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C++ 智能指针(shared_ptr/weak_ptr)源码分析

来源:互联网 作者:佚名 时间:2016-05-30 10:52
C11目前已经引入了unique_ptr, shared_ptr, weak_ptr等智能指针以及相关的模板类enable_shared_from_this等。 被广泛使用的是shared_ptr, shared_pt 具有C中一般指针(build-in/raw)的特性,同时它可以管理用户用new创建的对象,可以说,shared_ptr实现了C中

C++11目前已经引入了unique_ptr, shared_ptr, weak_ptr等智能指针以及相关的模板类enable_shared_from_this等。被广泛使用的是shared_ptr, shared_pt具有C++中一般指针(build-in/raw)的特性,同时它可以管理用户用new创建的对象,可以说,shared_ptr实现了C++中的RAII机制,让用户不用负责对象的内存回收,可以很方便的管理对象的生命周期,避免内存泄漏。一般的智能指针都定义为一个模板类,它的类型由被管理的对象类型初始化,内部包含了一个指向该对象的裸指针。


unique_ptr, shared_ptr, weak_ptr的特点如下:

  • unique_ptr独享被管理对象,同一时刻只能有一个unique_ptr拥有对象的所有权,当其被赋值时对象的所有权也发生转移,当其被销毁时对象也自动被销毁
  • shared_ptr共享被管理对象同一时刻可以有多个shared_ptr拥有对象的所有权,当最后一个shared_ptr对象销毁时,对象自动销毁
  • weak_ptr不拥有对象的所有权,但是它可以判断对象是否存在和返回指向对象的shared_ptr指针;它的用途之一是了解决多个对象内部含有shared_ptr引起的循环指向导致对象无法释放的问题

那么C++中是怎么实现这些特性的呢,我们可以在gcc的源码目录(gcc-6.1.0\gcc-6.1.0\libstdc++-v3\include\tr1)中找到智能指针的一种实现,通过分析其源码找到答案;其它例如boost::shared_ptr等的实现也是类似的。gcc中相关智能指针的实现源码如下

// <tr1/shared_ptr.h> -*- C++ -*-

// Copyright (C) 2007-2016 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 3, 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.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

//  shared_count.hpp
//  Copyright (c) 2001, 2002, 2003 Peter Dimov and Multi Media Ltd.

//  shared_ptr.hpp
//  Copyright (C) 1998, 1999 Greg Colvin and Beman Dawes.
//  Copyright (C) 2001, 2002, 2003 Peter Dimov

//  weak_ptr.hpp
//  Copyright (C) 2001, 2002, 2003 Peter Dimov

//  enable_shared_from_this.hpp
//  Copyright (C) 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)

// GCC Note:  based on version 1.32.0 of the Boost library.

/** @file tr1/shared_ptr.h
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{tr1/memory}
 */

#ifndef _TR1_SHARED_PTR_H
#define _TR1_SHARED_PTR_H 1

namespace std _GLIBCXX_VISIBILITY(default)
{
namespace tr1
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

 /**
   *  @brief  Exception possibly thrown by @c shared_ptr.
   *  @ingroup exceptions
   */
  class bad_weak_ptr : public std::exception
  {
  public:
    virtual char const*
    what() const throw()
    { return "tr1::bad_weak_ptr"; }
  };

  // Substitute for bad_weak_ptr object in the case of -fno-exceptions.
  inline void
  __throw_bad_weak_ptr()
  { _GLIBCXX_THROW_OR_ABORT(bad_weak_ptr()); }

  using __gnu_cxx::_Lock_policy;
  using __gnu_cxx::__default_lock_policy;
  using __gnu_cxx::_S_single;
  using __gnu_cxx::_S_mutex;
  using __gnu_cxx::_S_atomic;

  // Empty helper class except when the template argument is _S_mutex.
  template<_Lock_policy _Lp>
    class _Mutex_base
    {
    protected:
      // The atomic policy uses fully-fenced builtins, single doesn't care.
      enum { _S_need_barriers = 0 };
    };

  template<>
    class _Mutex_base<_S_mutex>
    : public __gnu_cxx::__mutex
    {
    protected:
      // This policy is used when atomic builtins are not available.
      // The replacement atomic operations might not have the necessary
      // memory barriers.
      enum { _S_need_barriers = 1 };
    };

  template<_Lock_policy _Lp = __default_lock_policy>
    class _Sp_counted_base
    : public _Mutex_base<_Lp>
    {
    public:  
      _Sp_counted_base()
      : _M_use_count(1), _M_weak_count(1) { }
      
      virtual
      ~_Sp_counted_base() // nothrow 
      { }
  
      // Called when _M_use_count drops to zero, to release the resources
      // managed by *this.
      virtual void
      _M_dispose() = 0; // nothrow
      
      // Called when _M_weak_count drops to zero.
      virtual void
      _M_destroy() // nothrow
      { delete this; }
      
      virtual void*
      _M_get_deleter(const std::type_info&) = 0;

      void
      _M_add_ref_copy()
      { __gnu_cxx::__atomic_add_dispatch(&_M_use_count, 1); }
  
      void
      _M_add_ref_lock();
      
      void
      _M_release() // nothrow
      {
        // Be race-detector-friendly.  For more info see bits/c++config.
        _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_use_count);
	if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, -1) == 1)
	  {
            _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_use_count);
	    _M_dispose();
	    // There must be a memory barrier between dispose() and destroy()
	    // to ensure that the effects of dispose() are observed in the
	    // thread that runs destroy().
	    // See http://gcc.gnu.org/ml/libstdc++/2005-11/msg00136.html
	    if (_Mutex_base<_Lp>::_S_need_barriers)
	      {
		__atomic_thread_fence (__ATOMIC_ACQ_REL);
	      }

