How to declare boost range adaptor (e.g. map_values)

Question

Say I have

class Value;
class Key;

class MyClass {
  private:
  std::map<Key,Value> my_map;
  ....
}

Inside of MyClass methods I have a very convenient way to iterate through values of my_map by saying

 for( auto& value: my_map | boost::adaptors::map_values) {
    ...
 }

However I would like to have a method of MyClass that would essentially output my_map | boost::adaptors::map_values and allow convenient value iteration outside of MyClass methods. How do I declare such a method? Would I need to implement some sort of pseudo container and corresponding iterator or there is a shortcut?

Answer 1

Basically, you have two good choices. You can provide users with a nicely decoupled interface by using boost's any_range, or else if you need the best performance, you can give clients direct access to the adapted range using c++11 decltypes. In the first case, clients won't have to change if you change implementation, but this comes at a cost of extra redirection.

using any_range:

#include <boost/range.hpp> 
#include <map> 
#include <boost/range/adaptor/map.hpp> 
#include <boost/range/any_range.hpp>

class Foo 
{ 
    std::map<int, int> my_map;
    boost::any_range<int, boost::forward_traversal_tag, int&, std::ptrdiff_t> Values()
    { return my_map | boost::adaptors::map_values; }
}; 

giving direct access (awkward syntax here is because you require VS2013, which doesn't support member variables in unevaluated context at class scope):

#include <boost/range.hpp> 
#include <map> 
#include <boost/range/adaptor/map.hpp> 

class Foo 
{ 
    std::map<int, int> my_map;
    auto Values() -> decltype(my_map | boost::adaptors::map_values)
    { return my_map | boost::adaptors::map_values; }
};