static_assert that a member variable is marked as 'static' in C++17?

Question

I've been playing around with templates and would like to enforce that a template type given to a function has a particular static member. Unfortunately, there is no std::is_static type trait.

A rough example of how I would use it (or an alternative):

template<typename T>
void SomeFunc(T& obj)
{
    static_assert(std::is_static_v<decltype(T::someVariable)>, "someVariable must be static");
}

Are there any ways to achieve this kind of behaviour? I could just write

template<typename T>
void SomeFunc(T& obj)
{
    // must be static
    T::someVariable;
}

But this wouldn't be anywhere near as nice or informative as a static_assert with a decent message. This is just a syntax error of sorts :(

Thanks!

Answer 1

If I understand correctly what you want, you're looking for std::is_member_pointer (or the opposite of the value of std::is_member_pointer).

An example (also C++11 and C++14 compliant)

#include <type_traits>

struct foo
 { int value; };

struct bar
 { static int value; };

int main()
 {
   static_assert(
      true == std::is_member_pointer<decltype(&foo::value)>::value, "!");
   static_assert(
      false == std::is_member_pointer<decltype(&bar::value)>::value, "!!");
 }

-- EDIT --

Not sure to understand the nakiya's objection but ... in the following example I've developed a type traits (hasSomeValueStatic) that set value to true if the template argument has a someValue static member and false otherwhise (if someValue isn't static of there isn't someValue.

#include <iostream>
#include <type_traits>

struct foo
 { int someValue; };

struct bar
 { static int someValue; };

template <typename, typename = int>
struct hasSomeValueStatic : std::false_type
 { };

template <typename T>
struct hasSomeValueStatic<T, decltype(T::someValue, 0)>
      : std::integral_constant<bool,
           ! std::is_member_pointer<decltype(&T::someValue)>::value>
 { };

int main()
 {
   std::cout << hasSomeValueStatic<foo>::value << std::endl; // print 0
   std::cout << hasSomeValueStatic<bar>::value << std::endl; // print 1
   std::cout << hasSomeValueStatic<int>::value << std::endl; // print 0
 }