shared_from_this() of an instance that is not yet owned by shared_ptr always returns null?

Question

I am trying to create a member function that returns whether it has stored to shared_ptr .

class X : public std::enable_shared_from_this {
    ...
    bool is_shared() const {
        return shared_from_this();
    }
};

...

(new X())->is_shared(); // -> false?

Is this legal? In this case, is shared_from_this() guaranteed to return null and not throw any exception?

Answer 1

shared_from_this can be used to obtain a shared_ptr from an object that is already managed by a shared_ptr .

Calling shared_from_this on a non-shared object is undefined behavior in C++11.

However, in C++17 shared_from_this will throw bad_weak_ptr and you can catch this exception:

#include <memory>
#include <iostream>

class X : public std::enable_shared_from_this<X> {
  public:
    bool is_shared() const {
        try {
            shared_from_this();
            return true;
        } catch (std::bad_weak_ptr&) {
            return false;
        }
    }
};

int main() {
    X x;
    std::cout << std::boolalpha << x.is_shared() << std::endl;
    auto y = std::make_shared<X>();
    std::cout << y->is_shared() << std::endl;
}

Output:

false
true

Furthermore you can use weak_from_this to get access to weak_ptr 's expired method:

bool is_shared() const {
    return !weak_from_this().expired();
}