you can afford. It follows that successful techniques rely on hiding allocation and deallocation inside more manageable types. Good examples are the standard containers. They manage memory for their elements better than you could without disproportionate effort. Consider writing this without the help of string and vector:
	#include
	#include
	#include
	#include
	using namespace std;

	int main()	// small program messing around with strings
	{
		cout << "enter some whitespace-separated words:\n";
		vector v;
		string s;
		while (cin>>s) v.push_back(s);

		sort(v.begin(),v.end());

		string cat;
		typedef vector::const_iterator Iter;
		for (Iter p = v.begin(); p!=v.end(); ++p) cat += *p+"+";
		cout << cat << '\n';
	}
What would be your chance of getting it right the first time? And how would you know you didn't have a leak?

Note the absence of explicit memory management, macros, casts, overflow checks, explicit size limits, and pointers. By using a function object and a standard algorithm, I could have eliminated the pointer-like use of the iterator, but that seemed overkill for such a tiny program.

These techniques are not perfect and it is not always easy to use them systematically. However, they apply surprisingly widely and by reducing the number of explicit allocations and deallocations you make the remaining examples much easier to keep track of. As early as 1981, I pointed out that by reducing the number of objects that I had to keep track of explicitly from many tens of thousands to a few dozens, I had reduced the intellectual effort needed to get the program right from a Herculean task to something manageable, or even easy.

If your application area doesn't have libraries that make programming that minimizes explicit memory management easy, then the fastest way of getting your program complete and correct might be to first build such a library.

Templates and the standard libraries make this use of containers, resource handles, etc., much easier than it was even a few years ago. The use of exceptions makes it close to essential.

If you cannot handle allocation/deallocation implicitly as part of an object you need in your application anyway, you can use a resource handle to minimize the chance of a leak. Here is an example where I need to return an object allocated on the free store from a function. This is an opportunity to forget to delete that object. After all, we cannot tell just looking at pointer whether it needs to be deallocated and if so who is responsible for that. Using a resource handle, here the standard library auto_ptr, makes it clear where the responsibility lies:

	#include
	#include
	using namespace std;

	struct S {
		S() { cout << "make an S\n"; }
		~S() { cout << "destroy an S\n"; }
		S(const S&) { cout << "copy initialize an S\n"; }
		S& operator=(const S&) { cout << "copy assign an S\n"; }
	};

	S* f()
	{
		return new S;	// who is responsible for deleting this S?
	};

	auto_ptr g()
	{
		return auto_ptr(new S);	// explicitly transfer responsibility for deleting this S
	}

	int main()
	{
		cout << "start main\n";
		S* p = f();
		cout << "after f() before g()\n";
	//	S* q = g();	// this error would be caught by the compiler
		auto_ptr q = g();
		cout << "exit main\n";
		// leaks *p
		// implicitly deletes *q
	}

Think about resources in general, rather than simply about memory.

If systematic application of these techniques is not possible in your environment (you have to use code from elsewhere, part of your program was written by Neanderthals, etc.), be sure to use a memory leak detector as part of your standard development procedure, or plug in a garbage collector.


In other words, why doesn't C++ provide a primitive for returning to the point from which an exception was thrown and continuing execution from there?

Basically, someone resuming from an exception handler can never be sure that the code after the point of throw was written to deal with the excecution just continuing as if nothing had happened. An exception handler cannot know how much context to "get right" before resuming. To get such code right, the writer of the throw and the writer of the catch need intimate knowledge of each others code and context. This creates a complicated mutual dependency that wherever it has been allowed has led to serious maintenance problems.

I seriously considered the possibility of allowing resumption when I designed the C++ exception handling mechanism and this issue was discussed in quite some detail during standardization. See the exception handling chapter of .

If you want to check to see if you can fix a problem before throwing an exception, call a function that checks and then throws only if the problem cannot be dealt with locally. A new_handler is an example of this.

I once answered this quesion in a somewhat different style as part of a discussion in a newsgroup.


