Dave Harris wrote:
In-Reply-To: <d1409a$kc1$1@sea.gmane.org> daniel@calamity.org.uk (Daniel James) wrote (abridged):
Think about how such a class would be stored in the proposed unordered associative containers. If you use unordered_set<base*> then the hash function will be called for the pointer, not the class.
I imagine you would use a hasher which dereferenced the pointer.
template<typename T> struct hash_ptr { size_t operator()(const T *p) const { return hash_value( *p ); } };
And perhaps this should be included in boost too? Eg this:
unordered_set< shared_ptr<MyType>, hash_ptr<MyType> >
looks reasonable to me.
You also need equal_ptr<MyType>. Also, it is easy to break the above unordered_set my modifying some of the MyTypes.
So it is tempting not to bother, and instead use:
size_t Derived::hash_value() const { // return combine_hash( Base::hash_value(), m_value ); size_t hash = Base::hash_value(); combine_hash( hash, m_value ); return hash; }
which is correct, but will produce more collisions than if we'd used the typeid.
size_t Derived::hash_value() const { size_t seed = my_unique_value; hash_combine( seed, Base::hash_value() ); hash_combine( seed, m_value ); return seed; }