why i can't cast nullptr to weak_ptr<>

问题:

class MyClass {
public:
     MyClass(std::weak_ptr<MyClass> parent){}
}

i want to do this:

auto newInstance = std::make_shared<MyClass>(nullptr);

or default value of weak_ptr argument is null, such as :

void function(int arg,std::weak_ptr<MyClass> obj = nullptr);

but, what i need is to do this instead:

auto newInstance = std::make_shared<MyClass>(std::shared_ptr<MyClass>(nullptr));

why is that?

回答1:

Because a weak_ptr in concept can only be constructed from another weak_ptr or shared_ptr. It just doesn't make sense to construct from a raw pointer, whether it's nullptr or not.

You can use a default constructed weak_ptr (std::weak_ptr<MyClass>()) where you are trying to use nullptr:

auto newInstance = std::make_shared<MyClass>(std::weak_ptr<MyClass>());
void function(int arg,std::weak_ptr<MyClass> obj = std::weak_ptr<MyClass>());

回答2:

A weak pointer's primary purpose is usually to know whether an object that might be destroyed by other code still exists. How could a weak pointer constructed from an ordinary pointer possibly know whether the object still exists? Can you even imagine a way this could possibly work?