Trivial cases of shared_ptr and weak_ptr failing

Question

I'm having trouble using shared_ptr and weak_ptr along with enable_shared_from_this.

When I google the symptoms of what I'm seeing, everybody suggests "you cannot use shared_from_this() when there are no shared_ptr instances owning your object.

But that's not my case.

Consider this code:

#include <memory>
#include <cassert>

class MyClass : std::enable_shared_from_this<MyClass>
{
public:
    void this_fails()
    {
        // Doesn't even assert(), because it throws bad_weak_ptr
        assert(shared_from_this());
    }
    void this_fails_too()
    {
        std::weak_ptr<MyClass> weak = weak_from_this();
        std::shared_ptr<MyClass> strong = weak.lock();
        // This assert fails
        assert(strong.get());
    }
};

int main()
{
    std::shared_ptr<MyClass> obj = std::make_shared<MyClass>();

    obj->this_fails();
    obj->this_fails_too();
}

Both methods in MyClass crash the program. I must be missing something obvious - what is it?

Answer 1

You must inherit publicly from std::enable_shared_from_this. Inheriting privately doesn't help - std::shared_ptr can't access the base class and set it up properly.