拷贝构造函数

•  what is copy constructor, and where it 's require?

A copy constructor is a special constructor in the C++ programming language used to create a new object as a copy of an existing object. The first argument of such a constructor is a reference to an object of the same type as is being constructed (const or non-const), which might be followed by parameters of any type (all having default values).

Normally the compiler automatically creates a copy constructor for each class (known as a default copy constructor) but for special cases the programmer creates the copy constructor, known as auser-defined copy constructor. In such cases, the compiler does not create one.

A user-defined copy constructor is generally needed when an object owns pointers or non-shareable references, such as to a file, in which case a destructor and an assignment operator should also be written (Rule of three).

string ( const string& str );
//Constructs a standard  object and initializes its content.
//Content is initialized to a copy of the  object str.

#include <iostream>
#include <string>
using namespace std;

int
main(void)
{
        string base = "hello world";
        string str (base);

        cout << str << endl;

        return (0);
}