Question about Introspection / static_assert (C++17)

Question

I have a (container like) class that accepts several of my other types/classes as the data member. Now I would like to specify some properties (edit: positive integer) of the data type as compile time information. This works fine, but I would like to static_assert that the provided type is correct (uint8_t), the actual value is different for most data type classes. I set the number as a std::integral_constant with uint8_t .

1) How do I do that (see example code provided)?
2) Is there a better approach?

I can use language features up to c++17.

( edited for clarification )

class DataClass
{
     public:
        // compile time information about char count
        using char_count = std::integral_constant<uint8_t, 2>;

     private:
        ...
}

class DataClass2
{
     public:
        // compile time information about char count
        using char_count = std::integral_constant<uint8_t, 4>;

     private:
        ...
}

template <typename DataType, ...>
class ContainerClass
{
     private:
        // does not work: error C2923: 'std::is_same_v':
        // 'value' is not a valid template type argument for parameter '<unnamed-symbol>'
        static_assert(std::is_same_v<DataType::char_count::value, uint8_t>, "not uint8_t");
        // this also does not work
        static_assert(std::is_same_v<decltype(DataType::char_count::value), uint8_t>, "also not uint8_t");

        // using the value actually works fine
        constexpr static auto char_count = DataType::char_count::value;

     public:
        ...
}

Answer 1

What you want to do depends on what exatly do you want to check.

1. Want to check that char_count has value 2? static_assert(DataType::char_count::value == 2, "char_count is not 2;");
2. Want to check that char_count is an integral constant (or something else) that has uint8_t as type member type alias? static_assert(std::is_same_v(typename DataType::char_count::value_type, "char_count is not uint8_t;");
3. Want to checkt that char_count is an integral constant with value 2 and type uint8_t ? static_assert(std::is_same_v<DataType::char_count, std::integral_constant<uint8_t, 2>>, "char_count is not 2 or not uint8_t;");