&* for raw pointer, iterator and … std::nullptr_t

Question

I have a template function that is enabled (through a std::enable_if) its parameter is a raw pointer, or has a std::iterator category or is a std::nullptr_t. In that function, a raw pointer (a data member) is set equal to the parameter, like that :

template<class T> void myFunction(T it) 
{
    _ptr = &*it;
}

The &* works well for pointer and iterator ... but it fails for a std::nullptr_t. Is there any solution available to avoid to write 2 different functions ?

Thank you.

Answer 1

最简单的方法是将函数更改为具有两个重载，一个重载仅用于存储指针的原始指针/ nullptr_t ，另一个重载由SFINAE为当前实现的迭代器选择，尽管您应注意，这在某些情况下会失败（特别是如果iterator::value_type重载一元运算operator& ）。

Answer 2

As always, we can solve this with a trait . This time, let's not write a full class, since a simple function overload suffices.

#include <cstddef>
#include <memory>

template <typename T>
T get_addr(T t) { return std::addressof(*t); } // #1

std::nullptr_t get_addr(std::nullptr_t np) { return np; }

Usage:

T _ptr = get_addr(it);

(This trait also works if the type pointed to by T overloads operator& .)

You're invited to guard the overload #1 with the same enable_if condition that you have in your main template.