C ++ STL堆栈：什么时候可以安全地pop（）

Question

Consider:

#include <iostream>
#include <stack>

class Abc {
    int x = 5;
    public:
    void display() {
        std::cout << x << std::endl;
    }
};

int main() {
    std::stack<Abc> S;
    S.emplace();

    auto obj = S.top();
    S.pop();
    obj.display();
    return 0;
}

From: http://www.cplusplus.com/reference/stack/stack/pop/ , "This calls the removed element's destructor". Also, from http://www.cplusplus.com/reference/stack/stack/top/ , stack.top() returns by reference.

If S.top() returned by reference and if S.pop() destructed the object, why doesn't obj.display() fail?

I know that stack calls the back() and pop_back() methods of the underlying container. By extension, why doesn't that fail?

Answer 1

auto obj = S.top(); copy initializes obj from S.top() . It's a copy of the then removed element.

You code snippet has well defined behavior.

It will be undefined if you were to take a reference, like auto& obj = S.top();