why i can't cast nullptr to weak_ptr<>

Question

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?

Answer 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>());