If you want to, you can of course use realloc(). However, realloc() is only guaranteed to work on arrays allocated by malloc() (and similar functions) containing objects without user-defined copy constructors. Also, please remember that contrary to naive expectations, realloc() occationally does copy its argument array.

In C++, a better way of dealing with reallocation is to use a standard library container, such as vector, and let it grow naturally.


See section 8.3, Chapter 14, and . The appendix focuses on techniques for writing exception-safe code in demanding applications, and is not written for novices.

In C++, exceptions is used to signal errors that cannot be handled locally, such as the failure to acquire a resource in a constructor. For example:

	class Vector {
		int sz;
		int* elem;
		class Range_error { };
	public:
		Vector(int s) : sz(s) { if (sz<0) throw Range_error(); /* ... */ }
		// ...
	};
Do not use exceptions as simply another way to return a value from a function. Most users assume - as the language definition encourages them to - that exception-handling code is error-handling code, and implementations are optimized to reflect that assumption.

A key technique is resource acquisiton is initialization (sometimes abbreviated to RAII), which uses classes with destructors to impose order on resource management. For example:

	void fct(string s)
	{
		File_handle f(s,"r");	// File_handle's constructor opens the file called "s"
		// use f
	} // here File_handle's destructor closes the file	
If the "use f" part of fct() throws an exception, the destructor is still thrown and the file is properly closed. This contrasts to the common unsafe usage:
	void old_fct(const char* s)
	{
		FILE* f = fopen(s,"r");	// open the file named "s"
		// use f
		fclose(f);	// close the file
	}
If the "use f" part of old_fct throws an exception - or simply does a return - the file isn't closed. In C programs, longjmp() is an additional hazard.


Because that would open a hole in the type system. For example:
	class Apple : public Fruit { void apple_fct(); /* ... */ };
	class Orange : public Fruit { /* ... */ }; // Orange doesn't have apple_fct()

	vector v;	// vector of Apples

	void f(vector& vf)		// innocent Fruit manipulating function
	{
		vf.push_back(new Orange);	// add orange to vector of fruit
	}

	void h()
	{
		f(v);	// error: cannot pass a vector as a vector
		for (int i=0; iapple_fct();
	}
Had the call f(v) been legal, we would have had an Orange pretending to be an Apple.

An alternative language design decision would have been to allow the unsafe conversion, but rely on dynamic checking. That would have required a run-time check for each access to v's members, and h() would have had to throw an exception upon encountering the last element of v.


  • We don't need one: generic programming provides statically type safe alternatives in most cases. Other cases are handled using multiple inheritance.
  • There is no useful universal class: a truly universal carries no semantics of its own.
  • A "universal" class encourages sloppy thinking about types and interfaces and leads to excess run-time checking.
  • Using a universal base class implies cost: Objects must be heap-allocated to be polymorphic; that implies memory and access cost. Heap objects don't naturally support copy semantics. Heap objects don't support simple scoped behavior (which complicates resource management). A universal base class encourages use of dynamic_cast and other run-time checking.
Yes. I have simplified the arguments, but this is a FAQ, not an academic paper.


You can read a single, whitespace terminated word like this:
	#include
	#include
	using namespace std;

	int main()
	{
		cout << "Please enter a word:\n";

		string s;
		cin>>s;
	
		cout << "You entered " << s << '\n';
	}
Note that there is no explicit memory management and no fixed-sized buffer that you could possibly overflow.