            // Be race-detector-friendly.  For more info see bits/c++config.
            _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count);
	    if (__gnu_cxx::__exchange_and_add_dispatch(&_M_weak_count,
						       -1) == 1)
              {
                _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count);
	        _M_destroy();
              }
	  }
      }
  
      void
      _M_weak_add_ref() // nothrow
      { __gnu_cxx::__atomic_add_dispatch(&_M_weak_count, 1); }

      void
      _M_weak_release() // nothrow
      {
        // Be race-detector-friendly. For more info see bits/c++config.
        _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count);
	if (__gnu_cxx::__exchange_and_add_dispatch(&_M_weak_count, -1) == 1)
	  {
            _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count);
	    if (_Mutex_base<_Lp>::_S_need_barriers)
	      {
	        // See _M_release(),
	        // destroy() must observe results of dispose()
		__atomic_thread_fence (__ATOMIC_ACQ_REL);
	      }
	    _M_destroy();
	  }
      }
  
      long
      _M_get_use_count() const // nothrow
      {
        // No memory barrier is used here so there is no synchronization
        // with other threads.
        return const_cast<const volatile _Atomic_word&>(_M_use_count);
      }

    private:  
      _Sp_counted_base(_Sp_counted_base const&);
      _Sp_counted_base& operator=(_Sp_counted_base const&);

      _Atomic_word  _M_use_count;     // #shared
      _Atomic_word  _M_weak_count;    // #weak + (#shared != 0)
    };

  template<>
    inline void
    _Sp_counted_base<_S_single>::
    _M_add_ref_lock()
    {
      if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, 1) == 0)
	{
	  _M_use_count = 0;
	  __throw_bad_weak_ptr();
	}
    }

  template<>
    inline void
    _Sp_counted_base<_S_mutex>::
    _M_add_ref_lock()
    {
      __gnu_cxx::__scoped_lock sentry(*this);
      if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, 1) == 0)
	{
	  _M_use_count = 0;
	  __throw_bad_weak_ptr();
	}
    }

  template<> 
    inline void
    _Sp_counted_base<_S_atomic>::
    _M_add_ref_lock()
    {
      // Perform lock-free add-if-not-zero operation.
      _Atomic_word __count = _M_use_count;
      do
	{
	  if (__count == 0)
	    __throw_bad_weak_ptr();
	  // Replace the current counter value with the old value + 1, as
	  // long as it's not changed meanwhile. 
	}
      while (!__atomic_compare_exchange_n(&_M_use_count, &__count, __count + 1,
					  true, __ATOMIC_ACQ_REL, 
					  __ATOMIC_RELAXED));
     }

  template<typename _Ptr, typename _Deleter, _Lock_policy _Lp>
    class _Sp_counted_base_impl
    : public _Sp_counted_base<_Lp>
    {
    public:
      // Precondition: __d(__p) must not throw.
      _Sp_counted_base_impl(_Ptr __p, _Deleter __d)
      : _M_ptr(__p), _M_del(__d) { }
    
      virtual void
      _M_dispose() // nothrow
      { _M_del(_M_ptr); }
      
      virtual void*
      _M_get_deleter(const std::type_info& __ti)
      {
#if __cpp_rtti
        return __ti == typeid(_Deleter) ? &_M_del : 0;
#else
        return 0;
#endif
      }
      
    private:
      _Sp_counted_base_impl(const _Sp_counted_base_impl&);
      _Sp_counted_base_impl& operator=(const _Sp_counted_base_impl&);
      
      _Ptr      _M_ptr;  // copy constructor must not throw
      _Deleter  _M_del;  // copy constructor must not throw
    };

  template<_Lock_policy _Lp = __default_lock_policy>
    class __weak_count;

  template<typename _Tp>
    struct _Sp_deleter
    {
      typedef void result_type;
      typedef _Tp* argument_type;
      void operator()(_Tp* __p) const { delete __p; }
    };

  template<_Lock_policy _Lp = __default_lock_policy>
    class __shared_count
    {
    public: 
      __shared_count()
      : _M_pi(0) // nothrow
      { }
  
      template<typename _Ptr>
        __shared_count(_Ptr __p) : _M_pi(0)
        {
	  __try
	    {
	      typedef typename std::tr1::remove_pointer<_Ptr>::type _Tp;
	      _M_pi = new _Sp_counted_base_impl<_Ptr, _Sp_deleter<_Tp>, _Lp>(
	          __p, _Sp_deleter<_Tp>());
	    }
	  __catch(...)
	    {
	      delete __p;
	      __throw_exception_again;
	    }
	}

      template<typename _Ptr, typename _Deleter>
        __shared_count(_Ptr __p, _Deleter __d) : _M_pi(0)
        {
	  __try
	    {
	      _M_pi = new _Sp_counted_base_impl<_Ptr, _Deleter, _Lp>(__p, __d);
	    }
	  __catch(...)
	    {
	      __d(__p); // Call _Deleter on __p.
	      __throw_exception_again;
	    }
	}

      // Special case for auto_ptr<_Tp> to provide the strong guarantee.
      template<typename _Tp>
        explicit
        __shared_count(std::auto_ptr<_Tp>& __r)
	: _M_pi(new _Sp_counted_base_impl<_Tp*,
		_Sp_deleter<_Tp>, _Lp >(__r.get(), _Sp_deleter<_Tp>()))
        { __r.release(); }

      // Throw bad_weak_ptr when __r._M_get_use_count() == 0.
      explicit
      __shared_count(const __weak_count<_Lp>& __r);
  
      ~__shared_count() // nothrow
      {
	if (_M_pi != 0)
	  _M_pi->_M_release();
      }
      
      __shared_count(const __shared_count& __r)
      : _M_pi(__r._M_pi) // nothrow
      {
	if (_M_pi != 0)
	  _M_pi->_M_add_ref_copy();
      }
  
