Martin Bonner wrote:
Consider the progression:
complex<double> struct { double x, double y; } pair<double, double> tuple<double, double> vector<double> vector<char> string
If we take the "feel" of operator< out of the equation, where should we draw the line, and based on what?
complex<double> should not have an operator <() because it models a mathematical concept (complex number) for which "less than" does not make sense.
How would you make a std::set of complex<>?
struct, pair, and tuple should not have an operator < because they are essentially unordered collections of data (I know that the elements of pair are called first and second, but it seems to me that they are essentially arbitrary labels).
How do you make a std::set of structs, pairs, or tuples?
vector is an interesting case. Some vectors are used to store lists of things (in which case ordering makes sense), but others are used to model things like mathematical vectors (in which case we are back to the complex case, and < does not make sense).
OK, do you define an operator< for vector or not? You don't know a priori what it will be used to hold.
string definitely should have operator <() because it models the real world concept (string of letters and other characters) for which "less than" definitely makes sense.
Still, there are several different "less than"s, all of which make sense. Based on the principle that operator< should be defined only if it's unique, string should not have operator<.