Nested Types

hash_map and hash_multimap define a host of nested types similar to standard containers. Several noteworthy points:

• key_type and value_type are not the same type. value_type is a pair<const Key, T>.
• key_hasher and value_hasher are not the same type. key_hasher is the template parameter Hash. value_hasher is a nested type which converts key_hasher into a function which accepts a value_type.
  • value_hasher has the public typedef's

    typedef value_type argument_type;
    typedef size_type   result_type;
    

This qualifies it as a std::unary_function (as defined in <functional>) and so could be used where other functionals are used.

• value_hasher has these public member functions:

  size_type operator()(const value_type& x) const;
  size_type operator()(const key_type& x) const;
  

  These simply return the result of key_hasher, but with the first operator extracting the key_type from the value_type before passing the key_type on to key_hasher.

  • Key_compare and value_compare are not the same type. key_compare is the template parameter Compare. value_compare is a nested type which converts key_compare into a function which accepts a value_type.
• value_compare has the public typedef's

  typedef value_type first_argument_type;
  typedef value_type second_argument_type;
  typedef bool          result_type;

  This qualifies it as a std:: binary_function (as defined in <functional>) and so could be used where other functionals are used.

• value_compare has these public member functions:

  bool operator()(const value_type& x, 
     const value_type& y) const;
  bool operator()(const key_type& x,
    const value_type& y) const;
  bool operator()(const value_type& x, 
    const key_type& y) const;
  

  These pass their arguments on to key_compare, extracting the key_type from value_type when necessary.