      __shared_count&
      operator=(const __shared_count& __r) // nothrow
      {
	_Sp_counted_base<_Lp>* __tmp = __r._M_pi;
	if (__tmp != _M_pi)
	  {
	    if (__tmp != 0)
	      __tmp->_M_add_ref_copy();
	    if (_M_pi != 0)
	      _M_pi->_M_release();
	    _M_pi = __tmp;
	  }
	return *this;
      }
  
      void
      _M_swap(__shared_count& __r) // nothrow
      {
	_Sp_counted_base<_Lp>* __tmp = __r._M_pi;
	__r._M_pi = _M_pi;
	_M_pi = __tmp;
      }
  
      long
      _M_get_use_count() const // nothrow
      { return _M_pi != 0 ? _M_pi->_M_get_use_count() : 0; }

      bool
      _M_unique() const // nothrow
      { return this->_M_get_use_count() == 1; }
      
      friend inline bool
      operator==(const __shared_count& __a, const __shared_count& __b)
      { return __a._M_pi == __b._M_pi; }
  
      friend inline bool
      operator<(const __shared_count& __a, const __shared_count& __b)
      { return std::less<_Sp_counted_base<_Lp>*>()(__a._M_pi, __b._M_pi); }
  
      void*
      _M_get_deleter(const std::type_info& __ti) const
      { return _M_pi ? _M_pi->_M_get_deleter(__ti) : 0; }

    private:
      friend class __weak_count<_Lp>;

      _Sp_counted_base<_Lp>*  _M_pi;
    };


  template<_Lock_policy _Lp>
    class __weak_count
    {
    public:
      __weak_count()
      : _M_pi(0) // nothrow
      { }
  
      __weak_count(const __shared_count<_Lp>& __r)
      : _M_pi(__r._M_pi) // nothrow
      {
	if (_M_pi != 0)
	  _M_pi->_M_weak_add_ref();
      }
      
      __weak_count(const __weak_count<_Lp>& __r)
      : _M_pi(__r._M_pi) // nothrow
      {
	if (_M_pi != 0)
	  _M_pi->_M_weak_add_ref();
      }
      
      ~__weak_count() // nothrow
      {
	if (_M_pi != 0)
	  _M_pi->_M_weak_release();
      }
      
      __weak_count<_Lp>&
      operator=(const __shared_count<_Lp>& __r) // nothrow
      {
	_Sp_counted_base<_Lp>* __tmp = __r._M_pi;
	if (__tmp != 0)
	  __tmp->_M_weak_add_ref();
	if (_M_pi != 0)
	  _M_pi->_M_weak_release();
	_M_pi = __tmp;  
	return *this;
      }
      
      __weak_count<_Lp>&
      operator=(const __weak_count<_Lp>& __r) // nothrow
      {
	_Sp_counted_base<_Lp>* __tmp = __r._M_pi;
	if (__tmp != 0)
	  __tmp->_M_weak_add_ref();
	if (_M_pi != 0)
	  _M_pi->_M_weak_release();
	_M_pi = __tmp;
	return *this;
      }

      void
      _M_swap(__weak_count<_Lp>& __r) // nothrow
      {
	_Sp_counted_base<_Lp>* __tmp = __r._M_pi;
	__r._M_pi = _M_pi;
	_M_pi = __tmp;
      }
  
      long
      _M_get_use_count() const // nothrow
      { return _M_pi != 0 ? _M_pi->_M_get_use_count() : 0; }

      friend inline bool
      operator==(const __weak_count<_Lp>& __a, const __weak_count<_Lp>& __b)
      { return __a._M_pi == __b._M_pi; }
      
      friend inline bool
      operator<(const __weak_count<_Lp>& __a, const __weak_count<_Lp>& __b)
      { return std::less<_Sp_counted_base<_Lp>*>()(__a._M_pi, __b._M_pi); }

    private:
      friend class __shared_count<_Lp>;

      _Sp_counted_base<_Lp>*  _M_pi;
    };

  // now that __weak_count is defined we can define this constructor:
  template<_Lock_policy _Lp>
    inline
    __shared_count<_Lp>::
    __shared_count(const __weak_count<_Lp>& __r)
    : _M_pi(__r._M_pi)
    {
      if (_M_pi != 0)
	_M_pi->_M_add_ref_lock();
      else
	__throw_bad_weak_ptr();
    }

  // Forward declarations.
  template<typename _Tp, _Lock_policy _Lp = __default_lock_policy>
    class __shared_ptr;
  
  template<typename _Tp, _Lock_policy _Lp = __default_lock_policy>
    class __weak_ptr;

  template<typename _Tp, _Lock_policy _Lp = __default_lock_policy>
    class __enable_shared_from_this;

  template<typename _Tp>
    class shared_ptr;
  
  template<typename _Tp>
    class weak_ptr;

  template<typename _Tp>
    class enable_shared_from_this;

  // Support for enable_shared_from_this.

  // Friend of __enable_shared_from_this.
  template<_Lock_policy _Lp, typename _Tp1, typename _Tp2>
    void
    __enable_shared_from_this_helper(const __shared_count<_Lp>&,
				     const __enable_shared_from_this<_Tp1,
				     _Lp>*, const _Tp2*);

  // Friend of enable_shared_from_this.
  template<typename _Tp1, typename _Tp2>
    void
    __enable_shared_from_this_helper(const __shared_count<>&,
				     const enable_shared_from_this<_Tp1>*,
				     const _Tp2*);

  template<_Lock_policy _Lp>
    inline void
    __enable_shared_from_this_helper(const __shared_count<_Lp>&, ...)
    { }


  struct __static_cast_tag { };
  struct __const_cast_tag { };
  struct __dynamic_cast_tag { };

  // A smart pointer with reference-counted copy semantics.  The
  // object pointed to is deleted when the last shared_ptr pointing to
  // it is destroyed or reset.
  template<typename _Tp, _Lock_policy _Lp>
    class __shared_ptr
    {
    public:
      typedef _Tp   element_type;
      
