Daryle Walker wrote:
Because operator== has only one purpose in C++: comparison for equality. Operator<, on the other hand, has two common roles.
For classes that have natural ordering, it should define this ordering, and then all the other operators (> <= >=) should be defined as well. Classes that doesn't have this natural ordering, often still define operator< just to be able to be used in ordered containers. In such cases, operator< doesn't define a natural ordering because there isn't any. Instead, it just defines an ordering "that works" for ordered containers, nothing more. It doesn't need to compatible with operator== and !=, and defining operators >, <=, >= is misleading and should be avoided.
The later role is invalid. The reason is that the version of STL that made it into the Standard specifies an extra parameter for ordered containers for a comparison operation. It allows two containers of the same element type to use different comparison criteria (at compile- and/or run-time). This invalidates the need for a fake "operator <" since you can always package the comparison routine in an extra parameter. Conversely, calling code must not hard code use of "operator <" or "std::less<>", unless that code requires types with a natural ordering. (The standard algorithms have variants that either assume "operator <" or take in a comparison parameter.)
Making a fake "operator <" just for ordered containers invalidates the work of those who added comparison parameters in standard classes/functions.
You have a point there, no doubt. On the other hand, Library writers should supply some kind of functor to be used with their types in conjunction with ordered containers (when appropriate, and boost::variant is of course appropriate). How should this functor be called? You might say there should be some convention, but there isn't any right now, and some libraries use operator< for this purpose. Is it that bad if operator< is the convention? I'm not sure. It's sure is convenient to use ordered containers like that... And it seems to me that there is no major drawback to this, because no other order operator is provided. But I guess it's highly dependent on taste...
The issue is different for the "==" and "!=" operators because two objects can have their state checked for equivalence without having a standardized order. The "std::complex<>" template classes are an example.
std::complex doesn't have operator<. std::tr1::shared_ptr does. So it seems the standard isn't very helpfully consistent here...
boost::shared_ptr is a good example of such a class with no natural ordering, with operator== and !=, with operator<, but without the other operators. And shared_ptr is of TR1 strength...
Ordering for pointers is generally defined if both pointers are part of the same array segment. (Either one or both of the pointers can be at the "one-past-the-end" point of said segment.) If two "shared_ptr" objects can be used for points in an array segment[1], then comparison can be allowed with ALL four operators ("<", ">", "<=", and ">="). Otherwise, none of those operators should be defined and any current existence of such operators should be considered a bug.
I didn't quite understand that last paragraph. shared_ptrs doesn't have to point to objects in array (actually, they better not), but there can be arrays of shared_ptrs. So should the shared_ptr class provide order operators or shouldn't it? Anyway, the committee already made up it mind about the answer to this question...