If you really need a whole line (and not just a single word) you can do this:

	#include
	#include
	using namespace std;

	int main()
	{
		cout << "Please enter a line:\n";

		string s;
		getline(cin,s);
	
		cout << "You entered " << s << '\n';
	}
For a brief introduction to standard library facilities, such as iostream and string, see Chaper 3 of (available online). For a detailed comparison of simple uses of C and C++ I/O, see "Learning Standard C++ as a New Language", which you can download from my


Because C++ supports an alternative that is almost always better: The "resource acquisition is initialization" technique (TC++PL3 section 14.4). The basic idea is to represent a resource by a local object, so that the local object's destructor will release the resource. That way, the programmer cannot forget to release the resource. For example:
	class File_handle {
		FILE* p;
	public:
		File_handle(const char* n, const char* a)
			{ p = fopen(n,a); if (p==0) throw Open_error(errno); }
		File_handle(FILE* pp)
			{ p = pp; if (p==0) throw Open_error(errno); }

		~File_handle() { fclose(p); }

		operator FILE*() { return p; }

		// ...
	};

	void f(const char* fn)
	{
		File_handle f(fn,"rw");	// open fn for reading and writing
		// use file through f
	}
In a system, we need a "resource handle" class for each resource. However, we don't have to have an "finally" clause for each acquisition of a resource. In realistic systems, there are far more resource acquisitions than kinds of resources, so the "resource acquisition is initialization" technique leads to less code than use of a "finally" construct.

Also, have a look at the examples of resource management in of .


An auto_ptr is an example of very simple handle class, defined in , supporting exception safety using the resource acquisition is initialization technique. An auto_ptr holds a pointer, can be used as a pointer, and deletes the object pointed to at the end of its scope. For example:
	#include
	using namespace std;

	struct X {
		int m;
		// ..
	};

	void f()
	{
		auto_ptr p(new X);
		X* q = new X;

		p->m++;		// use p just like a pointer
		q->m++;
		// ...

		delete q;
	}
If an exception is thrown in the ... part, the object held by p is correctly deleted by auto_ptr's destructor while the X pointed to by q is leaked. See TC++PL 14.4.2 for details.

Auto_ptr is a very lightweight class. In particular, it is *not* a reference counted pointer. If you "copy" one auto_ptr into another, the assigned to auto_ptr holds the pointer and the assigned auto_ptr holds 0. For example:

	#include
	#include
	using namespace std;

	struct X {
		int m;
		// ..
	};

	int main()
	{
		auto_ptr p(new X);
		auto_ptr q(p);
		cout << "p " << p.get() << " q " << q.get() << "\n";
	}
should print a 0 pointer followed by a non-0 pointer. For example:
	p 0x0 q 0x378d0
auto_ptr::get() returns the help pointer.

This "move semantics" differs from the usual "copy semantics", and can be surprising. In particular, never use an auto_ptr as a member of a standard container. The standard containers require the usual copy semantics. For example:

	std::vector >v;	// error

An auto_ptr holds a pointer to an indiviual element, not a pointer to an array:

	void f(int n)
	{
		auto_ptr p(new X[n]);	// error
		// ...
	}
This is an error because the destructor will delete the pointer using "delete" rather than "delete[]" and will fail to invoke the destructor for the last n-1 Xs.

So should we use an auto_array to hold arrays? No. There is no auto_array. The reason is that there isn't a need for one. A better solution is to use a vector:

	void f(int n)
	{
		vector v(n);
		// ...
	}
Should an exception occur in the ... part, v's destructor will be correctly invoked.

Yes, in the sense that you can use malloc() and new in the same program.

No, in the sense that you cannot allocate an object with malloc() and free it using delete. Nor can you allocate with new and delete with free() or use realloc() on an array allocated by new.

The C++ operators new and delete guarantee proper construction and destruction; where constructors or destructors need to be invoked, they are. The C-style functions malloc(), calloc(), free(), and realloc() doesn't ensure that. Furthermore, there is no guarantee that the mechanism use by new and delete to acquire and release raw memory is compatible with malloc() and free(). If mixing styles works on your system, you were simply "lucky" - for now.

If you feel the need for realloc() - and many do - then consider using a standard library vector. For example

	// read words from input into a vector of strings:

	vector words;
	string s;
	while (cin>>s && s!=".") words.push_back(s);
The vector expands as needed.

See also the examples and discussion in "Learning Standard C++ as a New Language", which you can download from my .