      __shared_ptr()
      : _M_ptr(0), _M_refcount() // never throws
      { }

      template<typename _Tp1>
        explicit
        __shared_ptr(_Tp1* __p)
	: _M_ptr(__p), _M_refcount(__p)
        {
	  __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)
	  typedef int _IsComplete[sizeof(_Tp1)];
	  __enable_shared_from_this_helper(_M_refcount, __p, __p);
	}

      template<typename _Tp1, typename _Deleter>
        __shared_ptr(_Tp1* __p, _Deleter __d)
        : _M_ptr(__p), _M_refcount(__p, __d)
        {
	  __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)
	  // TODO requires _Deleter CopyConstructible and __d(__p) well-formed
	  __enable_shared_from_this_helper(_M_refcount, __p, __p);
	}
      
      //  generated copy constructor, assignment, destructor are fine.
      
      template<typename _Tp1>
        __shared_ptr(const __shared_ptr<_Tp1, _Lp>& __r)
	: _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount) // never throws
        { __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>) }

      template<typename _Tp1>
        explicit
        __shared_ptr(const __weak_ptr<_Tp1, _Lp>& __r)
	: _M_refcount(__r._M_refcount) // may throw
        {
	  __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)
	  // It is now safe to copy __r._M_ptr, as _M_refcount(__r._M_refcount)
	  // did not throw.
	  _M_ptr = __r._M_ptr;
	}

#if (__cplusplus < 201103L) || _GLIBCXX_USE_DEPRECATED
      // Postcondition: use_count() == 1 and __r.get() == 0
      template<typename _Tp1>
        explicit
        __shared_ptr(std::auto_ptr<_Tp1>& __r)
	: _M_ptr(__r.get()), _M_refcount()
        { // TODO requries delete __r.release() well-formed
	  __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)
	  typedef int _IsComplete[sizeof(_Tp1)];
	  _Tp1* __tmp = __r.get();
	  _M_refcount = __shared_count<_Lp>(__r);
	  __enable_shared_from_this_helper(_M_refcount, __tmp, __tmp);
	}

#endif

      template<typename _Tp1>
        __shared_ptr(const __shared_ptr<_Tp1, _Lp>& __r, __static_cast_tag)
	: _M_ptr(static_cast<element_type*>(__r._M_ptr)),
	  _M_refcount(__r._M_refcount)
        { }

      template<typename _Tp1>
        __shared_ptr(const __shared_ptr<_Tp1, _Lp>& __r, __const_cast_tag)
	: _M_ptr(const_cast<element_type*>(__r._M_ptr)),
	  _M_refcount(__r._M_refcount)
        { }

      template<typename _Tp1>
        __shared_ptr(const __shared_ptr<_Tp1, _Lp>& __r, __dynamic_cast_tag)
	: _M_ptr(dynamic_cast<element_type*>(__r._M_ptr)),
	  _M_refcount(__r._M_refcount)
        {
	  if (_M_ptr == 0) // need to allocate new counter -- the cast failed
	    _M_refcount = __shared_count<_Lp>();
	}

      template<typename _Tp1>
        __shared_ptr&
        operator=(const __shared_ptr<_Tp1, _Lp>& __r) // never throws
        {
	  _M_ptr = __r._M_ptr;
	  _M_refcount = __r._M_refcount; // __shared_count::op= doesn't throw
	  return *this;
	}

#if (__cplusplus < 201103L) || _GLIBCXX_USE_DEPRECATED
      template<typename _Tp1>
        __shared_ptr&
        operator=(std::auto_ptr<_Tp1>& __r)
        {
	  __shared_ptr(__r).swap(*this);
	  return *this;
	}
#endif

      void
      reset() // never throws
      { __shared_ptr().swap(*this); }

      template<typename _Tp1>
        void
        reset(_Tp1* __p) // _Tp1 must be complete.
        {
	  // Catch self-reset errors.
	  _GLIBCXX_DEBUG_ASSERT(__p == 0 || __p != _M_ptr); 
	  __shared_ptr(__p).swap(*this);
	}

      template<typename _Tp1, typename _Deleter>
        void
        reset(_Tp1* __p, _Deleter __d)
        { __shared_ptr(__p, __d).swap(*this); }

      // Allow class instantiation when _Tp is [cv-qual] void.
      typename std::tr1::add_reference<_Tp>::type
      operator*() const // never throws
      {
	_GLIBCXX_DEBUG_ASSERT(_M_ptr != 0);
	return *_M_ptr;
      }

      _Tp*
      operator->() const // never throws
      {
	_GLIBCXX_DEBUG_ASSERT(_M_ptr != 0);
	return _M_ptr;
      }
    
      _Tp*
      get() const // never throws
      { return _M_ptr; }

      // Implicit conversion to "bool"
    private:
      typedef _Tp* __shared_ptr::*__unspecified_bool_type;

    public:
      operator __unspecified_bool_type() const // never throws
      { return _M_ptr == 0 ? 0 : &__shared_ptr::_M_ptr; }

      bool
      unique() const // never throws
      { return _M_refcount._M_unique(); }

      long
      use_count() const // never throws
      { return _M_refcount._M_get_use_count(); }

      void
      swap(__shared_ptr<_Tp, _Lp>& __other) // never throws
      {
	std::swap(_M_ptr, __other._M_ptr);
	_M_refcount._M_swap(__other._M_refcount);
      }

    private:
      void*
      _M_get_deleter(const std::type_info& __ti) const
      { return _M_refcount._M_get_deleter(__ti); }

      template<typename _Tp1, _Lock_policy _Lp1>
        bool
        _M_less(const __shared_ptr<_Tp1, _Lp1>& __rhs) const
        { return _M_refcount < __rhs._M_refcount; }

      template<typename _Tp1, _Lock_policy _Lp1> friend class __shared_ptr;
      template<typename _Tp1, _Lock_policy _Lp1> friend class __weak_ptr;

      template<typename _Del, typename _Tp1, _Lock_policy _Lp1>
        friend _Del* get_deleter(const __shared_ptr<_Tp1, _Lp1>&);

