大约"circular reference"，我用了weak_ptr但仍然发生内存泄漏

About "circular reference", I used weak_ptr but memory leak still happened

本文关键字：泄漏 内存 ptr weak reference circular 大约      更新时间：2023-10-16

我读到：

如何避免shared_ptr内存泄漏？

我知道我需要使用weak_ptr来避免循环引用。

所以我创建了一个小程序来播放循环引用。

将调用以下对象(spyder(

class spyder {
public:
spyder(std::string _name): 
m_name(_name), finger(nullptr)
{  }
inline const std::string ask_name() const{
return m_name;
}
std::shared_ptr<spyder> finger;
private:
std::string m_name;
};

我在主代码中调用spyder并shared_ptr和weak_ptr：

int main(){
auto sA = std::make_shared<spyder>("A");
auto sB = std::make_shared<spyder>("B");
std::weak_ptr<spyder> wp_sA(sA);
std::weak_ptr<spyder> wp_sB(sB);  
sA->finger = wp_sB.lock();
sB->finger = wp_sA.lock();
}

上面的代码发生了内存泄漏(使用 valgrind 进行检查(。

==20753== Memcheck, a memory error detector
==20753== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==20753== Using Valgrind-3.14.0 and LibVEX; rerun with -h for copyright info
==20753== Command: ./t
==20753== 
==20753== 
==20753== HEAP SUMMARY:
==20753==     in use at exit: 128 bytes in 2 blocks
==20753==   total heap usage: 3 allocs, 1 frees, 72,832 bytes allocated
==20753== 
==20753== LEAK SUMMARY:
==20753==    definitely lost: 64 bytes in 1 blocks
==20753==    indirectly lost: 64 bytes in 1 blocks
==20753==      possibly lost: 0 bytes in 0 blocks
==20753==    still reachable: 0 bytes in 0 blocks
==20753==         suppressed: 0 bytes in 0 blocks
==20753== Rerun with --leak-check=full to see details of leaked memory
==20753== 
==20753== For counts of detected and suppressed errors, rerun with: -v
==20753== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

但是在我将上面的代码修改为：

int main(){
spyder sA("A"), sB("B");
std::weak_ptr<spyder> wp_sA( std::make_shared<spyder>("A") ) ;
std::weak_ptr<spyder> wp_sB( std::make_shared<spyder>("B") );
sA.finger = wp_sB.lock();
sB.finger = wp_sA.lock();
}

内存泄漏尚未发生，

==20695== Memcheck, a memory error detector
==20695== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==20695== Using Valgrind-3.14.0 and LibVEX; rerun with -h for copyright info
==20695== Command: ./t
==20695== 
==20695== 
==20695== HEAP SUMMARY:
==20695==     in use at exit: 0 bytes in 0 blocks
==20695==   total heap usage: 3 allocs, 3 frees, 72,832 bytes allocated
==20695== 
==20695== All heap blocks were freed -- no leaks are possible
==20695== 
==20695== For counts of detected and suppressed errors, rerun with: -v
==20695== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

我对此感到困惑。

你根本没有解决循环依赖关系。

在您的第一个版本中，

sA由shared_ptr管理，并存储要sBshared_ptr。sB又由shared_ptr管理，并存储shared_ptrsA。这意味着它们的引用计数永远不会变为 0。

相反，finger应该是类型std::weak_ptr，并且您应该仅在需要使用它之前lock()它。

class spyder {
public:
spyder(std::string _name): 
m_name(_name), finger()
{  }
inline const std::string ask_name() const{
return m_name;
}
std::weak_ptr<spyder> finger;
private:
std::string m_name;
};
int main(){
auto sA = std::make_shared<spyder>("A");
auto sB = std::make_shared<spyder>("B");
sA->finger = sB;
sB->finger = sA;
}

您的第二个版本

从临时shared_ptr构造weak_ptr，这意味着在构造后立即weak_ptrexpired()。sA.finger和sB.finger都在此版本中存储nullptr(但没有内存泄漏(。

您的代码泄漏，因为它仍然具有循环引用。sA -> sA::finger -> sB -> sB::finger -> sAstd::weak_ptr的要点是存储对指针的引用。为了避免内存泄漏，您的类源应如下所示：

class spyder {
public:
spyder(std::string _name): 
m_name(_name), finger(nullptr)
{  }
inline const std::string ask_name() const{
return m_name;
}
std::weak_ptr<spyder> finger;
private:
std::string m_name;
};
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