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Boost.MultiIndex Future work

A number of new functionalities are considered for inclusion into future releases of Boost.MultiIndex. Some of them depend on the potential for extensibility of the library, which has been a guiding principle driving the current internal design of multi_index_container.


Ranked indices

Ordered indices are implemented using red-black trees; these trees can be augmented with additional information to obtain a type of data structure called order-statistics trees, allowing for logarithmic search of the n-th element. It has been proposed that order-statistics trees be used to devise a new type of ranked indices that support operator[] while retaining the functionality of ordered indices.

Notifying indices

Notifying indices can be implemented as decorators over preexistent index types, with the added functionality that internal events of the index (insertion, erasing, modifying of elements) are signalled to an external entity --for instance, by means of the Boost.Signals library. This functionality can have applications for:

  1. Logging,
  2. interfacing to GUI-based applications,
  3. synchronization between separate data structures.

The following is a sketch of a possible realization of notifying indices:

struct insert_log
  void operator()(int x)
    std::clog<<"insert: "<<x<<std::endl;

int main()
  typedef multi_index_container<
      notifying<ordered_unique<identity<int> > >, // notifying index
      ordered_non_unique<identity<int> >
  > indexed_t;

  indexed_t t;

  // on_insert is the signal associated to insertions


  return 0;

// output:
//   insert: 0
//   insert: 1


The notifying indices functionality described above exploits a powerful design pattern based on index adaptors, decorators over preexistent indices which add some functionality or somehow change the semantics of the underlying index. This pattern can be used for the implementation of constraints, adaptors that restrict the elements accepted by an index according to some validation predicate. The following is a possible realization of how constraints syntax may look like:

struct is_even
  bool operator()(int x)const{return x%2==0;}

typedef multi_index_container<
    constrained<ordered_unique<identity<int> >,is_even>
> indexed_t;

User-defined indices

The mechanisms by which Boost.MultiIndex orchestrates the operations of the indices held by a multi_index_container are simple enough to make them worth documenting so that the (bold) user can write implementations for her own indices.

Indexed maps

multi_index_container is rich enough to provide the basis for implementation of indexed maps, i.e. maps which can be looked upon several different keys. The motivation for having such a container is mainly aesthetic convenience, since it would not provide any additional feature to similar constructs based directly on multi_index_container.

The main challenge in writing an indexed map lies in the design of a reasonable interface that resembles that of std::map as much as possible. There seem to be fundamental difficulties in extending the syntax of a std::map to multiple keys. For one example, consider the situation:

indexed_map<int,string,double> m;
// keys are int and string, double is the mapped to value


cout<<m[0]<<endl;      // OK
cout<<m["zero"]<<endl; // OK
m[1]=1.0;              // !!

In the last sentence of the example, the user has no way of providing the string key mapping to the same value as m[1]. This and similar problems have to be devoted a careful study when designing the interface of a potential indexed map.

Revised July 6th 2013

© Copyright 2003-2013 Joaquín M López Muñoz. Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at