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

Piotr Skotnicki's answer is superb, but code like that makes me feel compelled to point out how much cleaner C++17 will be thanks to constexpr if and additional type traits like is_callable: Demo Demo^{*This version creates more warnings but is simpler}

template 
void apply_on_validity(FV&& valid_f, FI&& invalid_f, Args&&... args)
{
    if constexpr (std::is_callable_v)
        std::cout << "Apply valid_f by default\n";
    else
    {
        if constexpr (std::is_callable_v)
            std::cout << "Apply invalid_f if valid_f does not work\n";
        else
            std::cout << "Do nothing when neither valid_f nor invalid_f work\n";
    }
}