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 <

Answer 1

With operator overloading, you might be able to get the exact syntax that you want. But, as Adam points out, that could lead to excluding valid expressions.

Below is a template with operator overloading, a template function, and a macro to achieve a syntax similar to Mooing Duck's nicer solution, but without requiring C++11, and allowing the use of the || operator to denote the "haystack" collection.

template 
struct MultiOrComparable {
    mutable std::set vals;
    const MultiOrComparable & operator || (T v) const {
        vals.insert(v); return *this;
    }
    bool operator == (T v) const { return vals.find(v) != vals.end(); }
};

template 
MultiOrComparable MultiOrComparableStart (T) {
    return MultiOrComparable();
}

#define IsOneOf(x, y) ((MultiOrComparableStart(x)||y) == x)

Then, the following program "works":

enum Foo { A, B, C, D };

int
main ()
{
    if (!IsOneOf(A, B || C || D)) {
        std::cout << "!=" << std::endl;
    }
    if (IsOneOf('a', 'x' || 'y' || 'z' || 'a')) {
        std::cout << "==" << std::endl;
    }
}