hash.hh File Reference

Support for calculating hash values of objects. More...

#include <functional>
#include <dune/common/typetraits.hh>

Go to the source code of this file.

Classes
struct	Dune::hash< T >
	Functor for hashing objects of type T. More...

Namespaces
namespace	Dune
	Dune namespace.

Macros
#define	DUNE_DEFINE_HASH(template_args, type)
	Defines the required struct specialization to make type hashable via Dune::hash.
#define	DUNE_HASH_TEMPLATE_ARGS(...)
	Wrapper macro for the template arguments in DUNE_DEFINE_HASH.
#define	DUNE_HASH_TYPE(...)
	Wrapper macro for the type to be hashed in DUNE_DEFINE_HASH.

Functions
template<typename T>
void	Dune::hash_combine (std::size_t &seed, const T &arg)
	Calculates the hash value of arg and combines it in-place with seed.
template<typename It>
std::size_t	Dune::hash_range (It first, It last)
	Hashes all elements in the range [first,last) and returns the combined hash.
template<typename It>
void	Dune::hash_range (std::size_t &seed, It first, It last)
	Hashes all elements in the range [first,last) and combines the hashes in-place with seed.

Detailed Description

Support for calculating hash values of objects.

This file provides the functor Dune::hash to calculate hash values and some infrastructure to simplify extending Dune::hash for user-defined types, independent of the actual underlying implementation.

Macro Definition Documentation

DUNE_DEFINE_HASH

#define DUNE_DEFINE_HASH	(		template_args,
			type )

Defines the required struct specialization to make type hashable via Dune::hash.

In order to calculate the hash, operator() of the generated specialization will return the result of an unqualified call to the global function hash_value(const type&). As the call is not qualified, the function will be found using argument-dependent lookup, allowing implementors to conveniently place it inside the class body.

Consider the following type:

namespace ns {
  template<typename A, int i>
  class Foo
  {
    ...
  };
}

In order to add support for Dune::hash, you need to extend the definition like this:

namespace ns {
  template<typename A, int i>
  class Foo
  {
    ...
    // The keyword "friend" turns this into a global function that is a friend of Foo.
    inline friend std::size_t hash_value(const Foo& arg)
    {
      return ...;
    }
  };
}
 
// Define hash struct specialization
DUNE_DEFINE_HASH(DUNE_HASH_TEMPLATE_ARGS(typename A, int i),DUNE_HASH_TYPE(Foo<A,i>))

Warning

As the specialization has to be placed in the original namespace of the hash struct (e.g. std), this macro must be called from the global namespace!

Parameters

template_args	The template arguments required by the hash struct specialization, wrapped in a call to DUNE_HASH_TEMPLATE_ARGS. If this is a complete specialization, call DUNE_HASH_TEMPLATE_ARGS without arguments.
type	The exact type of the specialization, wrapped in a call to DUNE_HASH_TYPE.

DUNE_HASH_TEMPLATE_ARGS

#define DUNE_HASH_TEMPLATE_ARGS

(

...

)

Wrapper macro for the template arguments in DUNE_DEFINE_HASH.

This macro should always be used as a wrapper for the template arguments when calling DUNE_DEFINE_HASH. It works around some preprocessor limitations when the template arguments contain commas or the list is completely empty.

DUNE_HASH_TYPE

#define DUNE_HASH_TYPE

(

...

)

Wrapper macro for the type to be hashed in DUNE_DEFINE_HASH.

This macro should always be used as a wrapper for the type of the specialization when calling DUNE_DEFINE_HASH. It works around some preprocessor limitations when the type contains commas.