Choose function to apply based on the validity of an expression

Question

The problem is the following, in C++14:

• Let\'s have two functions FV&& valid_f, FI&& invalid_f, and argu

Answer 1

Here's an alternative answer, just for kicks. We need a static_if:

template  T&& static_if(std::true_type, T&& t, F&& ) { return std::forward(t); }
template  F&& static_if(std::false_type, T&& , F&& f) { return std::forward(f); }

And an is_callable. Since you're just supporting functions, we can do it as:

template 
struct is_callable : std::false_type { };

template 
struct is_callable()(std::declval()...))>>
: std::true_type
{ };

And then we can construct the logic in place:

template 
void apply_on_validity(FV&& valid_f, FI&& invalid_f, Args&&... args)
{
    auto noop = [](auto&&...) {};

    static_if(
        is_callable{},
        std::forward(valid_f),
        static_if(
            std::is_callable{},
            std::forward(invalid_f),
            noop
        )
    )(std::forward(args)...);
}