Using std::extent on std::array

Question

I have a templatized function and I want to static_assert that it\'s type has a size of three. This code illustrates what I\'m trying to do, but doesn\'t work:<

Answer 1

std::extent is defined for built-in arrays. For std::array use std::tuple_size instead. I don't know some trait that works on both, but it's easy to write one:

template
struct array_size : std::extent { };

template
struct array_size > : std::tuple_size > { };

and here's your foo, corrected/generalized (live example):

template < typename T >
void foo( T&& param )
{
    using U = typename std::remove_cv::type>::type;
    static_assert( 3 == array_size::value, "param must have a size of 3" );
}

Prefer a universal reference T&& param, otherwise only lvalues can be used.