In C, you can implicitly convert a void* to a T*. This is unsafe. Consider:

	#include

	int main()
	{
		char i = 0;
		char j = 0;
		char* p = &i;
		void* q = p;
		int* pp = q;	/* unsafe, legal C, not C++ */

		printf("%d %d\n",i,j);
		*pp = -1;	/* overwrite memory starting at &i */
		printf("%d %d\n",i,j);
	}
The effects of using a T* that doesn't point to a T can be disastrous. Consequently, in C++, to get a T* from a void* you need an explicit cast. For example, to get the undesirable effects of the program above, you have to write:
		int* pp = (int*)q;
or, using a new style cast to make the unchecked type conversion operation more visible:
		int* pp = static_cast(q);
Casts are best avoided.

One of the most common uses of this unsafe conversion in C is to assign the result of malloc() to a suitable pointer. For example:

	int* p = malloc(sizeof(int));
In C++, use the typesafe new operator:
	int* p = new int;
Incidentally, the new operator offers additional advantages over malloc():
  • new can't accidentally allocate the wrong amount of memory,
  • new implicitly checks for memory exhaustion, and
  • new provides for initialization
For example:
	typedef std::complex cmplx;

	/* C style: */
	cmplx* p = (cmplx*)malloc(sizeof(int));	/* error: wrong size */
							/* forgot to test for p==0 */
	if (*p == 7) { /* ... */ }			/* oops: forgot to initialize *p */

	// C++ style:
	cmplx* q = new cmplx(1,2); // will throw bad_alloc if memory is exhausted
	if (*q == 7) { /* ... */ }


If you want a constant that you can use in a constant expression, say as an array bound, you have two choices:

class X {
	static const int c1 = 7;
	enum { c2 = 19 };

	char v1[c1];
	char v2[c2];

	// ...
};
At first glance, the declaration of c1 seems cleaner, but note that to use that in-class initialization syntax, the constant must be a static const of integral or enumeration type initialized by a constant expression. That's quite restrictive:
class Y {
	const int c3 = 7;		// error: not static
	static int c4 = 7;		// error: not const
	static const float c5 = 7;	// error not integral
};
I tend to use the "enum trick" because it's portable and doesn't tempt me to use non-standard extensions of the in-class initialization syntax.

So why do these inconvenient restrictions exist? A class is typically declared in a header file and a header file is typically included into many translation units. However, to avoid complicated linker rules, C++ requires that every object has a unique definition. That rule would be broken if C++ allowed in-class definition of entities that needed to be stored in memory as objects. See for an explanation of C++'s design tradeoffs.

You have more flexibility if the const isn't needed for use in a constant expression:

	class Z {
		static char* p;		// initialize in definition
		const int i;		// initialize in constructor
	public:
		Z(int ii) :i(ii) { }
	};

	char* Z::p = "hello, there";
You can take the address of a static member if (and only if) it has an out-of-class definition:
	class AE {
		// ...
	public:
		static const int c6 = 7;
		static const int c7 = 31;
	};

	const int AE::c7;	// definition

	int f()
	{
		const int* p1 = &AE::c6;	// error: c6 not an lvalue
		const int* p2 = &AE::c7;	// ok
		// ...
	}


Consider
	delete p;
	// ...
	delete p;
If the ... part doesn't touch p then the second "delete p;" is a serious error that a C++ implementation cannot effectively protect itself against (without unusual precautions). Since deleting a zero pointer is harmless by definition, a simple solution would be for "delete p;" to do a "p=0;" after it has done whatever else is required. However, C++ doesn't gurantee that.

One reason is that the operand of delete need not be an lvalue. Consider:

	delete p+1;
	delete f(x);
Here, the implementation of delete does not have a pointer to which it can assign zero. These examples may be rare, but they do imply that it is not possible to guarantee that ``any pointer a to deleted object is 0.'' A simpler way of bypassing that ``rule'' is to have two pointers to an object:
	T* p = new T;
	T* q = p;
	delete p;
	delete q;	// ouch!
C++ explicitly allows an implementation of delete to zero out an lvalue operand, and I had hoped that implementations would do that, but that idea doesn't seem to have become popular with implementers.

