IfValidOr.md

<CppML/TypeTraits/IfValidOr.hpp>

IfValidOr

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

IfValidOr<Pipe>

Than IfValidOr<Pipe> is a metafunction that passes to Pipe the type T, which is either:

• T = ml::f<F, Args...>
  • if ml::f<F, Args...> is valid and not ill-formed, or
• T = Default
  • if ml::f<F, Args...> is not valid and is ill-formed

Pipe defaults to ml::Identity.

f::  Default F, Args... -> T <-> Pipe

Example

It can be used to implement logic based on introspection. To demonstrate, we define a metafunction, that extracts the ValueType alias from a type if it has one, or returns ml::None, if the type does not define such an alias.

struct A { using ValueType = int;};
struct B {};

template <typename _T>
using GetType = typename _T::ValueType;

using T0 = ml::f<ml::IfValidOr<>,
                 ml::None,       // Default
                 ml::F<GetType>, // make GetType a metafunction
                 A               // type for which to check
                 >;
using T1 = ml::f<ml::IfValidOr<>,
                 ml::None,       // Default
                 ml::F<GetType>, // make GetType a metafunction
                 B               // type for which to check
                 >;
static_assert( std::is_same_v<
                          T0, int>);
static_assert( std::is_same_v<
                          T1, ml::None>);