stdhash

Can the C++11 std::hash type be used to hash function pointers? There is a hash partial specialization defined as template <typename T> struct hash<T*>; but since function pointers are different from other pointer types in C++ (e.g. they can't be cast to void* ), I'm not sure whether it is safe to use it for types like int(*)() or void(*)(int, int) . Is this permitted? Is there any specific wording in the new ISO spec that supports or refutes this? Thanks! Great question. I don't know the answer for sure, and I'm happy to defer to anyone with better knowledge than me, but my thinking is that

So the library I use has an enum (say it's named LibEnum ). I need to have an std::unordered_set of LibEnum , but I get compilation error that there is no specialized std::hash for it. I could easily write it and just return the number of value (first element is 0, second 1 etc), but where exactly I should put this specialization and how should it look like? I can't modify the library sources. enum LibEnum { A, B, C, D}; std::unordered_set <LibEnum> mySet; //need std::hash for LibEnum //how should it look like? You can just specialise std::hash for your type: namespace std { template <> struct

I have an abstract base class Hashable that classes that can be hashed derive from. I would now like to extend std::hash to all classes that derive from Hashable . The following code is supposed to do exactly that. #include <functional> #include <type_traits> #include <iostream> class Hashable { public: virtual ~Hashable() {} virtual std::size_t Hash() const =0; }; class Derived : public Hashable { public: std::size_t Hash() const { return 0; } }; // Specialization of std::hash to operate on Hashable or any class derived from // Hashable. namespace std { template<class C> struct hash {