Switching keys in values in a map

A functor that reverses a pair: first becomes second,
second becomes first. Useful for switching
from a map to another map (or multimap in this example) where
the value becomes the key and the key the value.

[cc lang="cpp"]
std::map m;
std::multimap m2;
m["a"] = 0;
m["b"] = 1;
m["c"] = 0;
std::transform (m.begin(), m.end(), std::inserter (m2, m2.begin()),
pair_switch; ());
[/cc]

And the actual code:
[cc lang="cpp"]
template
struct pair_switcher : public std::unary_function >
{
typedef std::pair result_type;
typedef PairType argument_type;

result_type operator()(const argument_type &p) const
{
return result_type (p.second, p.first);
}
};
[/cc]