How to check if weak_ptr is empty (non-assigned)?

Question

Is there a way to distinguish between an assigned (possibly expired) weak_ptr and a non-assigned one.

weak_ptr w1;
weak_ptr w2 = ...;
         


        

Answer 1

You could try to create a shared pointer that accepts a weak pointer as a parameter and raises a std::bad_weak_ptr exception if a weak pointer is expired (or not assigned as in your case):

#include 
#include 
int main(){
    // with an assigned pointer
    std::shared_ptr p1(new int(42));
    std::weak_ptr w1(p1);
    try {
        std::shared_ptr p2(w1);
    }
    catch (const std::bad_weak_ptr& e) {
        std::cout << e.what() << '\n';
    }
    // with a non assigned pointer
    std::shared_ptr p2(new int(42));
    std::weak_ptr w2;
    try {
        std::shared_ptr p2(w2); // raises an exception
    }
    catch (const std::bad_weak_ptr& e) {
        std::cout << "Ptr 2: " << e.what() << '\n';
    }
}