Curious Curious - 8 months ago 43
C++ Question

How to specialize a type based on whether a template parameter has an alias

I want to be able to specialize a type based on whether a container has a specified typedef for example

class SomethingElse {};
class Something {
using Type = int;

static constexpr bool value = ChooseIfType<Something>::value;

Is there a way for
to return false when the type does not have the typedef

I feel like there is an easy way to do this but I cannot figure it out.



Just use std::void_t (or a C++11 alternative):

template<typename T, typename = std::void_t<>>
struct ChooseIfType : std::false_type {};

template<typename T>
struct ChooseIfType<T, std::void_t<typename T::Type>> : std::true_type {};

live demo

The solution makes use of SFINAE. The default is never malformed and creates a trait with value false. The compiler tries to match all template specializations (only one in this case). If T has a member type Type, then ChooseIfType<T, void_t<typename T::Type>> is more specialized than ChooseIfType<T, void_t<>>. If it doesn't, then it's not a viable specialization and the default is selected, but Substitution Failure Is Not An Error.

as per cppreference, a C++11 void_t implementation could look like this:

template<typename... Ts> struct make_void { typedef void type;};
template<typename... Ts> using void_t = typename make_void<Ts...>::type;