Sorting for equal_range()

[laserlight]

You could write something like this:

bool compareFoo(const Foo& f1, const Foo& f2)
{
    if (f1.s1 < f2.s1)
    {
        return true;
    }
    else if (f2.s1 < f1.s1)
    {
        return false;
    }
    else
    {
        return f1.s2 < f2.s2;
    }
}

or more compactly:

bool compareFoo(const Foo& f1, const Foo& f2)
{
    return f1.s1 < f2.s1 || !(f2.s1 < f1.s1) && f1.s2 < f2.s2;
}

Nonetheless, if you add a member variable to the struct, then this comparison function must change accordingly to cater to that.

Note that this is a comparison function that is used in the sorting. It is not a sort function by itself. Also, if this is comparison is canonical, then you could just overload operator< for Foo.