How to declare Magnitude type in C++?

Question

Say I have a template class that has to work with general types T complex and real but I also need a real type that I can store the abs(T) in. How do I get the appropriate type and make a type definition?

I tried the following:

template < typename T>
class sth{
typedef std::invoke_result<*std::abs, Value>::type real_type;

real_type sth_else(T x)
    return std::abs(f(x));
}

Answer 1

If you need to define in through result of expression, then it would be something like

using real_type = decltype (std::abs(std::declval<T>()));

declval would provide a proper argument of type T to function