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

Take:

template  struct rank : rank {};
template <> struct rank<0> {};

and then:

template 
auto apply_on_validity_impl(rank<2>, FV&& valid_f, FI&& invalid_f, Args&&... args)
    -> decltype(std::forward(valid_f)(std::forward(args)...), void())
{
    std::forward(valid_f)(std::forward(args)...);
}

template 
auto apply_on_validity_impl(rank<1>, FV&& valid_f, FI&& invalid_f, Args&&... args)
    -> decltype(std::forward(invalid_f)(std::forward(args)...), void())
{
    std::forward(invalid_f)(std::forward(args)...);
}

template 
void apply_on_validity_impl(rank<0>, FV&& valid_f, FI&& invalid_f, Args&&... args)
{

}

template 
void apply_on_validity(FV&& valid_f, FI&& invalid_f, Args&&... args)
{
    return apply_on_validity_impl(rank<2>{}, std::forward(valid_f), std::forward(invalid_f), std::forward(args)...);
}

DEMO