shared_ptr能和基于引用计数的智能指针混用吗?
编程难
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·
2020-12-22 18:45
前言
我在上一篇文章中介绍了几种误用智能指针的例子。还有一种比较典型,就是混用两种类型的智能指针。直接看代码吧!
示例代码
#include "stdafx.h"
#include
#include
#include "RefCountedPtr.h"
class PBObject : public RefCounted
{
public:
virtual double Volume() { return 0.0; }
};
class PBWall : public PBObject
{
public:
virtual double Volume() { return 100.0; }
~PBWall() { std::cout << __FUNCTION__ << std::endl; }
};
void Test()
{
RefCountedPtr pWall(new PBWall()) ;
std::shared_ptr pSharedWall(pWall.get()) ;
}
int _tmain(int argc, _TCHAR* argv[])
{
Test();
return 0;
}
pWall
在析构的时候会调用 Release()
递减引用计数到 0
,从而调用 delete
。pSharedWall
拿着同样的指针,在析构的时候也会调用 delete
删除同一块地址。于是暴雷了!
那么有办法把 RefCountPtr
类型的智能指针交给 shared_ptr
管理吗?答案是肯定的,只需要把 Test()
改成看下面这样即可:
void Test()
{
RefCountedPtr pWall(new PBWall()) ;
std::shared_ptr pSharedWall(pWall.get(),
[](PBObject* p){ /* p->Release(); */ } // use customized deleter, do nothing, can't call p->Release();
) ;
}
在构造 shared_ptr
类型的智能指针的时候,可以传递一个自定义的 Deleter
,这个 Deleter
不需要做任何事情。
基于引用计数的智能指针的一个实现
基于引用计数的智能指针常用于 COM
编程中,下面的代码是一份简要的实现,感兴趣的小伙伴儿可以看一看。毕竟,手机不太适合看代码,也可以下载后在电脑上查看。
#include "stdafx.h"
#include
#include
// http://en.wikipedia.org/wiki/Curiously_recurring_template_pattern
template <class Base> class RefCounted : public Base
{
protected:
virtual ~RefCounted() {}
public:
RefCounted() { m_refCount = 0; }
RefCounted(RefCounted const& rhs) { m_refCount = 0; }
RefCounted& operator=(RefCounted const& rhs) { if (this != &rhs) { Base::operator=(rhs); } return *this; }
int GetRefCount() const { return m_refCount; }
int AddRef() const { return ++m_refCount; }
int Release() const
{
int refCount = --m_refCount;
if (0 < refCount)
{
return refCount;
}
delete this;
return 0;
}
private:
mutable int m_refCount;
};
class IRefCounted
{
protected:
virtual ~IRefCounted() {}
public:
virtual int AddRef() const = 0;
virtual int Release() const = 0;
};
template<class T> class RefCountedPtr
{
private:
typedef RefCountedPtr this_type;
public:
RefCountedPtr() : p_(0) {}
RefCountedPtr(T * p, bool add_ref = true) : p_(p)
{
if (p_ != 0 && add_ref) p_->AddRef();
}
template RefCountedPtr(RefCountedPtr const & rhs) : p_(rhs.get())
{
if (p_ != 0) p_->AddRef();
}
RefCountedPtr(RefCountedPtr const & rhs) : p_(rhs.p_)
{
if (p_ != 0) p_->AddRef();
}
~RefCountedPtr()
{
if (p_ != 0) p_->Release();
}
template<class U> RefCountedPtr & operator=(RefCountedPtr const & rhs)
{
this_type(rhs).swap(*this);
return *this;
}
bool operator== (RefCountedPtr const& rhs) const
{
return p_ == rhs.p_;
}
template<class U> bool operator== (RefCountedPtr const& rhs) const
{
return p_ == rhs.get();
}
bool operator!= (RefCountedPtr const& rhs) const
{
return p_ != rhs.p_;
}
template<class U> bool operator!= (RefCountedPtr const& rhs) const
{
return p_ != rhs.get();
}
RefCountedPtr & operator=(RefCountedPtr const & rhs)
{
this_type(rhs).swap(*this);
return *this;
}
RefCountedPtr & operator=(T * rhs)
{
this_type(rhs).swap(*this);
return *this;
}
T * get() const
{
return p_;
}
T& operator*() const
{
return *p_;
}
T * operator->() const
{
return p_;
}
void swap(RefCountedPtr & rhs)
{
T * tmp = p_;
p_ = rhs.p_;
rhs.p_ = tmp;
}
private:
T * p_;
};
附件
文中的 RefCounterPtr
实现可以参考附件,下载地址如下:
百度云盘链接: https://pan.baidu.com/s/1HpFELQf97pkximT5oGm2pg 提取码: h7w8
CSDN:https://download.csdn.net/download/xiaoyanilw/13679955
总结
shared_ptr
的 Deleter
妙用无穷,本文只是其中的一个用处。
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