为什么在锁定weak_ptr时出现访问冲突?

Why am I getting an access violation when locking a weak_ptr?

本文关键字：访问冲突 ptr 锁定 weak 为什么      更新时间：2023-10-16

我有一个std::weak_ptr。在尝试使用底层对象之前，我锁定它以获得shared_ptr:

auto foo_sharedptr = foo_weakptr.lock();
if (foo_sharedptr != nullptr)
{
    // do stuff with foo
}

通常这很好。但是，有时我在调用lock时遇到访问冲突:

Unhandled exception at 0x00007FF91F411BC3 (My.dll) in My.exe:  
0xC0000005: Access violation reading location 0xFFFFFFFFFFFFFFFF.

我的猜测是底层指针已被删除，但我对weak_ptr的理解是，在这种情况下，lock应该返回一个nullptr。我用错类型了吗?如果不是，我应该如何调试它?

EDIT:虽然被赞，但这似乎不是正确的答案，抱歉:

http://en.cppreference.com/w/cpp/memory/shared_ptr/operator_cmp 

template< class T >
bool operator==( const shared_ptr<T>& lhs, std::nullptr_t rhs );
    (7)     (since C++11)
template< class T >
bool operator!=( const shared_ptr<T>& lhs, std::nullptr_t rhs );
    (9)     (since C++11)

7) !韩
9) (bool) lh

…破碎的实现? ?真的不知道。

测试gcc -std=c++11:(取自http://en.cppreference.com/w/cpp/memory/weak_ptr并改编)

#include <iostream>
#include <memory>
std::weak_ptr<int> gw;
void f()
{
    auto spt = gw.lock();
    if (spt != nullptr) {
        std::cout << *spt << "n";
    }
    else {
        std::cout << "gw is expiredn";
    }
}
int main()
{
    {
        auto sp = std::make_shared<int>(42);
        gw = sp;
        f();
    }
    f();
}

输出如预期:

<>之前42Gw已过期之前

必须在别处

原始:

简而言之:将其检查为bool，不要与nullptr进行比较(这将尝试在nullptr上使用rhs = shared_ptr失败的lhs.get() == rhs.get()):

auto foo_sharedptr = foo_weakptr.lock();
if (foo_sharedptr)
{
    // do stuff with foo
}

参见文档:

#include <iostream>
#include <memory>
#include <thread>
void observe(std::weak_ptr<int> weak) 
{
    std::shared_ptr<int> observe(weak.lock());
    if (observe) {
        std::cout << "tobserve() able to lock weak_ptr<>, value=" << *observe << "n";
    } else {
        std::cout << "tobserve() unable to lock weak_ptr<>n";
    }
}
int main()
{
    std::weak_ptr<int> weak;
    std::cout << "weak_ptr<> not yet initializedn";
    observe(weak);
    {
        std::shared_ptr<int> shared(new int(42));
        weak = shared;
        std::cout << "weak_ptr<> initialized with shared_ptr.n";
        observe(weak);
    }
    std::cout << "shared_ptr<> has been destructed due to scope exit.n";
    observe(weak);
}
相关文章：