Pointer to class data member “::*”

Question

I came across this strange code snippet which compiles fine:

class Car
{
    public:
    int speed;
};

int main()
{
    int Car::*pSpeed = &Car::speed;
         


        

Answer 1

A realworld example of a pointer-to-member could be a more narrow aliasing constructor for std::shared_ptr:

template 
template 
shared_ptr::shared_ptr(const shared_ptr, T U::*member);

What that constructor would be good for

assume you have a struct foo:

struct foo {
    int ival;
    float fval;
};

If you have given a shared_ptr to a foo, you could then retrieve shared_ptr's to its members ival or fval using that constructor:

auto foo_shared = std::make_shared();
auto ival_shared = std::shared_ptr(foo_shared, &foo::ival);

This would be useful if want to pass the pointer foo_shared->ival to some function which expects a shared_ptr

https://en.cppreference.com/w/cpp/memory/shared_ptr/shared_ptr