Re: Why overloaded < operator doesn't work with std::map

Ulrich Eckhardt <>
21 Nov 2006 07:13:05 -0500
listboss wrote:

class X {
X() {};

         ^ unnecessary

~X() {};

          ^ unnecessary

X(const X&other);
bool operator<(const X& rhs);
X& operator=(const X& other);
int n;

Try to compile this, understand the errors you get and, once fixed, the map
will work, too. Code:

bool wrapper( X const& x1, X const& x2)
{ return x1<x2; }

X x;
wrapper( x, x);

I figured out that either I had to define operator< as a friend

This has nothing to do with friendship. The point is that a binary operator
(except operator=) can be defined either as a free function taking two
parameters or as memberfunction taking one explicit parameter and
the 'this' pointer. Often, it makes sense to use the free function version
of operators, in particular when the arguments are mixed. In your example,
you could e.g. compare the X to an X, the X to an int or an int to an X.

or use a functor as a key_compare, something like:
std::map<X,bool, cmpX>

Well, by default it uses std::less, which in turn uses operator<. If you
implement operator< (which you should only do if it makes some intuitive
sense), it works as-is, otherwise you should rather use an explicit, named
comparator struct.


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