Check for multiple values when using comparison operators

Question

I've always been under the impression that for any comparison statement, i.e. X == Y or X != Y is the format, and you chain statements together with && or ||.

Is there not some way to write X == (Y || Z) instead of X == Y || X == Z?

Edit: Since it has been established that this is not possible to do cleanly, how else could it be done?

Answer 1

There's no clean way to do what you ask in C++.

What trips many people up is that X == (Y || Z) may be a legal expression and the compiler will not complain. It will just be a bug. Each C++ statement must evaluate to true/false on its own and the operators just string them together. What you're suggesting would require some intrinsic list structure. Many languages have that (like Python), but C++ does not.

Answer 2

#include <algorithm>
#include <array>
#include <string>
#include <iostream>
#include <initializer_list>
 
template<class Type, class Next>
bool is_one_of(const Type& needle, const Next& next)
{return needle==next;}
template<class Type, class Next, class ... Rest>
bool is_one_of(const Type& needle, const Next& next, Rest... haystack)
{return needle==next || is_one_of(needle, haystack...);}
 
int main() {
    std::string X, Y;
    if (is_one_of(X, Y, "HI"))
        std::cout << "it is!";
    else
        std::cout << "it isn't!";
    return 0;
}

proof of compilation. Xeo also observes that std::any_of, std::all_of and std::none_of may have been useful, depending on your actual needs and desires.