id,summary,reporter,owner,description,type,status,milestone,component,version,severity,resolution,keywords,cc
9755,Simple way to get hash.,Rainer Deyke <rainerd@…>,Daniel James,"When writing custom hash_value functions, it is often useful to forward to the hash of another value.  However, there is currently no short and simple way to do this.  Consider:

template<class T> struct C {
  some_complex_mpl_expression<T>::type value;
  friend std::size_t hash_value() {
    // First attempt: requires knowledge of the type of the value to hash.
    return boost::hash<some_complex_mpl_expression<T>::type>()(this->value);

    // Second attempt: two lines, not recommended.
    using namespace boost;
    return hash_value(this->value);

    // Third attempt: works, but requires three lines of code and doesn't yield the same hash as the first two attempts.
    std::size_t seed = 0;
    boost::hash_combine(seed, this->value);
    return seed;
  }
};

I propose the addition of a simple function to calculate the hash of any arbitrary value, defined below:

template<class T> std::size_t get_hash(T const& v) {
  return hash<T>()(v);  
}

This would allow me to write the above hash_value function with a single line of
simple and correct code:

  friend std::size_t hash_value() {
     return boost::get_hash(this->value);
  }
",Feature Requests,new,To Be Determined,hash,Boost 1.55.0,Cosmetic,,,