You are expressly allowed and encouraged to add specializations to namespace std
*. The correct (and basically only) way to add a hash function is this:
namespace std {
template <> struct hash<Foo>
{
size_t operator()(const Foo & x) const
{
/* your code here, e.g. "return hash<int>()(x.value);" */
}
};
}
(Other popular specializations that you might consider supporting are std::less
, std::equal_to
and std::swap
.)
*) as long as one of the involved types is user-defined, I suppose.