static_pointer_cast for weak_ptr

Question

In c++0x, there is a std::static_pointer_cast for std::shared_ptr, but there is no equivalent method for std::weak_ptr. Is this intentional, or an oversight? If an oversight, how would I define an appropriate function?

Answer 1

This ought to do it for you:

template<class T, class U>
std::weak_ptr<T>
static_pointer_cast(std::weak_ptr<U> const& r)
{
    return std::static_pointer_cast<T>(std::shared_ptr<U>(r));
}

This will throw an exception if the weak_ptr has expired. If you would rather get a null weak_ptr, then use r.lock() instead.

Answer 2

Howard's version is correct, but in many cases it makes sense to simply pass weakptr.lock() as parameter to std::static_pointer_cast:

std::weak_ptr<A> a = ...;  
std::weak_ptr<B> b = std::static_pointer_cast<B>(a.lock());

This syntax explicitly shows what is going on, and makes code easy to read.

Answer 3

The omission is intentional because despite it's name, std::weak_ptr is not a pointer type and does not provide pointer interface (operator ->, operator *, static_pointer_cast, etc.).