Boost源码学习---scoped_ptr.hpp

scoped_ptr是一个智能指针，包装了指向堆上内存的指针。它对指针所有权加以限制，不能转让指针所有权，一旦scoped_ptr获取了指针的管理权，便不再释放，无法再从其取回来，就像scope意思一样，指针智能在作用域使用，不能转让出去。一旦离开scoped_ptr的作用域，将调用它的析构函数，释放指针，不用手动释放。下面是它的源代码：

#ifndef BOOST_SMART_PTR_SCOPED_PTR_HPP_INCLUDED
#define BOOST_SMART_PTR_SCOPED_PTR_HPP_INCLUDED

//  (C) Copyright Greg Colvin and Beman Dawes 1998, 1999.
//  Copyright (c) 2001, 2002 Peter Dimov
//
//  Distributed under the Boost Software License, Version 1.0. (See
//  accompanying file LICENSE_1_0.txt or copy at
//  http://www.boost.org/LICENSE_1_0.txt)
//
//  http://www.boost.org/libs/smart_ptr/scoped_ptr.htm
//

#include <boost/config.hpp>
#include <boost/assert.hpp>
#include <boost/checked_delete.hpp>
#include <boost/smart_ptr/detail/sp_nullptr_t.hpp>
#include <boost/detail/workaround.hpp>

#ifndef BOOST_NO_AUTO_PTR
# include <memory>          // for std::auto_ptr
#endif

namespace boost
{

// Debug hooks

#if defined(BOOST_SP_ENABLE_DEBUG_HOOKS)

void sp_scalar_constructor_hook(void * p);
void sp_scalar_destructor_hook(void * p);

#endif

//  scoped_ptr mimics a built-in pointer except that it guarantees deletion
//  of the object pointed to, either on destruction of the scoped_ptr or via
//  an explicit reset(). scoped_ptr is a simple solution for simple needs;
//  use shared_ptr or std::auto_ptr if your needs are more complex.

template<class T> class scoped_ptr // noncopyable
{
private:

    T * px;//原生态指针
		//将复制构造函数和赋值操作符声明为私有，不允许在类外使用，这样就限制了其所有权的转让
    scoped_ptr(scoped_ptr const &);
    scoped_ptr & operator=(scoped_ptr const &);

    typedef scoped_ptr<T> this_type;
		
		//重载为私有，不支持比较
    void operator==( scoped_ptr const& ) const;
    void operator!=( scoped_ptr const& ) const;

public:

    typedef T element_type;
		//显示构造函数，必须显示调用
    explicit scoped_ptr( T * p = 0 ): px( p ) // never throws
    {
#if defined(BOOST_SP_ENABLE_DEBUG_HOOKS)
        boost::sp_scalar_constructor_hook( px );
#endif
    }

#ifndef BOOST_NO_AUTO_PTR

		//参数为auto_ptr时的构造函数
		//p.release()设auto_ptr为NULL，返回其内部指针
    explicit scoped_ptr( std::auto_ptr<T> p ) BOOST_NOEXCEPT : px( p.release() )
    {
#if defined(BOOST_SP_ENABLE_DEBUG_HOOKS)
        boost::sp_scalar_constructor_hook( px );
#endif
    }

#endif
		//析构函数，析构对象，释放内存
    ~scoped_ptr() // never throws
    {
#if defined(BOOST_SP_ENABLE_DEBUG_HOOKS)
        boost::sp_scalar_destructor_hook( px );
#endif
        boost::checked_delete( px );
    }

    void reset(T * p = 0) // never throws
    {
        BOOST_ASSERT( p == 0 || p != px ); // catch self-reset errors
        //把p转换为scoped_prt，再调用swap交换
        this_type(p).swap(*this);
    }
		//重载解引用，返回指针指向的对象
    T & operator*() const // never throws
    {
        BOOST_ASSERT( px != 0 );
        return *px;
    }
		//重载->，返回指针
    T * operator->() const // never throws
    {
        BOOST_ASSERT( px != 0 );
        return px;
    }
		//返回指针
    T * get() const BOOST_NOEXCEPT
    {
        return px;
    }

// implicit conversion to "bool"
#include <boost/smart_ptr/detail/operator_bool.hpp>
		//交换两个指针
    void swap(scoped_ptr & b) BOOST_NOEXCEPT
    {
        T * tmp = b.px;
        b.px = px;
        px = tmp;
    }
};

#if !defined( BOOST_NO_CXX11_NULLPTR )

template<class T> inline bool operator==( scoped_ptr<T> const & p, boost::detail::sp_nullptr_t ) BOOST_NOEXCEPT
{
    return p.get() == 0;
}

template<class T> inline bool operator==( boost::detail::sp_nullptr_t, scoped_ptr<T> const & p ) BOOST_NOEXCEPT
{
    return p.get() == 0;
}

template<class T> inline bool operator!=( scoped_ptr<T> const & p, boost::detail::sp_nullptr_t ) BOOST_NOEXCEPT
{
    return p.get() != 0;
}

template<class T> inline bool operator!=( boost::detail::sp_nullptr_t, scoped_ptr<T> const & p ) BOOST_NOEXCEPT
{
    return p.get() != 0;
}

#endif

template<class T> inline void swap(scoped_ptr<T> & a, scoped_ptr<T> & b) BOOST_NOEXCEPT
{
    a.swap(b);
}

// get_pointer(p) is a generic way to say p.get()

template<class T> inline T * get_pointer(scoped_ptr<T> const & p) BOOST_NOEXCEPT
{
    return p.get();
}

} // namespace boost

#endif // #ifndef BOOST_SMART_PTR_SCOPED_PTR_HPP_INCLUDED

scoped_ptr与auto_ptr不同的是：auto_ptr可以转让使用权，在转让的时候，上一个指针失去使用权。而scoped_ptr不允许转让使用权。

scoped_ptr和auto_ptr都不能放在容器，auto_ptr是因为它只允许一个auto_prt使用指针，而scoped_ptr是因为不允许转让使用权，不支持拷贝和复制，不符合容器对元素的要求。