IsValid.md

<CppML/TypeTraits/IsValid.hpp>

IsValid

template <typename Pipe = ml::Identity>
struct IsValid {
template<typename F, typename ...Args>
  using f = /* .... */;
};

IsValid<Pipe>

Than IsValid<Pipe> is a metafunction that passes to Pipe an ml::Bool<truthValue>, where truthValue is a bool marking whether the metafunction F can be invoked with Args... (i.e. if the expression is not ill-formed, and compiles). Pipe defaults to ml::Identity.

f::  F, Args... -> ml::Bool<truthValue> <-> Pipe

Example

It can be used to implement introspection. To demonstrate, we write a metafunction that checks if the type has a size() method.

template <typename _T>
using HasSize = decltype(std::declval<_T>().size());
using T0 = ml::f<
                ml::IsValid<>,
                ml::F<HasSize>,   // make HasSize a metafunction
                std::vector<int>>;
using T1 = ml::f<
                ml::IsValid<>,
                ml::F<HasSize>,   // make HasSize a metafunction
                int>;
static_assert( std::is_same_v<
                          T0, ml::Bool<true>>);
static_assert( std::is_same_v<
                          T1, ml::Bool<false>>);