      // Friends injected into enclosing namespace and found by ADL:
      template<typename _Tp1>
        friend inline bool
        operator==(const __shared_ptr& __a, const __shared_ptr<_Tp1, _Lp>& __b)
        { return __a.get() == __b.get(); }

      template<typename _Tp1>
        friend inline bool
        operator!=(const __shared_ptr& __a, const __shared_ptr<_Tp1, _Lp>& __b)
        { return __a.get() != __b.get(); }

      template<typename _Tp1>
        friend inline bool
        operator<(const __shared_ptr& __a, const __shared_ptr<_Tp1, _Lp>& __b)
        { return __a._M_less(__b); }

      _Tp*         	   _M_ptr;         // Contained pointer.
      __shared_count<_Lp>  _M_refcount;    // Reference counter.
    };

  // 2.2.3.8 shared_ptr specialized algorithms.
  template<typename _Tp, _Lock_policy _Lp>
    inline void
    swap(__shared_ptr<_Tp, _Lp>& __a, __shared_ptr<_Tp, _Lp>& __b)
    { __a.swap(__b); }

  // 2.2.3.9 shared_ptr casts
  /*  The seemingly equivalent
   *           shared_ptr<_Tp, _Lp>(static_cast<_Tp*>(__r.get()))
   *  will eventually result in undefined behaviour,
   *  attempting to delete the same object twice.
   */
  template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
    inline __shared_ptr<_Tp, _Lp>
    static_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r)
    { return __shared_ptr<_Tp, _Lp>(__r, __static_cast_tag()); }

  /*  The seemingly equivalent
   *           shared_ptr<_Tp, _Lp>(const_cast<_Tp*>(__r.get()))
   *  will eventually result in undefined behaviour,
   *  attempting to delete the same object twice.
   */
  template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
    inline __shared_ptr<_Tp, _Lp>
    const_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r)
    { return __shared_ptr<_Tp, _Lp>(__r, __const_cast_tag()); }

  /*  The seemingly equivalent
   *           shared_ptr<_Tp, _Lp>(dynamic_cast<_Tp*>(__r.get()))
   *  will eventually result in undefined behaviour,
   *  attempting to delete the same object twice.
   */
  template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
    inline __shared_ptr<_Tp, _Lp>
    dynamic_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r)
    { return __shared_ptr<_Tp, _Lp>(__r, __dynamic_cast_tag()); }

  // 2.2.3.7 shared_ptr I/O
  template<typename _Ch, typename _Tr, typename _Tp, _Lock_policy _Lp>
    std::basic_ostream<_Ch, _Tr>&
    operator<<(std::basic_ostream<_Ch, _Tr>& __os, 
	       const __shared_ptr<_Tp, _Lp>& __p)
    {
      __os << __p.get();
      return __os;
    }

  // 2.2.3.10 shared_ptr get_deleter (experimental)
  template<typename _Del, typename _Tp, _Lock_policy _Lp>
    inline _Del*
    get_deleter(const __shared_ptr<_Tp, _Lp>& __p)
    {
#if __cpp_rtti
      return static_cast<_Del*>(__p._M_get_deleter(typeid(_Del)));
#else
      return 0;
#endif
    }


  template<typename _Tp, _Lock_policy _Lp>
    class __weak_ptr
    {
    public:
      typedef _Tp element_type;
      
      __weak_ptr()
      : _M_ptr(0), _M_refcount() // never throws
      { }

      // Generated copy constructor, assignment, destructor are fine.
      
      // The "obvious" converting constructor implementation:
      //
      //  template<typename _Tp1>
      //    __weak_ptr(const __weak_ptr<_Tp1, _Lp>& __r)
      //    : _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount) // never throws
      //    { }
      //
      // has a serious problem.
      //
      //  __r._M_ptr may already have been invalidated. The _M_ptr(__r._M_ptr)
      //  conversion may require access to *__r._M_ptr (virtual inheritance).
      //
      // It is not possible to avoid spurious access violations since
      // in multithreaded programs __r._M_ptr may be invalidated at any point.
      template<typename _Tp1>
        __weak_ptr(const __weak_ptr<_Tp1, _Lp>& __r)
	: _M_refcount(__r._M_refcount) // never throws
        {
	  __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)
	  _M_ptr = __r.lock().get();
	}

      template<typename _Tp1>
        __weak_ptr(const __shared_ptr<_Tp1, _Lp>& __r)
	: _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount) // never throws
        { __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>) }

      template<typename _Tp1>
        __weak_ptr&
        operator=(const __weak_ptr<_Tp1, _Lp>& __r) // never throws
        {
	  _M_ptr = __r.lock().get();
	  _M_refcount = __r._M_refcount;
	  return *this;
	}
      
      template<typename _Tp1>
        __weak_ptr&
        operator=(const __shared_ptr<_Tp1, _Lp>& __r) // never throws
        {
	  _M_ptr = __r._M_ptr;
	  _M_refcount = __r._M_refcount;
	  return *this;
	}

      __shared_ptr<_Tp, _Lp>
      lock() const // never throws
      {
#ifdef __GTHREADS
	// Optimization: avoid throw overhead.
	if (expired())
	  return __shared_ptr<element_type, _Lp>();

	__try
	  {
	    return __shared_ptr<element_type, _Lp>(*this);
	  }
	__catch(const bad_weak_ptr&)
	  {
	    // Q: How can we get here?
	    // A: Another thread may have invalidated r after the
	    //    use_count test above.
	    return __shared_ptr<element_type, _Lp>();
	  }
	
#else
	// Optimization: avoid try/catch overhead when single threaded.
	return expired() ? __shared_ptr<element_type, _Lp>()
	                 : __shared_ptr<element_type, _Lp>(*this);

