Question

I need to hash a combination of four different things, comprising 20 bytes, as such I defined this:

struct holder
{
  char a;
  uint16_t b;
  uint64_t c;
  char d[9];
} __attribute((__packed__));

and then I can load one of the above and pass it to a hash function. So then I want my class Foo to have a std::tr1::unordered_map<holder, int> map. but in order to declare that in the header file for Foo, I need to include the definition of holder, the template specialization inside std::tr1 for hash for my type, and as a result the full hash function. Is there a way to not have all this code up in the header but still give my class this hashmap?

Answer 1

Simply declare the function in a header file, and define it at a cpp file.

This would look like:

// Header
namespace std { namespace tr1

// Define specialization
template<>
struct hash<holder>: unary_function<holder, size_t> {
    // Declare member
    size_t operator()(holder const&) const;
};

} }

// Source

// Define member
std::size_t std::tr1::hash<holder>::operator()(holder const&) const
{ /* implementation */ }