Question

Could anybody explain me what is the difference between overload == and <?

For example, if I use a map:

map<Type, int> a();

friend bool operator<(const Type& lhs, const Type& rhs);

friend bool operator==(const Type& lhs, const Type& rhs);

And I implement both operators, when I call:

a.find(value);

The operator function of == is being called? I think not. I debugged and saw that < is called, but why? What should be the behavior of the operator function of <?

I come from Java where the method equals is called and is pretty simple to understand the behavior.

Thanks.

Answer 1

operator== overloads the == operator (and no other); operator<
overloads the < operator (and no other).

std::map is defined to use std::less (and only std::less) by
default, and std::less is defined to use < by default. In general,
however, I would recommend not overloading operator< unless
ordered comparison makes sense for your class, in which case, you should
overload all six comparison operators, in a coherent fashion.
Otherwise, you can specify a comparison functional type as an additional
template argument to std::map; the comparison functional object should
define a strict weak ordering relationship. If the type is designed to
be used as a key, but the ordering is still purely arbitrary, you might
specialize std::less.

As for Java, without operator overloading, it obviously can’t use <;
by default, SortedMap (the Java equivalent to std::map) requires the
keys to be Comparable, however, which in turn requires a compare
function, which returns a value <, == or > 0, depending on whether
this is <, == or > than other. I’ll admit that I find this a
little bit more logical, but the difference is very, very small. (The
rationale behind the C++ decision is that build-in types like int or
double can be used as keys. In Java, you’ld have to box them.)