#endif
      } // XXX MT

      long
      use_count() const // never throws
      { return _M_refcount._M_get_use_count(); }

      bool
      expired() const // never throws
      { return _M_refcount._M_get_use_count() == 0; }
      
      void
      reset() // never throws
      { __weak_ptr().swap(*this); }

      void
      swap(__weak_ptr& __s) // never throws
      {
	std::swap(_M_ptr, __s._M_ptr);
	_M_refcount._M_swap(__s._M_refcount);
      }

    private:
      // Used by __enable_shared_from_this.
      void
      _M_assign(_Tp* __ptr, const __shared_count<_Lp>& __refcount)
      {
	_M_ptr = __ptr;
	_M_refcount = __refcount;
      }

      template<typename _Tp1>
        bool
        _M_less(const __weak_ptr<_Tp1, _Lp>& __rhs) const
        { return _M_refcount < __rhs._M_refcount; }

      template<typename _Tp1, _Lock_policy _Lp1> friend class __shared_ptr;
      template<typename _Tp1, _Lock_policy _Lp1> friend class __weak_ptr;
      friend class __enable_shared_from_this<_Tp, _Lp>;
      friend class enable_shared_from_this<_Tp>;

      // Friend injected into namespace and found by ADL.
      template<typename _Tp1>
        friend inline bool
        operator<(const __weak_ptr& __lhs, const __weak_ptr<_Tp1, _Lp>& __rhs)
        { return __lhs._M_less(__rhs); }

      _Tp*       	 _M_ptr;         // Contained pointer.
      __weak_count<_Lp>  _M_refcount;    // Reference counter.
    };

  // 2.2.4.7 weak_ptr specialized algorithms.
  template<typename _Tp, _Lock_policy _Lp>
    inline void
    swap(__weak_ptr<_Tp, _Lp>& __a, __weak_ptr<_Tp, _Lp>& __b)
    { __a.swap(__b); }


  template<typename _Tp, _Lock_policy _Lp>
    class __enable_shared_from_this
    {
    protected:
      __enable_shared_from_this() { }
      
      __enable_shared_from_this(const __enable_shared_from_this&) { }
      
      __enable_shared_from_this&
      operator=(const __enable_shared_from_this&)
      { return *this; }

      ~__enable_shared_from_this() { }
      
    public:
      __shared_ptr<_Tp, _Lp>
      shared_from_this()
      { return __shared_ptr<_Tp, _Lp>(this->_M_weak_this); }

      __shared_ptr<const _Tp, _Lp>
      shared_from_this() const
      { return __shared_ptr<const _Tp, _Lp>(this->_M_weak_this); }

    private:
      template<typename _Tp1>
        void
        _M_weak_assign(_Tp1* __p, const __shared_count<_Lp>& __n) const
        { _M_weak_this._M_assign(__p, __n); }

      template<typename _Tp1>
        friend void
        __enable_shared_from_this_helper(const __shared_count<_Lp>& __pn,
					 const __enable_shared_from_this* __pe,
					 const _Tp1* __px)
        {
	  if (__pe != 0)
	    __pe->_M_weak_assign(const_cast<_Tp1*>(__px), __pn);
	}

      mutable __weak_ptr<_Tp, _Lp>  _M_weak_this;
    };


  // The actual shared_ptr, with forwarding constructors and
  // assignment operators.
  template<typename _Tp>
    class shared_ptr
    : public __shared_ptr<_Tp>
    {
    public:
      shared_ptr()
      : __shared_ptr<_Tp>() { }

      template<typename _Tp1>
        explicit
        shared_ptr(_Tp1* __p)
	: __shared_ptr<_Tp>(__p) { }

      template<typename _Tp1, typename _Deleter>
        shared_ptr(_Tp1* __p, _Deleter __d)
	: __shared_ptr<_Tp>(__p, __d) { }

      template<typename _Tp1>
        shared_ptr(const shared_ptr<_Tp1>& __r)
	: __shared_ptr<_Tp>(__r) { }

      template<typename _Tp1>
        explicit
        shared_ptr(const weak_ptr<_Tp1>& __r)
	: __shared_ptr<_Tp>(__r) { }

#if (__cplusplus < 201103L) || _GLIBCXX_USE_DEPRECATED
      template<typename _Tp1>
        explicit
        shared_ptr(std::auto_ptr<_Tp1>& __r)
	: __shared_ptr<_Tp>(__r) { }
#endif

      template<typename _Tp1>
        shared_ptr(const shared_ptr<_Tp1>& __r, __static_cast_tag)
	: __shared_ptr<_Tp>(__r, __static_cast_tag()) { }

      template<typename _Tp1>
        shared_ptr(const shared_ptr<_Tp1>& __r, __const_cast_tag)
	: __shared_ptr<_Tp>(__r, __const_cast_tag()) { }

      template<typename _Tp1>
        shared_ptr(const shared_ptr<_Tp1>& __r, __dynamic_cast_tag)
	: __shared_ptr<_Tp>(__r, __dynamic_cast_tag()) { }

      template<typename _Tp1>
        shared_ptr&
        operator=(const shared_ptr<_Tp1>& __r) // never throws
        {
	  this->__shared_ptr<_Tp>::operator=(__r);
	  return *this;
	}

#if (__cplusplus < 201103L) || _GLIBCXX_USE_DEPRECATED
      template<typename _Tp1>
        shared_ptr&
        operator=(std::auto_ptr<_Tp1>& __r)
        {
	  this->__shared_ptr<_Tp>::operator=(__r);
	  return *this;
	}
#endif
    };

