C++20 std::invocable syntax

Question

I want to write this

    template <typename F, typename... Args> requires std::invocable<F&, Args...>
    void applyFunction(F&& f, Args&&... args) {

    }

without requires clause, so I want something like

template <std::invocable<F, Args...>>
    void applyFunction(F&& f, Args&&... args) {

    }

...but it fails to compile. I searched StackOverflow but couldn't find how to use the correct syntax. How can I use terse syntax correctly?

UPDATE: I found that this works:

    template <typename... Args, std::invocable<Args...> F>
    void applyFunction(F&& f, Args&&... args) {

    }

Still ugly. Any better way?

Answer 1

Using a concept in place of typename in a template parameter list only works when you are replacing one of the concept's template parameters. Specifically, the first one.

If you have two (or more) template parameters that share a relationship, odds are good that you will need to have a requires clause. And you need to make your peace with that. Indeed, putting the parameter pack in front of the function as you have done in the template argument list has consequences. It pretty much impossible for a user to specify the callable type parameter directly when calling the function, and thus the user would be forced to rely on template argument deduction.

Don't try to force syntax to work just because it takes a couple fewer keystrokes to type. Write in the way that makes the most sense for what you're doing, that makes it the most clear as to what is going on.