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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：
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)由具有原子属性的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。
C++ 智能指针（shared_ptr/weak_ptr）源码分析
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原文地址：http://blog.csdn.net/ithiker/article/details/51532484
