bimap i boost

Discovering the bimap framework - Boost 1.49.0

Discovering
the bimap framework

Interpreting
bidirectional maps

One way to interpret bidirectional maps is as a function between two collections
of data, lets call them the left and the right collection. An element in
this bimap is a relation between an element from the left collection and
an element from the right collection. The types of both collections defines
the bimap behaviour. We can view the stored data from the left side, as
a mapping between keys from the left collection and data from the right
one, or from the right side, as a mapping between keys from the right collection
and data from the left collection.

Standard
mapping framework

Relationships between data in the STL are represented by maps. A standard
map is a directed relation of keys from a left collection and data from
a right unconstrained collection. The following diagram shows the relationship
represented and the user's viewpoint.

The left collection type depends on the selected map type. For example
if the the map type is std::multimap
the collection type of X is a multiset_of.
The following table shows the equivalent types for the std associative
containers.

Table 1.1. std associative containers

container	left collection type	right collection type
map	set_of	no constraints
multimap	multiset_of	no constraints
unordered_map	unordered_set_of	no constraints
unordered_multimap	unordered_multiset_of	no constraints

Bimap
mapping framework

Boost.Bimap design is based on the STL, and extends the framework in a
natural way. The following diagram represents the new situation.

Notice that now the std::maps
are a particular case of a Boost.Bimap container, where you can view only
one side of the relationship and can control the constraints of only one
of the collections. Boost.Bimap allows the user to view the relationship
from three viewpoints. You can view it from one side, obtaining a std::map compatible container, or you can
work directly with the whole relation.

The next diagram shows the layout of the relation and pairs of a bimap.
It is the one from the one minute tutorial

Bimap pairs are signature-compatible with standard pairs but are different
from them. As you will see in other sections they can be tagged with user
defined names and additional information can be attached to them. You can
convert from std::pairs to bimap pairs directly but the
reverse conversion is not provided. This mean that you can insert elements
in a bimap using algorithms like std::copy
from containers like std::map,
or use std::make_pair to add new elements. However
it is best to use bm.left.insert( bm_type::left_value_type(f,s) ) instead
of bm.insert( std::make_pair(f,s) ) to avoid
an extra call to the copy constructor of each type.

The following code snippet shows the relation between a bimap and standard
maps.

	Note
	You have to used references to views, and not directly views object. Views cannot be constructed as separate objects from the container they belong to, so the following: // Wrong: we forgot the & after bm_type::left_type bm_type::left_map lm = bm.left; does not compile, since it is trying to construct the view object lm. This is a common source of errors in user code.

Go to source code

 

template< class Map, class CompatibleKey, class CompatibleData >
void use_it( Map & m,
             const CompatibleKey  & key,
             const CompatibleData & data )
{
    typedef typename Map::value_type value_type;
    typedef typename Map::const_iterator const_iterator;

    m.insert( value_type(key,data) );
    const_iterator iter = m.find(key);
    if( iter != m.end() )
    {
        assert( iter->first  == key  );
        assert( iter->second == data );

        std::cout << iter->first << " --> " << iter->second;
    }
    m.erase(key);
}

int main()
{
    typedef bimap< set_of<std::string>, set_of<int> > bimap_type;
    bimap_type bm;

    // Standard map
    {
        typedef std::map< std::string, int > map_type;
        map_type m;

        use_it( m, "one", 1 );
    }

    // Left map view
    {
        typedef bimap_type::left_map map_type;
        map_type & m = bm.left;

        use_it( m, "one", 1 );
    }

    // Reverse standard map
    {
        typedef std::map< int, std::string > reverse_map_type;
        reverse_map_type rm;

        use_it( rm, 1, "one" );
    }

    // Right map view
    {
        typedef bimap_type::right_map reverse_map_type;
        reverse_map_type & rm = bm.right;

        use_it( rm, 1, "one" );
    }

    return 0;
}

 

Copyright © 2006 -2007 Matias Capeletto 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)