I want to use a POD struct as a hash key in a map, e.g. struct A { int x; int y; }; std::unordered_map<A, int> my_map; but I can't do this, since no hash function is auto-generatable for such structs. Why does the C++ standard not require a default hash for a POD struct? Why do compilers (specifically, GCC 4.x / 5.x) offer such a hash, even if the standard doesn't mandate one? How can I generate a hash function, using a template, in a portable way, for all of my POD structures (I'm willing to make semantic assumptions if necessary)?

Why no default hash for C++ POD structs?

I want to use a POD struct as a hash key in a map, e.g.

struct A { int x; int y; };
std::unordered_map<A, int> my_map;

but I can't do this, since no hash function is auto-generatable for such structs.

Why does the C++ standard not require a default hash for a POD struct?
Why do compilers (specifically, GCC 4.x / 5.x) offer such a hash, even if the standard doesn't mandate one?
How can I generate a hash function, using a template, in a portable way, for all of my POD structures (I'm willing to make semantic assumptions if necessary)?