learncpp std::shared_ptr

NOTE: 下面是一个正确使用的例子

#include <iostream>
#include <memory> // for std::shared_ptr

class Resource
{
public:
    Resource() { std::cout << "Resource acquired\n"; }
    ~Resource() { std::cout << "Resource destroyed\n"; }
};

int main()
{
    // allocate a Resource object and have it owned by std::shared_ptr
    Resource *res = new Resource;
    std::shared_ptr<Resource> ptr1(res);
    {
        std::shared_ptr<Resource> ptr2(ptr1); // use copy initialization to make another std::shared_ptr pointing to the same thing

        std::cout << "Killing one shared pointer\n";
    } // ptr2 goes out of scope here, but nothing happens

    std::cout << "Killing another shared pointer\n";

    return 0;
} // ptr1 goes out of scope here, and the allocated Resource is destroyed


// 编译: g++ --std=c++11 test.cpp 

NOTE: 下面是一个错误使用的例子

#include <iostream>
#include <memory> // for std::shared_ptr

class Resource
{
public:
    Resource() { std::cout << "Resource acquired\n"; }
    ~Resource() { std::cout << "Resource destroyed\n"; }
};

int main()
{
    Resource *res = new Resource;
    std::shared_ptr<Resource> ptr1(res);
    {
        std::shared_ptr<Resource> ptr2(res); // create ptr2 directly from res (instead of ptr1)

        std::cout << "Killing one shared pointer\n";
    } // ptr2 goes out of scope here, and the allocated Resource is destroyed

    std::cout << "Killing another shared pointer\n";

    return 0;
} // ptr1 goes out of scope here, and the allocated Resource is destroyed again

and then crashes (at least on the author’s machine).

The difference here is that we created two std::shared_ptr independently from each other. As a consequence, even though they’re both pointing to the same Resource, they aren’t aware of each other. When ptr2 goes out of scope, it thinks it’s the only owner of the Resource, and deallocates it. When ptr1 later goes out of the scope, it thinks the same thing, and tries to delete the Resource again. Then bad things happen.