GaudiUtils::Hash< T > Class Template Reference

Simple hash function. More...

#include <GaudiKernel/Hash.h>

Inheritance diagram for GaudiUtils::Hash< T >:

Public Member Functions
std::size_t	operator() (const T &key) const
	the hash-function More...
template<>
size_t	operator() (const GaudiAlg::ID &key) const
	Hash-function for class GaudiAlg::ID. More...
template<>
std::size_t	operator() (const char *key) const
	(very)specific for C-strings More...

Detailed Description

template<class T>
class GaudiUtils::Hash< T >

Simple hash function.

The default implementation is based on boost::hash

Attention

There is no generic a'la Stroustrup implementation valid for "any" objects.

See also

GaudiUtils::GenericHash

Attention

The hashing of pointers does not rely on hash for objects

The usage requires to provide the specialization The default specialization are provided by boost::bash_value for

• all primitive fundamental types
• strings and wide strings
• std::complex
• std::pair
• std::vector, std::list, std::deque
• std::set, std::multiset, std::map, std::multimap
• C-arrays, including C-strings
• generic pointer types

To provide own specialization for type TYPE or TYPE* there are four ways:

• provide the specialization of Gaudi::Utils::Hash<TYPE>::operator() ( const TYPE& val ) const or Gaudi::Utils::Hash<TYPE*>::operator() ( const TYPE* val ) const
• provide the full specialization of Gaudi::Utils::Hash<TYPE>
• provide the function std::size_t hash_value ( const TYPE& val ) ;

The first and the last ways are recommended. The second way can be needed if e.g. TYPE has non trivial properties, e.g. one need to use non-const or non-ref forms of hashing value For the last way the function must reside in the same namespace, where TYPE is defines, therefore if TYPE is in a global namespace it is better to avoid this way. The most natural place for this function is a "friend function" of the class. In this way it can have access to some useful internals of the class. However this is a bit intrusive way. The third way is not recommended, since sooner or later we'll need to migrate from boost::hash to std::tr1::hash.

Comparing the previous Gaudi implementation of GaudiUtils::Hash and boost::bash specializations, one can expect:

• some (tiny?) CPU penalty for floating point types
• some (tiny?) CPU penalty for long-long integer types
• better CPU performance for generic pointer types
• guaranteed coherence of hash for std::string & C-string

Probably the first two items are compensated by better "hashing"-performance

Author

Marco Clemencic

Vanya BELYAEV Ivan..nosp@m.Bely.nosp@m.aev@n.nosp@m.ikhe.nosp@m.f.nl

Date

2005-10-07

Definition at line 95 of file Hash.h.

Member Function Documentation

template<class T >

std::size_t GaudiUtils::Hash< T >::operator() ( const T &  key ) const

inline

the hash-function

the generic implementations of hash-function

Definition at line 147 of file Hash.h.

  {
    using namespace boost;
    return hash_value( key );
  }

template<>

size_t GaudiUtils::Hash< GaudiAlg::ID >::operator() ( const GaudiAlg::ID &  key ) const

inline

Hash-function for class GaudiAlg::ID.

Definition at line 145 of file GaudiHistoID.h.

  {
    return key.hash();
  }

template<>

std::size_t GaudiUtils::Hash< char * >::operator() ( const char *  key ) const

inline

(very)specific for C-strings

Definition at line 161 of file Hash.h.

  {
    std::size_t seed = 0;
    while ( key ) {
      boost::hash_combine( seed, *key );
      ++key;
    }
    return seed;
  }

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The documentation for this class was generated from the following file:

• GaudiKernel/GaudiKernel/Hash.h