id,summary,reporter,owner,description,type,status,milestone,component,version,severity,resolution,keywords,cc
11994,Support intrusive container key extractors that return the key by value,fdegros@…,Ion Gaztañaga,"[http://www.boost.org/doc/libs/1_60_0/doc/html/intrusive/map_multimap.html Map and multimap-like interface for associative containers] specifies that the ""key extractor"" passed by `key_of_value` must provide a member function to obtain a const reference to the key stored inside a `value_type`.

According to my experience, if the key extractor returns the key by value instead of const reference, then the code compiles but the program crashes a run time. This is a real pitfall.

Consider supporting key extraction by value as well. It would allow to use keys that are not stored directly in the value_type, or not directly accessible by reference.

For example:
{{{#!c++

using boost::intrusive;

struct my_node: set_base_hook<> {
  string s;
};

// The key is the length of my_node::s.
struct by_length {
  using key_type = std::size_t;

  // Return key by value.
  std::size_t operator()(const my_node& n) const {
    return n.s.length();
  }
};

multiset<std::string, key_of_value<by_length>> v;
v.insert(...);

// Find a string by length.
const auto it = v.find(3);
...

}}}

If this is not possible, then add a static assertion to make sure that the code doesn't compile at all.",Bugs,closed,To Be Determined,intrusive,Boost 1.60.0,Problem,fixed,,