Can a C++ template match (only) a list of one type, or a single other type?

Question

I want to write templates to construct let's say an input port. That can be done from either a single input-output port, or from a list of input pins. I prefer those templates to have the same name. I have concepts for both an input-output port and for input pins. I can write

template< typename... arguments > 
struct port_in ....

template< pin_in T > 
struct port_in<> ....

But now the list version will accept any type. I can check that in the implementation, but that will degrade the error message the user gets when he passes types that are not suited. Can I somehow restrict the list to one type, yet allow a single template parameter of the other type?

Answer 1

If you want to ensure that users always get a reasonable error message, then you need to constrain the base template. Assuming your existing concepts are named InputPin and InputOutputPort, you could constrain the base template to accept either a sequence of input pins or a single input/output port as follows:

template<class... Ts>
concept bool AllInputPins = (InputPin<Ts> && ...);

template<class... Ts>
concept bool OneInputOutputPort = sizeof...(Ts) == 1 && (InputOutputPort<Ts> && ...);

template <class... Args>
  requires AllInputPins<Args...> || OneInputOutputPort<Args...>
struct port_in {
  // ...
};

Answer 2

If the list version is supposed to take numbers you could make it:

template<uint16_t ... Ports>
struct port_in ....

I'm not sure there is a clean way to do this if it can take non-integers and you want an actual type list (that is not all the arguments need be of the same type). If you can require that all the types be the same I think you could do something like:

template<typename T, std::enable_if_t<T> * = nullptr>
struct port_in_base{};
template<typename T, T ... ports>
struct port_in : port_in_base<T> ....