Convenient way to define all comparison operators for class with one numeric data member?

Question

If I have a type that consists of a single numeric data member (say, an int) and various methods, is there a convenient way to tell the compiler to automaticall

Answer 1

The C++ way of doing this is to use a tag type and ADL. Here is a quick example:

namespace relational {
    struct tag {};

    template 
    bool operator== (T const& lhs, T const& rhs) { return !(rhs < lhs) && !(lhs < rhs); }
    template 
    bool operator!= (T const& lhs, T const& rhs) { return !(lhs == rhs); }

    template 
    bool operator> (T const& lhs, T const& rhs) { return rhs < lhs; }
    template 
    bool operator<= (T const& lhs, T const& rhs) { return !(rhs < lhs); }
    template 
    bool operator>= (T const& lhs, T const& rhs) { return !(lhs < rhs); }
}

struct foo: relational::tag {
    int value;
    foo(int value): value(value) {}
    bool operator< (foo const& other) const { return this->value < other.value; }
};

#include 
void compare(foo f0, foo f1) {
    std::cout << std::boolalpha
              << f0.value << " == " << f1.value << " => " << (f0 == f1) << '\n'
              << f0.value << " != " << f1.value << " => " << (f0 != f1) << '\n'
              << f0.value << " <  " << f1.value << " => " << (f0 <  f1) << '\n'
              << f0.value << " <= " << f1.value << " => " << (f0 <= f1) << '\n'
              << f0.value << " >  " << f1.value << " => " << (f0 >  f1) << '\n'
              << f0.value << " >= " << f1.value << " => " << (f0 >= f1) << '\n'
        ;
}
int main() {
    compare(foo(1), foo(2));
    compare(foo(2), foo(2));
}