weak_ptr是辅助shared_ptr的智能指针。就像它的名字一样,是个“弱”指针;仅有几个接口,仅能完成很少工作。它可以从一个shared_ptr或weak_ptr对象构造,获取对资源的观测权,它是没有共享资源的,所以它的对象的创建不会引起指针引用计数的增加,它的对象的析构也不会引起计数器的减少。
它没有重载*和->,不能使用指针,主要的几个接口如下:
long use_count() const//返回计数器数值 bool expired() const//判断指针是否失效,也就是判断计数器是否为零 shared_ptr<T> lock() const//返回一个shared_ptr<T>指针 void reset()//重置weak_ptr void swap(weak_ptr<T> &b)//交换两个若指针
weak_ptr的一个重要用途是获得this指针的shared_ptr,使对象能够生产shared_ptr管理自己:对象使用weak_ptr观测this指针(不影响计数器),在需要使用shared_ptr的时候调用lock()函数。
有一个类enable_shared_from_this<T>来帮助实现上述做法,它的头文件在<boost/enable_shared_from_this.hpp>中,主要声明:
template<class T> class enable_shared_from_this { public: shared_ptr<T> shared_from_this(); };
在使用的时候继承它既可使用shared_from_this来获取shared_ptr对象。(要注意不能使用非shared_ptr对象类使用这个函数,这样会导致shared_from_this返回的对象在析构时使用delete来释放栈上内存)。
#ifndef BOOST_SMART_PTR_WEAK_PTR_HPP_INCLUDED #define BOOST_SMART_PTR_WEAK_PTR_HPP_INCLUDED // // weak_ptr.hpp // // Copyright (c) 2001, 2002, 2003 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) // // See http://www.boost.org/libs/smart_ptr/weak_ptr.htm for documentation. // #include <memory> // boost.TR1 include order fix #include <boost/smart_ptr/detail/shared_count.hpp> #include <boost/smart_ptr/shared_ptr.hpp> namespace boost { template<class T> class weak_ptr { private: // Borland 5.5.1 specific workarounds typedef weak_ptr<T> this_type; public: typedef typename boost::detail::sp_element< T >::type element_type; //默认构造函数 weak_ptr() BOOST_NOEXCEPT : px(0), pn() // never throws in 1.30+ { } // generated copy constructor, assignment, destructor are fine... #if !defined( BOOST_NO_CXX11_RVALUE_REFERENCES ) // ... except in C++0x, move disables the implicit copy //复制构造函数 weak_ptr( weak_ptr const & r ) BOOST_NOEXCEPT : px( r.px ), pn( r.pn ) { } //赋值操作符 weak_ptr & operator=( weak_ptr const & r ) BOOST_NOEXCEPT { px = r.px; pn = r.pn; return *this; } #endif // // The "obvious" converting constructor implementation: // // template<class Y> // weak_ptr(weak_ptr<Y> const & r): px(r.px), pn(r.pn) // never throws // { // } // // has a serious problem. // // r.px may already have been invalidated. The px(r.px) // conversion may require access to *r.px (virtual inheritance). // // It is not possible to avoid spurious access violations since // in multithreaded programs r.px may be invalidated at any point. // template<class Y> #if !defined( BOOST_SP_NO_SP_CONVERTIBLE ) weak_ptr( weak_ptr<Y> const & r, typename boost::detail::sp_enable_if_convertible<Y,T>::type = boost::detail::sp_empty() ) #else weak_ptr( weak_ptr<Y> const & r ) #endif BOOST_NOEXCEPT : px(r.lock().get()), pn(r.pn) { boost::detail::sp_assert_convertible< Y, T >(); } #if !defined( BOOST_NO_CXX11_RVALUE_REFERENCES ) template<class Y> #if !defined( BOOST_SP_NO_SP_CONVERTIBLE ) weak_ptr( weak_ptr<Y> && r, typename boost::detail::sp_enable_if_convertible<Y,T>::type = boost::detail::sp_empty() ) #else weak_ptr( weak_ptr<Y> && r ) #endif BOOST_NOEXCEPT : px( r.lock().get() ), pn( static_cast< boost::detail::weak_count && >( r.pn ) ) { boost::detail::sp_assert_convertible< Y, T >(); r.px = 0; } // for better efficiency in the T == Y case weak_ptr( weak_ptr && r ) BOOST_NOEXCEPT : px( r.px ), pn( static_cast< boost::detail::weak_count && >( r.pn ) ) { r.px = 0; } // for better efficiency in the T == Y case weak_ptr & operator=( weak_ptr && r ) BOOST_NOEXCEPT { this_type( static_cast< weak_ptr && >( r ) ).swap( *this ); return *this; } #endif template<class Y> #if !defined( BOOST_SP_NO_SP_CONVERTIBLE ) weak_ptr( shared_ptr<Y> const & r, typename boost::detail::sp_enable_if_convertible<Y,T>::type = boost::detail::sp_empty() ) #else //由shared_ptr创建weak_ptr weak_ptr( shared_ptr<Y> const & r ) #endif BOOST_NOEXCEPT : px( r.px ), pn( r.pn ) { boost::detail::sp_assert_convertible< Y, T >(); } #if !defined(BOOST_MSVC) || (BOOST_MSVC >= 1300) template<class Y> weak_ptr & operator=( weak_ptr<Y> const & r ) BOOST_NOEXCEPT { boost::detail::sp_assert_convertible< Y, T >(); px = r.lock().get(); pn = r.pn; return *this; } #if !defined( BOOST_NO_CXX11_RVALUE_REFERENCES ) template<class Y> weak_ptr & operator=( weak_ptr<Y> && r ) BOOST_NOEXCEPT { this_type( static_cast< weak_ptr<Y> && >( r ) ).swap( *this ); return *this; } #endif //赋值操作符,右值为shared_ptr template<class Y> weak_ptr & operator=( shared_ptr<Y> const & r ) BOOST_NOEXCEPT { boost::detail::sp_assert_convertible< Y, T >(); px = r.px; pn = r.pn; return *this; } #endif //主要的几个接口如下:: //返回shared_ptr对象 shared_ptr<T> lock() const BOOST_NOEXCEPT { return shared_ptr<T>( *this, boost::detail::sp_nothrow_tag() ); } //返回计数器数值 long use_count() const BOOST_NOEXCEPT { return pn.use_count(); } //判断指针是否有效,判断其计数器是否为零 bool expired() const BOOST_NOEXCEPT { return pn.use_count() == 0; } bool _empty() const // extension, not in std::weak_ptr { return pn.empty(); } void reset() BOOST_NOEXCEPT // never throws in 1.30+ { this_type().swap(*this); } void swap(this_type & other) BOOST_NOEXCEPT { std::swap(px, other.px); pn.swap(other.pn); } template<typename Y> void _internal_aliasing_assign(weak_ptr<Y> const & r, element_type * px2) { px = px2; pn = r.pn; } template<class Y> bool owner_before( weak_ptr<Y> const & rhs ) const BOOST_NOEXCEPT { return pn < rhs.pn; } template<class Y> bool owner_before( shared_ptr<Y> const & rhs ) const BOOST_NOEXCEPT { return pn < rhs.pn; } // Tasteless as this may seem, making all members public allows member templates // to work in the absence of member template friends. (Matthew Langston) #ifndef BOOST_NO_MEMBER_TEMPLATE_FRIENDS private: //设为友元 template<class Y> friend class weak_ptr; template<class Y> friend class shared_ptr; #endif //类的成员变量 element_type * px; // contained pointer boost::detail::weak_count pn; // reference counter }; // weak_ptr template<class T, class U> inline bool operator<(weak_ptr<T> const & a, weak_ptr<U> const & b) BOOST_NOEXCEPT { return a.owner_before( b ); } template<class T> void swap(weak_ptr<T> & a, weak_ptr<T> & b) BOOST_NOEXCEPT { a.swap(b); } } // namespace boost #endif // #ifndef BOOST_SMART_PTR_WEAK_PTR_HPP_INCLUDED
时间: 2024-10-05 23:09:16