  // 2.2.3.8 shared_ptr specialized algorithms.
  template<typename _Tp>
    inline void
    swap(__shared_ptr<_Tp>& __a, __shared_ptr<_Tp>& __b)
    { __a.swap(__b); }

  template<typename _Tp, typename _Tp1>
    inline shared_ptr<_Tp>
    static_pointer_cast(const shared_ptr<_Tp1>& __r)
    { return shared_ptr<_Tp>(__r, __static_cast_tag()); }

  template<typename _Tp, typename _Tp1>
    inline shared_ptr<_Tp>
    const_pointer_cast(const shared_ptr<_Tp1>& __r)
    { return shared_ptr<_Tp>(__r, __const_cast_tag()); }

  template<typename _Tp, typename _Tp1>
    inline shared_ptr<_Tp>
    dynamic_pointer_cast(const shared_ptr<_Tp1>& __r)
    { return shared_ptr<_Tp>(__r, __dynamic_cast_tag()); }


  // The actual weak_ptr, with forwarding constructors and
  // assignment operators.
  template<typename _Tp>
    class weak_ptr
    : public __weak_ptr<_Tp>
    {
    public:
      weak_ptr()
      : __weak_ptr<_Tp>() { }
      
      template<typename _Tp1>
        weak_ptr(const weak_ptr<_Tp1>& __r)
	: __weak_ptr<_Tp>(__r) { }

      template<typename _Tp1>
        weak_ptr(const shared_ptr<_Tp1>& __r)
	: __weak_ptr<_Tp>(__r) { }

      template<typename _Tp1>
        weak_ptr&
        operator=(const weak_ptr<_Tp1>& __r) // never throws
        {
	  this->__weak_ptr<_Tp>::operator=(__r);
	  return *this;
	}

      template<typename _Tp1>
        weak_ptr&
        operator=(const shared_ptr<_Tp1>& __r) // never throws
        {
	  this->__weak_ptr<_Tp>::operator=(__r);
	  return *this;
	}

      shared_ptr<_Tp>
      lock() const // never throws
      {
#ifdef __GTHREADS
	if (this->expired())
	  return shared_ptr<_Tp>();

	__try
	  {
	    return shared_ptr<_Tp>(*this);
	  }
	__catch(const bad_weak_ptr&)
	  {
	    return shared_ptr<_Tp>();
	  }
#else
	return this->expired() ? shared_ptr<_Tp>()
	                       : shared_ptr<_Tp>(*this);
#endif
      }
    };

  template<typename _Tp>
    class enable_shared_from_this
    {
    protected:
      enable_shared_from_this() { }
      
      enable_shared_from_this(const enable_shared_from_this&) { }

      enable_shared_from_this&
      operator=(const enable_shared_from_this&)
      { return *this; }

      ~enable_shared_from_this() { }

    public:
      shared_ptr<_Tp>
      shared_from_this()
      { return shared_ptr<_Tp>(this->_M_weak_this); }

      shared_ptr<const _Tp>
      shared_from_this() const
      { return shared_ptr<const _Tp>(this->_M_weak_this); }

    private:
      template<typename _Tp1>
        void
        _M_weak_assign(_Tp1* __p, const __shared_count<>& __n) const
        { _M_weak_this._M_assign(__p, __n); }

      template<typename _Tp1>
        friend void
        __enable_shared_from_this_helper(const __shared_count<>& __pn,
					 const enable_shared_from_this* __pe,
					 const _Tp1* __px)
        {
	  if (__pe != 0)
	    __pe->_M_weak_assign(const_cast<_Tp1*>(__px), __pn);
	}

      mutable weak_ptr<_Tp>  _M_weak_this;
    };

_GLIBCXX_END_NAMESPACE_VERSION
}
}

#endif // _TR1_SHARED_PTR_H
其主要的类关系如下所示(省略相关的类模板参数):


从上面的类图我们可以很清楚的看出shared_ptr内部,含有一个指向被管理对象(managed object)T的指针以及一个__shared_count对象,__shared_count对象包含一个指向管理模块(manager object)的基类指针,管理模块(manager object)具有原子属性的use_count和weak_count、指向被管理对象(managed object)T的指针、以及用来销毁被管理对象的deleter组成:


weak_ptr内部组成与shared_ptr类似,内部同样含有一个指向被管理对象T的指针以及一个__weak_count对象:


很明显,shared_ptr与weak_ptr的差异主要是由__shared_ptr与__weak_ptr体现出来的,而__shared_ptr与__weak_ptr的差异则主要是由__shared_count与__weak_count体现出来。


通过shared_ptr的构造函数,可以发现,在创建一个shared_ptr的时候需要一个new 操作符返回的被管理对象的地址来初始化shared_ptr, shared_ptr在内部会构建一个_shared_count对象,由_shared_count对象的构造函数可知,创建shared_ptr的时候也动态的创建了一个管理对象_Sp_counted_base_impl:

template<typename _Tp1> explicit __shared_ptr(_Tp1* __p)
: _M_ptr(__p), _M_refcount(__p) {
    __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)
    typedef int _IsComplete[sizeof(_Tp1)];
    __enable_shared_from_this_helper(_M_refcount, __p, __p);
}

template<typename _Ptr>
__shared_count(_Ptr __p) : _M_pi(0)
{
    __try
   {
	  typedef typename std::tr1::remove_pointer<_Ptr>::type _Tp;
	  _M_pi = new _Sp_counted_base_impl<_Ptr, _Sp_deleter<_Tp>, _Lp>(__p, _Sp_deleter<_Tp>());
    }
    __catch(...)
    {
        delete __p;
	__throw_exception_again;
    }
}


由上面我们不难发现,shared_ptr内部包含一个指向被管理对象的指针_M_ptr,_Sp_counted_base_impl内部也含有一个指向被管理对象的指针_M_ptr, 它们是不是重复多余了呢?实际上没有。这首先要从shared_ptr的拷贝构造或者赋值构造说起,当一个shared_ptr对象sp2是由sp1拷贝构或者赋值构造得来的时候,实际上构造完成后sp1内部的__shared_count对象包含的指向管理对象的指针与sp2内部的__shared_count对象包含的指向管理对象的指针是相等的,也就是说当多个shared_ptr对象来管理同一个对象时,它们共同使用同一个动态分配的管理对象。这可以从下面的__share_ptr的构造函数和__shared_count的构造函数清楚的看出。

template<typename _Tp1>
 __shared_ptr(const __shared_ptr<_Tp1, _Lp>& __r)
 : _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount) // never throws
{__glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)}


