Idiomatic use of std::rel_ops

Question

What is the preferred method of using std::rel_ops to add the full set of relational operators to a class?

This documentation suggests a using nam

Answer 1

It's not the nicest, but you can use using namespace std::rel_ops as an implementation detail for implementing the comparison operators on your type. For example:

template 
struct MyType
{
    T value;

    friend bool operator<(MyType const& lhs, MyType const& rhs)
    {
        // The type must define `operator<`; std::rel_ops doesn't do that
        return lhs.value < rhs.value;
    }

    friend bool operator<=(MyType const& lhs, MyType const& rhs)
    {
        using namespace std::rel_ops;
        return lhs.value <= rhs.value;
    }

    // ... all the other comparison operators
};

By using using namespace std::rel_ops;, we allow ADL to lookup operator<= if it is defined for the type, but fall back onto the one defined in std::rel_ops otherwise.

This is still a pain, though, as you still have to write a function for each of the comparison operators.