If you consider zeroing out pointers important, consider using a destroy function:

	template inline void destroy(T*& p) { delete p; p = 0; }

Consider this yet-another reason to minimize explicit use of new and delete by relying on stadard library containers, handles, etc.

Note that passing the pointer as a reference (to allow the pointer to be zero'd out) has the added benefit of preventing destroy() from being called for an rvalue:

	int* f();
	int* p;
	// ...
	destroy(f());	// error: trying to pass an rvalue by non-const reference
	destroy(p+1);	// error: trying to pass an rvalue by non-const reference

The simple answer is "of course it is!", but have a look at the kind of example that often accompany that question:
	void f()
	{
		X* p = new X;
		// use p
	}
That is, there was some (mistaken) assumption that the object created by "new" would be destroyed at the end of a function.

Basically, you should only use "new" if you want an object to live beyond the lifetime of the scope you create it in. That done, you need to use "delete" to destroy it. For example:

	X* g(int i) { /* ... */ return new X(i); }	// the X outlives the call of g()

	void h(int i)
	{
		X* p = g(i);
		// ...
		delete p;
	}
If you want an object to live in a scope only, don't use "new" but simply define a variable:
        {
                ClassName x;
                // use x
        }
The variable is implicitly destroyed at the end of the scope.

Code that creates an object using new and then deletes it at the end of the same scope is ugly, error-prone, and inefficient. For example:

	void fct()	// ugly, error-prone, and inefficient
	{
		X* p = new X;
		// use p
		delete p;
	}

The definition
	void main() { /* ... */ }
is not and never has been C++, nor has it even been C. See the ISO C++ standard 3.6.1[2] or the ISO C standard 5.1.2.2.1. A conforming implementation accepts
	int main() { /* ... */ }
and
	int main(int argc, char* argv[]) { /* ... */ }
A conforming implementation may provide more versions of main(), but they must all have return type int. The int returned by main() is a way for a program to return a value to "the system" that invokes it. On systems that doesn't provide such a facility the return value is ignored, but that doesn't make "void main()" legal C++ or legal C. Even if your compiler accepts "void main()" avoid it, or risk being considered ignorant by C and C++ programmers.

In C++, main() need not contain an explicit return statement. In that case, the value returned is 0, meaning successful execution. For example:

	#include

	int main()
	{
		std::cout << "This program returns the integer value 0\n";
	}
Note also that neither ISO C++ nor C99 allows you to leave the type out of a declaration. That is, in contrast to C89 and ARM C++ ,"int" is not assumed where a type is missing in a declaration. Consequently:
	#include

	main() { /* ... */ }
is an error because the return type of main() is missing.

Most operators can be overloaded by a programmer. The exceptions are
	. (dot)  ::  ?:  sizeof
There is no fundamental reason to disallow overloading of ?:. I just didn't see the need to introduce the special case of overloading a ternary operator. Note that a function overloading expr1?expr2:expr3 would not be able to guarantee that only one of exper2 and expr3 was executed.

Sizeof cannot be overloaded because built-in operations, such as incrementing a pointer into an array implicitly depends on it. Consider:

	X a[10];
	X* p = &a[3];
	X* q = &a[3];
	p++;	// p points to a[4]
		// thus the integer value of p must be
		// sizeof(X) larger than the integer value of q
Thus, sizeof(X) could not be given a new and different meaning by the programmer without violating basic language rules.

In N::m neither N nor m are expressions with values; N and m are names known to the compiler and :: performs a (compile time) scope resolution rather than an expression evaluation. One could imagine allowing overloading of x::y where x is an object rather than a namespace or a class, but that would - contrary to first appearences - involve introducing new syntax (to allow expr::expr). It is not obvious what benefits such a complication would bring.

