# Boost.MultiIndex Future work

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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`.

## Contents

- Notifying indices
- Constraints
- User-defined indices
- Indexed maps

## 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<
>         int,
>         indexed_by<
>           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
>       t.on_insert.connect(insert_log());

>       t.insert(0);
>       t.insert(1);

>       return 0;
>     }

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

## Constraints

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<
>       int,
>       indexed_by<
>         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.

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Release notes

Revised April 19th 2015

© Copyright 2003-2015 Joaquín M López Muñoz. Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)