// auto_ptr implementation -*- C++ -*-
// Copyright (C) 2007, 2008 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library. This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 2, or (at your option)
// any later version.
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
// You should have received a copy of the GNU General Public License
// along with this library; see the file COPYING. If not, write to
// the Free Software Foundation, 51 Franklin Street, Fifth Floor,
// Boston, MA 02110-1301, USA.
// As a special exception, you may use this file as part of a free software
// library without restriction. Specifically, if other files instantiate
// templates or use macros or inline functions from this file, or you compile
// this file and link it with other files to produce an executable, this
// file does not by itself cause the resulting executable to be covered by
// the GNU General Public License. This exception does not however
// invalidate any other reasons why the executable file might be covered by
// the GNU General Public License.
/** @file backward/auto_ptr.h
* This is an internal header file, included by other library headers.
* You should not attempt to use it directly.
*/
#ifndef _STL_AUTO_PTR_H
#define _STL_AUTO_PTR_H 1
#include
#include
_GLIBCXX_BEGIN_NAMESPACE(std)
/**
* A wrapper class to provide auto_ptr with reference semantics.
* For example, an auto_ptr can be assigned (or constructed from)
* the result of a function which returns an auto_ptr by value.
*
* All the auto_ptr_ref stuff should happen behind the scenes.
*/
template
struct auto_ptr_ref
{
_Tp1* _M_ptr;
explicit
auto_ptr_ref(_Tp1* __p): _M_ptr(__p) { }
} _GLIBCXX_DEPRECATED_ATTR;
/**
* @brief A simple smart pointer providing strict ownership semantics.
*
* The Standard says:
*
* An @c auto_ptr owns the object it holds a pointer to. Copying
* an @c auto_ptr copies the pointer and transfers ownership to the
* destination. If more than one @c auto_ptr owns the same object
* at the same time the behavior of the program is undefined.
*
* The uses of @c auto_ptr include providing temporary
* exception-safety for dynamically allocated memory, passing
* ownership of dynamically allocated memory to a function, and
* returning dynamically allocated memory from a function. @c
* auto_ptr does not meet the CopyConstructible and Assignable
* requirements for Standard Library * href="tables.html#65">container elements and thus
* instantiating a Standard Library container with an @c auto_ptr
* results in undefined behavior.
*
* Quoted from [20.4.5]/3.
*
* Good examples of what can and cannot be done with auto_ptr can
* be found in the libstdc++ testsuite.
*
* _GLIBCXX_RESOLVE_LIB_DEFECTS
* 127. auto_ptr<> conversion issues
* These resolutions have all been incorporated.
*/
template
class auto_ptr
{
private:
_Tp* _M_ptr;
public:
/// The pointed-to type.
typedef _Tp element_type;
/**
* @brief An %auto_ptr is usually constructed from a raw pointer.
* @param p A pointer (defaults to NULL).
*
* This object now @e owns the object pointed to by @a p.
*/
explicit
auto_ptr(element_type* __p = 0) throw() : _M_ptr(__p) { } /**
* @brief An %auto_ptr can be constructed from another %auto_ptr.
* @param a Another %auto_ptr of the same type.
*
* This object now @e owns the object previously owned by @a a,
* which has given up ownership.
*/
auto_ptr(auto_ptr& __a) throw() : _M_ptr(__a.release()) { }
/**
* @brief An %auto_ptr can be constructed from another %auto_ptr.
* @param a Another %auto_ptr of a different but related type.
*
* A pointer-to-Tp1 must be convertible to a
* pointer-to-Tp/element_type.
*
* This object now @e owns the object previously owned by @a a,
* which has given up ownership.
*/
template
auto_ptr(auto_ptr<_Tp1>& __a) throw() : _M_ptr(__a.release()) { }
/**
* @brief %auto_ptr assignment operator.
* @param a Another %auto_ptr of the same type.
*
* This object now @e owns the object previously owned by @a a,
* which has given up ownership. The object that this one @e
* used to own and track has been deleted.
*/
auto_ptr&
operator=(auto_ptr& __a) throw()
{
reset(__a.release());
return *this;
}
/**
* @brief %auto_ptr assignment operator.
* @param a Another %auto_ptr of a different but related type.
*
* A pointer-to-Tp1 must be convertible to a pointer-to-Tp/element_type.
*
* This object now @e owns the object previously owned by @a a,
* which has given up ownership. The object that this one @e
* used to own and track has been deleted.
*/
template
auto_ptr&
operator=(auto_ptr<_Tp1>& __a) throw()
{
reset(__a.release());
return *this;
}
/**
* When the %auto_ptr goes out of scope, the object it owns is
* deleted. If it no longer owns anything (i.e., @c get() is
* @c NULL), then this has no effect.
*
* The C++ standard says there is supposed to be an empty throw
* specification here, but omitting it is standard conforming. Its
* presence can be detected only if _Tp::~_Tp() throws, but this is
* prohibited. [17.4.3.6]/2
*/
~auto_ptr() { delete _M_ptr; }
/**
* @brief Smart pointer dereferencing.