Operator . (dot) could in principle be overloaded using the same technique as used for ->. However, doing so can lead to questions about whether an operation is meant for the object overloading . or an object referred to by . For example:

	class Y {
	public:
		void f();
		// ...
	};

	class X {	// assume that you can overload .
		Y* p;
		Y& operator.() { return *p; }
		void f();
		// ...
	};

	void g(X& x)
	{
		x.f();	// X::f or Y::f or error?
	}

This problem can be solved in several ways. At the time of standardization, it was not obvious which way would be best. For more details, see .


The simplest way is to use a stringstream:
	#include
	#include
	#include
	using namespace std;

	string itos(int i)	// convert int to string
	{
		stringstream s;
		s << i;
		return s.str();
	}

	int main()
	{
		int i = 127;
		string ss = itos(i);
		const char* p = ss.c_str();

		cout << ss << " " << p << "\n";
	}
Naturally, this technique works for converting any type that you can output using << to a string. For a description of string streams, see 21.5.3 of .

Just declare the C function ``extern "C"'' (in your C++ code) and call it (from your C or C++ code). For example:
	// C++ code

	extern "C" void f(int);	// one way

	extern "C" {	// another way
		int g(double);
		double h();
	};

	void code(int i, double d)
	{
		f(i);
		int ii = g(d);
		double dd = h();
		// ...
	}
The definitions of the functions may look like this:
	/* C code: */

	void f(int i)
	{
		/* ... */
	}

	int g(double d)
	{
		/* ... */
	}

	double h()
	{
		/* ... */
	}

Note that C++ type rules, not C rules, are used. So you can't call function declared ``extern "C"'' with the wrong number of argument. For example:
	// C++ code

	void more_code(int i, double d)
	{
		double dd = h(i,d);	// error: unexpected arguments
		// ...
	}

Just declare the C++ function ``extern "C"'' (in your C++ code) and call it (from your C or C++ code). For example:
	// C++ code:

	extern "C" void f(int);

	void f(int i)
	{
		// ...
	}
Now f() can be used like this:
	/* C code: */

	void f(int);
	
	void cc(int i)
	{
		f(i);
		/* ... */
	}
Naturally, this works only for non-member functions. If you want to call member functions (incl. virtual functions) from C, you need to provide a simple wrapper. For example:
	// C++ code:

	class C {
		// ...
		virtual double f(int);
	};

	extern "C" double call_C_f(C* p, int i)	// wrapper function
	{
		return p->f(i);
	}
Now C::f() can be used like this:
	/* C code: */

	double call_C_f(struct C* p, int i);
	
	void ccc(struct C* p, int i)
	{
		double d = call_C_f(p,i);
		/* ... */
	}
If you want to call overloaded functions from C, you must provide wrappers with distinct names for the C code to use. For example:
	// C++ code:

	void f(int);
	void f(double);

	extern "C" void f_i(int i) { f(i); }
	extern "C" void f_d(double d) { f(d); }
Now the f() functions can be used like this:
	/* C code: */

	void f_i(int);
	void f_d(double);
	
	void cccc(int i,double d)
	{
		f_i(i);
		f_d(d);
		/* ... */
	}
Note that these techniques can be used to call a C++ library from C code even if you cannot (or do not want to) modify the C++ headers.

Both are "right" in the sense that both are valid C and C++ and both have exactly the same meaning. As far as the language definitions and the compilers are concerned we could just as well say ``int*p;'' or ``int * p;''

The choice between ``int* p;'' and ``int *p;'' is not about right and wrong, but about style and emphasis. C emphasized expressions; declarations were often considered little more than a necessary evil. C++, on the other hand, has a heavy emphasis on types.

A ``typical C programmer'' writes ``int *p;'' and explains it ``*p is what is the int'' emphasizing syntax, and may point to the C (and C++) declaration grammar to argue for the correctness of the style. Indeed, the * binds to the name p in the grammar.

