c++ - 在lambda中锁定std::shared_ptr的复制操作

Question

对于此示例代码:

#include <iostream>
#include <thread>
#include <mutex>
#include <memory>

struct A
{
    int _i;
    A(int i):_i(i)
    {
        std::cout<<"A() "<<_i<<std::endl;
    }
    ~A()
    {
        std::cout<<"~A() "<<_i<<std::endl;
    }
    void Print()
    {
        std::cout<<"Print() "<<_i<<std::endl;
    }
};

struct B
{
    std::shared_ptr<A> Asp;
    std::mutex AspMutex;

    void SetA()
    {
        static int i = 0;
        std::unique_lock<std::mutex> lock(AspMutex);
        Asp = std::make_shared<A>(i);
    }

    void AccessA1()
    {
        std::shared_ptr<A> aspCopy;
        {
            std::unique_lock<std::mutex> lock(AspMutex);
            aspCopy = Asp;
        }
        (*aspCopy).Print();
    }

    void AccessA2()
    {
        auto aspCopy = [&]()
        {
            std::unique_lock<std::mutex> lock(AspMutex);
            return Asp;
        }();
        (*aspCopy).Print();
    }

    void AccessA3()
    {
        (*[&]()
        {
            std::unique_lock<std::mutex> lock(AspMutex);
            return Asp;
        }()
        ).Print();
    }

};

int main()
{
    B b;
    b.SetA();
    std::thread t([&]{b.SetA();});
    b.AccessA1();
    b.AccessA2();
    b.AccessA3();
    t.join();
}

我很好奇c++ 17(或更高版本)标准是否可以保证 A::Access1和 A::Access2方法是线程安全的( std::shared_ptr的副本将受 lock保护)。

Answer 1

是。该锁使A::Access1和A::Access2线程与并发SetA安全。在C++ 17中仍然如此。