Question

I have tried searching for this but couldn’t really find a proper answer. I have a stl::set of a custom class I made.

The class looks like

class myObject{

public:
  int a;
  int b;
  int c;

// constructor and operator < ()
}

The < comparison is based on b and c but I want to find elements in the set by a. Is there a way to do that other than performing a linear search by iterating over the set and checking for myObject.a? I am trying to get a sorted container of myObject but I need to be able to find elements in that container by the identifier a which is not really involved in the < comparison.

Answer 1

You can do it using boost::multi_index_container

class myObject
{
public:
    int a;
    int b;
    int c;
    bool operator < (const myObject& obj) const
    { return b < obj.b; }
};

using namespace boost::multi_index;
typedef multi_index_container<
    myObject,
    indexed_by<
        ordered_unique<identity<myObject> >, // index by operator <
        ordered_unique<member<myObject, int, &myObject::a> > // index by member a
  > 
> SetOfmyObjects;

typedef SetOfmyObjects::nth_index<0>::type SetIndex_b;
typedef SetOfmyObjects::nth_index<1>::type SetIndex_a;

…

    SetOfmyObjects s;
    const SetIndex_b& index_b = s.get<0>();
    SetIndex_b::iterator it_b;
    for (it_b = index_b.begin(); it_b != index_b.end(); ++it_b)
        // ordered by b
    const SetIndex_a& index_a = s.get<1>();
    SetIndex_a::iterator it_a;
    for (it_a = index_a.begin(); it_a != index_a.end(); ++it_a)
        // ordered by a