A ``typical C++ programmer'' writes ``int* p;'' and explains it ``p is a pointer to an int'' emphasizing type. Indeed the type of p is int*. I clearly prefer that emphasis and see it as important for using the more advanced parts of C++ well.

The critical confusion comes (only) when people try to declare several pointers with a single declaration:

	int* p, p1;	// probable error: p1 is not an int*
Placing the * closer to the name does not make this kind of error significantly less likely.
	int *p, p1;	// probable error?
Declaring one name per declaration minimizes the problem - in particular when we initialize the variables. People are far less likely to write:
	int* p = &i;
	int p1 = p;	// error: int initialized by int*
And if they do, the compiler will complain.

Whenever something can be done in two ways, someone will be confused. Whenever something is a matter of taste, discussions can drag on forever. Stick to one pointer per declaration and always initialize variables and the source of confusion disappears. See for a longer discussion of the C declaration syntax.


Such style issues are a matter of personal taste. Often, opinions about code layout are strongly held, but probably consistency matters more than any particular style. Like most people, I'd have a hard time constructing a solid logical argument for my preferences.

I personally use what is often called "K&R" style. When you add conventions for constructs not found in C, that becomes what is sometimes called "Stroustrup" style. For example:

class C : public B {
public:
	// ...
};

void f(int* p, int max)
{
	if (p) {
		// ...
	}

	for (int i = 0; iThis style conserves vertical space better than most layout styles, and I like to fit as much as is reasonable onto a screen. Placing the opening brace of a function on a new line helps me distinguish function definition from class definitions at a glance. 

Indentation is very important.

Design issues, such as the use of abstract classes for major interfaces, use of templates to present flexible type-safe abstractions, and proper use of exceptions to represent errors, are far more important than the choice of layout style.


I put it before, but that's a matter of taste. "const T" and "T const" were always (both) allowed and equivalent. For example:
	const int a = 1;	// ok
	int const b = 2;	// also ok
My guess is that using the first version will confuse fewer programmers (``is more idiomatic'').

Why? When I invented "const" (initially named "readonly" and had a corresponding "writeonly"), I allowed it to go before or after the type because I could do so without ambiguity. Pre-standard C and C++ imposed few (if any) ordering rules on specifiers.

I don't remember any deep thoughts or involved discussions about the order at the time. A few of the early users - notably me - simply liked the look of

	const int c = 10;
better than
	int const c = 10;
at the time.

I may have been influenced by the fact that my earliest examples were written using "readonly" and

	readonly int c = 10;
does read better than
	int readonly c = 10;
The earliest (C or C++) code using "const" appears to have been created (by me) by a global substitution of "const" for "readonly".

I remember discussing syntax alternatives with several people - incl. Dennis Ritchie - but I don't remember which languages I looked at then.

Note that in const pointers, "const" always comes after the "*". For example:

	int *const p1 = q;	// constant pointer to int variable
	int const* p2 = q;	// pointer to constant int
	const int* p3 = q;	// pointer to constant int


Casts are generally best avoided. With the exception of dynamic_cast, their use implies the possibility of a type error or the truncation of a numeric value. Even an innocent-looking cast can become a serious problem if, during development or maintenance, one of the types involved is changed. For example, what does this mean?:
	x = (T)y;
We don't know. It depends on the type T and the types of x and y. T could be the name of a class, a typedef, or maybe a template parameter. Maybe x and y are scalar variables and (T) represents a value conversion. Maybe x is of a class derived from y's class and (T) is a downcast. Maybe x and y are unrelated pointer types. Because the C-style cast (T) can be used to express many logically different operations, the compiler has only the barest chance to catch misuses. For the same reason, a programmer may not know exactly what a cast does. This is sometimes considered an advantage by novice programmers and is a source of subtle errors when the novice guessed wrong.

