Boost.MultiIndex Documentation - Examples

# Boost.MultiIndex Examples

Performance
Index
Tests

## Contents

- Example 1: basic usage
- Example 2: using member functions as keys
- Example 3: constructing `multi_index_container`s with `ctor_args_list`
- Example 4: bidirectional map
- Example 5: sequenced indices
- Example 6: complex searches and foreign keys
- Example 7: composite keys
- Example 8: hashed indices
- Example 9: serialization and MRU lists

## Example 1: basic usage

See source code.

Basic program showing the multi-indexing capabilities of Boost.MultiIndex with an admittedly boring set of `employee` records.

## Example 2: using member functions as keys

See source code.

Usually keys assigned to an index are based on a member variable of the element, but key extractors can be defined which take their value from a member function. This has some similarity with the concept of calculated keys supported by some relational database engines. The example shows how to use the predefined `const_mem_fun` key extractor to deal with this situation.

Keys based on member functions usually will not be actual references, but rather the temporary values resulting from the invocation of the member function used. This implies that `modify_key` cannot be applied to this type of extractors, which is a perfectly logical constraint anyway.

## Example 3: constructing `multi_index_container`s with `ctor_args_list`

See source code.

We show a practical example of usage of `multi_index_container::ctor_arg_list`, whose definition and purpose are explained in the Advanced topics section. The program groups a sorted collection of numbers based on identification through modulo arithmetics, by which `x` and `y` are equivalent if `(x%n)==(y%n)`, for some fixed `n`.

## Example 4: bidirectional map

See source code.

This example shows how to construct a bidirectional map with `multi_index_container`. By a bidirectional map we mean a container of elements of `std::pair<const FromType,const ToType>` such that no two elements exists with the same `first` or `second` value (`std::map` only guarantees uniqueness of the first member). Fast lookup is provided for both keys. The program features a tiny Spanish-English dictionary with online query of words in both languages.

## Example 5: sequenced indices

See source code.

The combination of a sequenced index with an index of type `ordered_non_unique` yields a `list`-like structure with fast lookup capabilities. The example performs some operations on a given text, like word counting and selective deletion of some words.

## Example 6: complex searches and foreign keys

See source code.

This program illustrates some advanced techniques that can be applied for complex data structures using `multi_index_container`. Consider a `car_model` class for storing information about automobiles. On a first approach, `car_model` can be defined as:

>     struct car_model
>     {
>       std::string model;
>       std::string manufacturer;
>       int         price;
>     };

This definition has a design flaw that any reader acquainted with relational databases can easily spot: The `manufacturer` member is duplicated among all cars having the same manufacturer. This is a waste of space and poses difficulties when, for instance, the name of a manufacturer has to be changed. Following the usual principles in relational database design, the appropriate design involves having the manufactures stored in a separate `multi_index_container` and store pointers to these in `car_model`:

>     struct car_manufacturer
>     {
>       std::string name;
>     };

>     struct car_model
>     {
>       std::string       model;
>       car_manufacturer* manufacturer;
>       int               price;
>     };

Although predefined Boost.MultiIndex key extractors can handle many situations involving pointers (see advanced features of Boost.MultiIndex key extractors in the Advanced topics section), this case is complex enough that a suitable key extractor has to be defined. The following utility cascades two key extractors:

>     template<class KeyExtractor1,class KeyExtractor2>
>     struct key_from_key
>     {
>     public:
>       typedef typename KeyExtractor1::result_type result_type;

>       key_from_key(
>         const KeyExtractor1& key1_=KeyExtractor1(),
>         const KeyExtractor2& key2_=KeyExtractor2()):
>         key1(key1_),key2(key2_)
>       {}

>       template<typename Arg>
>       result_type operator()(Arg& arg)const
>       {
>         return key1(key2(arg));
>       }

>     private:
>       KeyExtractor1 key1;
>       KeyExtractor2 key2;
>     };

so that access from a `car_model` to the `name` field of its associated `car_manufacturer` can be accomplished with

>     key_from_key<
>       member<car_manufacturer,const std::string,&car_manufacturer::name>,
>       member<car_model,const car_manufacturer *,car_model::manufacturer>
>     >

The program asks the user for a car manufacturer and a range of prices and returns the car models satisfying these requirements. This is a complex search that cannot be performed on a single operation. Broadly sketched, one procedure for executing the selection is:

1. Select the elements with the given manufacturer by means of `equal_range`,
2. feed these elements into a `multi_index_container` sorted by price,
3. select by price using `lower_bound` and `upper_bound`;

or alternatively:

1. Select the elements within the price range with `lower_bound` and `upper_bound`,
2. feed these elements into a `multi_index_container` sorted by manufacturer,
3. locate the elements with given manufacturer using `equal_range`.

An interesting technique developed in the example lies in the construction of the intermediate `multi_index_container`. In order to avoid object copying, appropriate view types are defined with `multi_index_container`s having as elements pointers to `car_model`s instead of actual objects. These views have to be supplemented with appropriate dereferencing key extractors.

## Example 7: composite keys

See source code.

Boost.MultiIndex `composite_key` construct provides a flexible tool for creating indices with non-trivial sorting criteria. The program features a rudimentary simulation of a file system along with an interactive Unix-like shell. A file entry is represented by the following structure:

>     struct file_entry
>     {
>       std::string       name;
>       unsigned          size;
>       bool              is_dir; // true if the entry is a directory
>       const file_entry* dir;    // directory this entry belongs in
>     };

Entries are kept in a `multi_index_container` maintaining two indices with composite keys:

- A primary index ordered by directory and name,
- a secondary index ordered by directory and size.

The reason that the order is made firstly by the directory in which the files are located obeys to the local nature of the shell commands, like for instance `ls`. The shell simulation only has three commands:

- `cd [.|..|<directory>]`
- `ls [-s]` (`-s` orders the output by size)
- `mkdir <directory>`

The program exits when the user presses the Enter key at the command prompt.

The reader is challenged to add more functionality to the program; for instance:

- Implement additional commands, like `cp`.
- Add handling of absolute paths.
- Use serialization to store and retrieve the filesystem state between program runs.

## Example 8: hashed indices

See source code.

Hashed indices can be used as an alternative to ordered indices when fast lookup is needed and sorting information is of no interest. The example features a word counter where duplicate entries are checked by means of a hashed index. Confront the word counting algorithm with that of example 5.

## Example 9: serialization and MRU lists

See source code.

A typical application of serialization capabilities allows a program to restore the user context between executions. The example program asks the user for words and keeps a record of the ten most recently entered ones, in the current or in previous sessions. The serialized data structure, sometimes called an MRU (most recently used) list, has some interest on its own: an MRU list behaves as a regular FIFO queue, with the exception that, when inserting a preexistent entry, this does not appear twice, but instead the entry is moved to the front of the list. You can observe this behavior in many programs featuring a "Recent files" menu command. This data structure is implemented with `multi_index_container` by combining a sequenced index and an index of type `hashed_unique`.

Performance
Index
Tests

Revised August 22nd 2005

© Copyright 2003-2005 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)