*
* If this %auto_ptr no longer owns anything, then this
* operation will crash. (For a smart pointer, "no longer owns
* anything" is the same as being a null pointer, and you know
* what happens when you dereference one of those...)
*/
element_type&
operator*() const throw()
{
_GLIBCXX_DEBUG_ASSERT(_M_ptr != 0);
return *_M_ptr;
}
/**
* @brief Smart pointer dereferencing.
*
* This returns the pointer itself, which the language then will
* automatically cause to be dereferenced.
*/
element_type*
operator->() const throw()
{
_GLIBCXX_DEBUG_ASSERT(_M_ptr != 0);
return _M_ptr;
}
/**
* @brief Bypassing the smart pointer.
* @return The raw pointer being managed.
*
* You can get a copy of the pointer that this object owns, for
* situations such as passing to a function which only accepts
* a raw pointer.
*
* @note This %auto_ptr still owns the memory.
*/
element_type*
get() const throw() { return _M_ptr; }
/**
* @brief Bypassing the smart pointer.
* @return The raw pointer being managed.
*
* You can get a copy of the pointer that this object owns, for
* situations such as passing to a function which only accepts
* a raw pointer.
*
* @note This %auto_ptr no longer owns the memory. When this object
* goes out of scope, nothing will happen.
*/
element_type*
release() throw()
{
element_type* __tmp = _M_ptr;
_M_ptr = 0;
return __tmp;
}
/**
* @brief Forcibly deletes the managed object.
* @param p A pointer (defaults to NULL).
*
* This object now @e owns the object pointed to by @a p. The
* previous object has been deleted.
*/
void
reset(element_type* __p = 0) throw()
{
if (__p != _M_ptr)
{
delete _M_ptr;
_M_ptr = __p;
}
}
/**
* @brief Automatic conversions
*
* These operations convert an %auto_ptr into and from an auto_ptr_ref
* automatically as needed. This allows constructs such as
* @code
* auto_ptr func_returning_auto_ptr(.....);
* ...
* auto_ptr ptr = func_returning_auto_ptr(.....);
* @endcode
*/
auto_ptr(auto_ptr_ref __ref) throw()
: _M_ptr(__ref._M_ptr) { }
auto_ptr&
operator=(auto_ptr_ref __ref) throw()
{
if (__ref._M_ptr != this->get())
{
delete _M_ptr;
_M_ptr = __ref._M_ptr;
}
return *this;
}
template
operator auto_ptr_ref<_Tp1>() throw()
{ return auto_ptr_ref<_Tp1>(this->release()); }
template
operator auto_ptr<_Tp1>() throw()
{ return auto_ptr<_Tp1>(this->release()); }
} _GLIBCXX_DEPRECATED_ATTR;
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 541. shared_ptr template assignment and void
template<>
class auto_ptr
{
public:
typedef void element_type;
} _GLIBCXX_DEPRECATED_ATTR;
_GLIBCXX_END_NAMESPACE
#endif /* _STL_AUTO_PTR_H */
从代码中可以看出
1. auto_ptr可以辅助我们管理内存
2. auto_ptr在‘复制’的时候，管理权会放弃
3. auto_ptr_ref的引入 巧妙解决了右值引用的问题。
对于第3点，详细说明一下
在标准C++中 右值引用时不可以的。比如：
如果有函数 int f(void);
我们采用： int &p =f(void);这样的方式，在标准C++中是不允许的
必须要 const int &p = f(void); //之所以要说是标准C++ 因为在Vc中是支持的。呵呵
从源代码上可以看出 auto_ptr的拷贝构造函数“不是”我们常看见的 auto_ptr(const atuo_ptr &p);
而是设计为 auto_ptr(atuo_ptr p)这样的样式(原因是在这个拷贝构造函数中，对于传入的参数我们必须使其放弃管理权)
在这种设计下：我们就不能写这样的函数 atuo_ptr p( f(void) )
所以引入了auto_ptr_ref这个辅助结构来帮助我们的auto_ptr可以执行这样的函数 atuo_ptr p( f(void));
具体步骤为：
===============================================================================
auto_ptr p(auto_ptr(new int(0)))
便可以成功，过程如下：
1. 构造临时对象 auto_ptr(new int(0))
2. 想将临时对象通过拷贝构造函数传给p,却发现没有合适的拷贝构造函数，因为只有auto_ptr(auto_ptr& rhs）这个不能用，又没有auto_ptr(const auto_ptr& rhs) （因为用了在所有权转移中会出错），呵呵！
3. 编译器只能曲线救国，看看类型转换后能不能传递。
4. 由于我们定义了 operator auto_ptr_ref() 所以编译器自然就可以试一下转为 auto_ptr_ref类型。
5. 编译器猛然间发现，我们定义了 auto_ptr(auto_ptr_ref rhs):ap(rhs.p){} 的构造函数，可以传递。
6. 顺利构造p,任务完成。
