boost::multi_index index by function call with parameter(s)

Question

Hi Everyone

I'm trying to make a boost::multi_index container that uses member functions w/ parameters as keys.

class Data {
public:
   std::string get(const std::string & _attr) { return _internals_fetch_data(_attr); }

/*
   assume some implementation for storing data in some structure(s)
*/
};

Suppose I have a rectangular list of these data items that I want to multiple indicies over. rectangular means all items in list have the same attributes via get()

The boost::multi_index declaration is something like

typedef multi_index_container<
  Data,
  indexed_by<
   ordered_unique<
    BOOST_MULTI_INDEX_CONST_MEM_FUN(Data,String,get)
   >
  >
> my_container;

Except that BOOST_MULTI_INDEX_CONST_MEM_FUNCT() does not have these features. Composite keys still work with member variables.

How do I get around this ? It doesn't look like I can give ordered_unique<> a boost::function1

EDIT:

After some thought, here is the gist of what I'm trying to do.

boost::multi_index determines it's indexing features during compile time. How do I circumvent these features and use run-time determined indexes ?

Answer 1

What do you suppose to pass as argument to function? What is const std::string & _attr?

Anyway, BOOST_MULTI_INDEX_CONST_MEM_FUN is just a macro for const_mem_fun functor. You could write your own functor const_mem_fun1. But I have no idea how you going to use it. Implementation of const_mem_fun receives object by reference (or wrapper, e.g. shared_ptr) and then it calls member function with no arguments.

boost::multi_index::indexed_by expects type, not an object, as well as ordered_unique expects type. That's why you cannot write const_mem_fun1<Data, std::string, &Data::get>("string"). Why do you cannot use function get with no parameters and create special string field in Data struct?

struct Data {
  std::string _attr;
public:
  std::string get() const { return _internals_fetch_data(_attr); }
};