STL auto_ptr

先贴下auto_ptr的SGI实现

/*
 * Copyright (c) 1997-1999
 * Silicon Graphics Computer Systems, Inc.
 *
 * Permission to use, copy, modify, distribute and sell this software
 * and its documentation for any purpose is hereby granted without fee,
 * provided that the above copyright notice appear in all copies and
 * that both that copyright notice and this permission notice appear
 * in supporting documentation.  Silicon Graphics makes no
 * representations about the suitability of this software for any
 * purpose.  It is provided "as is" without express or implied warranty.
 *
 */

#ifndef __SGI_STL_MEMORY
#define __SGI_STL_MEMORY

#include <stl_algobase.h>
#include <stl_alloc.h>
#include <stl_construct.h>
#include <stl_tempbuf.h>
#include <stl_uninitialized.h>
#include <stl_raw_storage_iter.h>


__STL_BEGIN_NAMESPACE

#if defined(__SGI_STL_USE_AUTO_PTR_CONVERSIONS) && \
    defined(__STL_MEMBER_TEMPLATES)

template<class _Tp1> struct auto_ptr_ref {
  _Tp1* _M_ptr;
  auto_ptr_ref(_Tp1* __p) : _M_ptr(__p) {}
};

#endif

template <class _Tp> class auto_ptr {
private:
  _Tp* _M_ptr;

public:
  typedef _Tp element_type;

  explicit auto_ptr(_Tp* __p = 0) __STL_NOTHROW : _M_ptr(__p) {}
  auto_ptr(auto_ptr& __a) __STL_NOTHROW : _M_ptr(__a.release()) {}

#ifdef __STL_MEMBER_TEMPLATES
  template <class _Tp1> auto_ptr(auto_ptr<_Tp1>& __a) __STL_NOTHROW
    : _M_ptr(__a.release()) {}
#endif /* __STL_MEMBER_TEMPLATES */

  auto_ptr& operator=(auto_ptr& __a) __STL_NOTHROW {
    if (&__a != this) {
      delete _M_ptr;
      _M_ptr = __a.release();
    }
    return *this;
  }

#ifdef __STL_MEMBER_TEMPLATES
  template <class _Tp1>
  auto_ptr& operator=(auto_ptr<_Tp1>& __a) __STL_NOTHROW {
    if (__a.get() != this->get()) {
      delete _M_ptr;
      _M_ptr = __a.release();
    }
    return *this;
  }
#endif /* __STL_MEMBER_TEMPLATES */

  // Note: 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 (17.4.3.6/2)
  // this is prohibited.
  ~auto_ptr() { delete _M_ptr; }

  _Tp& operator*() const __STL_NOTHROW {
    return *_M_ptr;
  }
  _Tp* operator->() const __STL_NOTHROW {
    return _M_ptr;
  }
  _Tp* get() const __STL_NOTHROW {
    return _M_ptr;
  }
  _Tp* release() __STL_NOTHROW {
    _Tp* __tmp = _M_ptr;
    _M_ptr = 0;
    return __tmp;
  }
  void reset(_Tp* __p = 0) __STL_NOTHROW {
    if (__p != _M_ptr) {
      delete _M_ptr;
      _M_ptr = __p;
    }
  }

  // According to the C++ standard, these conversions are required.  Most
  // present-day compilers, however, do not enforce that requirement---and, 
  // in fact, most present-day compilers do not support the language 
  // features that these conversions rely on.
  
#if defined(__SGI_STL_USE_AUTO_PTR_CONVERSIONS) && \
    defined(__STL_MEMBER_TEMPLATES)

public:
  auto_ptr(auto_ptr_ref<_Tp> __ref) __STL_NOTHROW
    : _M_ptr(__ref._M_ptr) {}

  auto_ptr& operator=(auto_ptr_ref<_Tp> __ref) __STL_NOTHROW {
    if (__ref._M_ptr != this->get()) {
      delete _M_ptr;
      _M_ptr = __ref._M_ptr;
    }
    return *this;
  }

  template <class _Tp1> operator auto_ptr_ref<_Tp1>() __STL_NOTHROW 
    { return auto_ptr_ref<_Tp1>(this->release()); }
  template <class _Tp1> operator auto_ptr<_Tp1>() __STL_NOTHROW
    { return auto_ptr<_Tp1>(this->release()); }

#endif /* auto ptr conversions && member templates */
};

__STL_END_NAMESPACE

#endif /* __SGI_STL_MEMORY */


// Local Variables:
// mode:C++
/

auto_ptr的几个限制：

1、auto_ptr不能共享所有权。 一个指针对象，只能属于一个auto_ptr对象。从源码中的拷贝构造函数和赋值函数可以看出，当用一个auto_ptr的对象ap1去构造或者赋值给另一个auto_ptr的对象ap2时，会先调用ap1.release()。 ap1.release()里面，把_M_ptr置为了0.
2、auto_ptr不能指向数组。从源码可以看出，auto_ptr的析构函数，用的是delete，而不是delete[]
3、auto_ptr不能作为容器的成员。
4、不能通过赋值操作来初始化auto_ptr
std::auto_ptr<int> p(new int(42));     //OK
std::auto_ptr<int> p = new int(42);    //ERROR
这是因为auto_ptr 的构造函数被定义为了explicit