The "new-style casts" were introduced to give programmers a chance to state their intentions more clearly and for the compiler to catch more errors. For example:

	int a = 7;
	double* p1 = (double*) &a;		// ok (but a doesn't point to a double)
	double* p2 = static_cast(&a);		// error
	double* p2 = reinterpret_cast(&a);	// ok: I really mean it

	const int c = 7;
	int* q1 = &c;			// error
	int* q2 = (int*)&c;		// ok (but *q2=2; is still invalid code and may fail)
	int* q3 = static_cast(&c);	// error: static_cast doesn't cast away const
	int* q4 = const_cast(&c);	// I really mean it
The idea is that conversions allowed by static_cast are somewhat less likely to lead to errors than those that require reinterpret_cast. In principle, it is possible to use the result of a static_cast without casting it back to its original type, whereas you should always cast the result of a reinterpret_cast back to its original type before using it to ensure portability.

A secondary reason for introducing the new-style cast was that C-style casts are very hard to spot in a program. For example, you can't conveniently search for casts using an ordinary editor or word processor. This near-invisibility of C-style casts is especially unfortunate because they are so potentially damaging. An ugly operation should have an ugly syntactic form. That observation was part of the reason for chosing the syntax for the new-style casts. A further reason was for the new-style casts to match the template notation, so that programmers can write their own casts, especially run-time checked casts.

Maybe, because static_cast is so ugly and so relatively hard to type, you're more likely to think twice before using one? That would be good, because casts really are mostly avoidable in modern C++.


Macros do not obey the C++ scope and type rules. This is often the cause of subtle and not-so-subtle problems. Consequently, C++ provides alternatives that fit better with the rest of C++, such as inline functions, templates, and namespaces.

Consider:

	#include "someheader.h"

	struct S {
		int alpha;
		int beta;
	};
If someone (unwisely) has written a macro called "alpha" or a macro called "beta" this may not compile or (worse) compile into something unexpected. For example, "someheader.h" may contain:
	#define alpha 'a'
	#define beta b[2]
Conventions such as having macros (and only macros) in ALLCAPS helps, but there is no language-level protection against macros. For example, the fact that the member names were in the scope of the struct didn't help: Macros operate on a program as a stream of characters before the compiler proper sees it. This, incidentally, is a major reason for C and C++ program development environments and tools have been unsophisticated: the human and the compiler see different things.

Unfortunately, you cannot assume that other programmers consistently avoid what you consider "really stupid". For example, someone recently reported to me that they had encountered a macro containing a goto. I have seen that also and heard arguments that might - in a weak moment - appear to make sense. For example:

	#define prefix get_ready(); int ret__
	#define Return(i) ret__=i; do_something(); goto exit
	#define suffix exit: cleanup(); return ret__

	void f()
	{
		prefix;
		// ...
		Return(10);
		// ...
		Return(x++);
		//...
		suffix;
	}
Imagine being presented with that as a maintenance programmer; "hiding" the macros in a header - as is not uncommon - makes this kind of "magic" harder to spot.

One of the most common subtle problems is that a function-style macro doesn't obey the rules of function argument passing. For example:

	#define square(x) (x*x)

	void f(double d, int i)
	{
		square(d);	// fine
		square(i++);	// ouch: means (i++*i++)
		square(d+1);	// ouch: means (d+1*d+1); that is, (d+d+1)
		// ...
	}
The "d+1" problem is solved by adding parentheses in the "call" or in the macro definition:
	#define square(x) ((x)*(x))	/* better */
However, the problem with the (presumably unintended) double evaluation of i++ remains.

And yes, I do know that there are things known as macros that doesn't suffer the problems of C/C++ preprocessor macros. However, I have no ambitions for improving C++ macros. Instead, I recommend the use of facilities from the C++ language proper, such as inline functions, templates, constructors (for initialization), destructors (for cleanup), exceptions (for exiting contexts), etc.


"cout" is pronounced "see-out". The "c" stands for "character" because iostreams map values to and from byte (char) representations.


"char" is usually pronounced "tchar", not "kar". This may seem illogical because "character" is pronounced "ka-rak-ter", but nobody ever accused English pronounciation and spelling of being logical.
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