__shared_count&
operator=(const __shared_count& __r) // nothrow
{
    _Sp_counted_base<_Lp>* __tmp = __r._M_pi;
    if (__tmp != _M_pi)
    {
        if (__tmp != 0)
            __tmp->_M_add_ref_copy();
	if (_M_pi != 0)
	    _M_pi->_M_release();
	
        _M_pi = __tmp;
    }
}


上面说说当多个shared_ptr对象来管理同一个对象时,它们共同使用同一个动态分配的管理对象,为什么上面给出的_shared_count的构造函数中出现了__tmp != _M_pi的情形呢?这在sp2未初始化时(_M_pi为0,_r._M_pi非0)便是这样的情形。

更一般的,也可以考虑这样的情形:shared_ptr实例sp1开始指向类A的实例对象a1, 另外一个shared_ptr实例sp2指向类A的实例对象a2(a1 != a2),这样当把sp2赋值给sp1时便会出现上面的情形。假设初始时有且仅有一个sp1指向a1, 有且仅有一个sp2指向a2; 则赋值结束时sp1与sp2均指向a2, 没有指针指向a1, sp1指向的a1以及其对应的管理对象均应该被析构。这在上面的代码中我们可以很清楚的看到:因为__tmp != _M_pi,  __tmp->_M_add_ref_copy()将会增加a2的use_count的引用计数;由于a1内部的_M_pi != 0, 将会调用其_M_release()函数:

//************_Sp_counted_base*****************//
void
_M_add_ref_copy()
{ __gnu_cxx::__atomic_add_dispatch(&_M_use_count, 1); }


//************_Sp_counted_base*****************//
void
_M_release() // nothrow
{
    // Be race-detector-friendly.  For more info see bits/c++config.
    _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_use_count);
	if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, -1) == 1)
	{
            _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_use_count);
	    _M_dispose();
	    // There must be a memory barrier between dispose() and destroy()
	    // to ensure that the effects of dispose() are observed in the
	    // thread that runs destroy().
	    // See http://gcc.gnu.org/ml/libstdc++/2005-11/msg00136.html
	    if (_Mutex_base<_Lp>::_S_need_barriers)
	    {
		    __atomic_thread_fence (__ATOMIC_ACQ_REL);
	    }

            // Be race-detector-friendly.  For more info see bits/c++config.
            _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count);
	    if (__gnu_cxx::__exchange_and_add_dispatch(&_M_weak_count, -1) == 1)
            {
		_GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count);
	        _M_destroy();
             }
	}
}

//************_Sp_counted_base*****************//
// Called when _M_use_count drops to zero, to release the resources
// managed by *this.
virtual void
_M_dispose() = 0; // nothrow

// Called when _M_weak_count drops to zero.
virtual void
_M_destroy() // nothrow
{ delete this; }

//************_Sp_counted_base_impl*************//
virtual void
_M_dispose() // nothrow
{ _M_del(_M_ptr); }

_M_release()函数首先对a1的use_count减去1,并对比减操作之前的值,如果减之前是1,说明减后是0,a1没有shared_ptr指针指向它了,应该将a1对象销毁,于是调用_M_dispose()函数销毁a1; 同时对a1的weak_count减去1,也对比减操作之前的值,如果减之前是1,说明减后是0,a1没有weak_ptr指向它了,应该将管理对象销毁,于是调用_M_destroy()销毁了管理对象。


从上面可以看出,use_count主要用来标记被管理对象的生命周期,weak_count主要用来标记管理对象的生命周期
当一个shared_ptr超出作用域被销毁时,它也会调用其_share_count的_M_release()对use_count和weak_count进行自减并判断是否需要释放资源:

~__shared_count() // nothrow
 {
	 if (_M_pi != 0)
	  _M_pi->_M_release();
 }


对于weak_ptr, 其对应的__weak_count的拷贝构造函数如下

//************_Sp_counted_base*****************//
 void
 _M_weak_add_ref() // nothrow
{ __gnu_cxx::__atomic_add_dispatch(&_M_weak_count, 1); }

//************_Sp_counted_base*****************//
void
_M_weak_release() // nothrow
{
    // Be race-detector-friendly. For more info see bits/c++config.
    _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count);
    if (__gnu_cxx::__exchange_and_add_dispatch(&_M_weak_count, -1) == 1)
    {
        _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count);
	if (_Mutex_base<_Lp>::_S_need_barriers)
	{
	    // See _M_release(),
	    // destroy() must observe results of dispose()
            __atomic_thread_fence (__ATOMIC_ACQ_REL);
	}
	_M_destroy();
    }
}
 
__weak_count<_Lp>&
operator=(const __shared_count<_Lp>& __r) // nothrow
{
    _Sp_counted_base<_Lp>* __tmp = __r._M_pi;
    if (__tmp != 0)
        __tmp->_M_weak_add_ref();
  
    if (_M_pi != 0)
        _M_pi->_M_weak_release();
  
    _M_pi = __tmp;  
	
    return *this;
}
      
__weak_count<_Lp>&
operator=(const __weak_count<_Lp>& __r) // nothrow
{
    _Sp_counted_base<_Lp>* __tmp = __r._M_pi;
    if (__tmp != 0)
        __tmp->_M_weak_add_ref();
    if (_M_pi != 0)
        _M_pi->_M_weak_release();
    _M_pi = __tmp;
	
    return *this;
}

~__weak_count() // nothrow
{
    if (_M_pi != 0)
        _M_pi->_M_weak_release();
}


从上面可以看出,__weak_count相关的赋值拷贝以及析构函数均只会影响到weak_count的值,当weak_count为0时,释放管理对象。




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