c++ - Defining operator< for a struct

Question

I sometimes use small structs as keys in maps, and so I have to define an operator< for them. Usually, this ends up looking something like this:

struct MyStruct
{
    A a;
    B b;
    C c;

    bool operator<(const MyStruct& rhs) const
    {
        if (a < rhs.a)
        {
           return true;
        }
        else if (a == rhs.a)
        {
            if (b < rhs.b)
            {
                return true;
            }
            else if (b == rhs.b)
            {
                return c < rhs.c;
            }
        }

        return false;
    }
};

This seems awfully verbose and error-prone. Is there a better way, or some easy way to automate definition of operator< for a struct or class?

I know some people like to just use something like memcmp(this, &rhs, sizeof(MyStruct)) < 0, but this may not work correctly if there are padding bytes between the members, or if there are char string arrays that may contain garbage after the null terminators.

Answer 1

This is quite an old question and as a consequence all answers here are obsolete. C++11 allows a more elegant and efficient solution:

bool operator <(const MyStruct& x, const MyStruct& y) {
    return std::tie(x.a, x.b, x.c) < std::tie(y.a, y.b, y.c);
}

Why is this better than using boost::make_tuple? Because make_tuple will create copies of all the data members, which can be costly. std::tie, by contrast, will just create a thin wrapper of references (which the compiler will probably optimise away entirely).

In fact, the above code should now be considered the idiomatic solution to implementing a lexicographical compare for structures with several data members.