...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
A Bimap is a data structure that represents bidirectional relations between
elements of two collections. The container is designed to work as two opposed
STL maps. A bimap between a collection X
and a collection Y
can be viewed
as a map from X
to Y
(this view will be called the left
map view) or as a map from Y
to X
(known as the right
map view). Additionally, the bimap can also be viewed as a set of
relations between X
and Y
(named the collection of relations
view).
The following code creates an empty bimap container:
typedef bimap<X,Y> bm_type; bm_type bm;
Given this code, the following is the complete description of the resulting bimap. [1]
bm.left
is signature-compatible with std::map<X,Y>
bm.right
is signature-compatible with std::map<Y,X>
bm
is signature-compatible
with std::set< relation<X,Y> >
You can see how a bimap container offers three views over the same collection of bidirectional relations.
If we have any generic function that work with maps
template< class MapType > void print_map(const MapType & m) { typedef typename MapType::const_iterator const_iterator; for( const_iterator iter = m.begin(), iend = m.end(); iter != iend; ++iter ) { std::cout << iter->first << "-->" << iter->second << std::endl; } }
We can use the left map view and the right map view with it
bimap< int, std::string > bm; ... print_map( bm.left ); print_map( bm.right );
And the output will be
1 --> one
2 --> two
...one --> 1
two --> 2
...
The relation
class represents
two related elements. The two values are named left and right to express the
symmetry of this type. The bimap pair classes are signature-compatible with
std::pairs
.
A convenience header is available in the boost directory:
#include <boost/bimap.hpp>
Lets define a bidirectional map between integers and strings:
typedef boost::bimap< int, std::string > bm_type; bm_type bm;
Remember that bm
alone can
be used as a set of relations. We can insert elements or iterate over them
using this view.
bm.insert( bm_type::value_type(1, "one" ) ); bm.insert( bm_type::value_type(2, "two" ) ); std::cout << "There are " << bm.size() << "relations" << std::endl; for( bm_type::const_iterator iter = bm.begin(), iend = bm.end(); iter != iend; ++iter ) { // iter->left : data : int // iter->right : data : std::string std::cout << iter->left << " <--> " << iter->right << std::endl; }
bm.left
works like a std::map<
int, std::string
>
. We use it in the same way we will
use a standard map.
typedef bm_type::left_map::const_iterator left_const_iterator; for( left_const_iterator left_iter = bm.left.begin(), iend = bm.left.end(); left_iter != iend; ++left_iter ) { // left_iter->first : key : int // left_iter->second : data : std::string std::cout << left_iter->first << " --> " << left_iter->second << std::endl; } bm_type::left_const_iterator left_iter = bm.left.find(2); assert( left_iter->second == "two" ); bm.left.insert( bm_type::left_value_type( 3, "three" ) );
The type of |
|
|
|
This line produces the same effect of |
bm.right
works like a std::map<
std::string, int >
. It
is important to note that the key is the first type and the data is the second
one, exactly as with standard maps.
bm_type::right_const_iterator right_iter = bm.right.find("two"); // right_iter->first : key : std::string // right_iter->second : data : int assert( right_iter->second == 2 ); assert( bm.right.at("one") == 1 ); bm.right.erase("two"); bm.right.insert( bm_type::right_value_type( "four", 4 ) );
The main difference between bimap views and their standard containers counterparts
is that, because of the bidirectional nature of a bimap, the values stored
in it can not be modified directly using iterators. For example, when a std::map<X,Y>
iterator
is dereferenced the return type is std::pair<const X,
Y>
,
so the following code is valid: m.begin()->second = new_value;
.
However dereferencing a bimap<X,Y>::left_iterator
returns a type that is signature-compatible with a std::pair<const X, const Y>
bm.left.find(1)->second = "1"; // Compilation error
If you insert (1,"one")
and (1,"1")
in a std::map<int,std::string>
the second insertion will have no effect.
In a bimap<X,Y>
both keys have to remain unique. The insertion
may fail in other situations too. Lets see an example
bm.clear(); bm.insert( bm_type::value_type( 1, "one" ) ); bm.insert( bm_type::value_type( 1, "1" ) ); // No effect! bm.insert( bm_type::value_type( 2, "one" ) ); // No effect! assert( bm.size() == 1 );
Look how you can reuse code that is intend to be used with std::maps, like the print_map function in this example.
#include <string> #include <iostream> #include <boost/bimap.hpp> template< class MapType > void print_map(const MapType & map, const std::string & separator, std::ostream & os ) { typedef typename MapType::const_iterator const_iterator; for( const_iterator i = map.begin(), iend = map.end(); i != iend; ++i ) { os << i->first << separator << i->second << std::endl; } } int main() { // Soccer World cup typedef boost::bimap< std::string, int > results_bimap; typedef results_bimap::value_type position; results_bimap results; results.insert( position("Argentina" ,1) ); results.insert( position("Spain" ,2) ); results.insert( position("Germany" ,3) ); results.insert( position("France" ,4) ); std::cout << "The number of countries is " << results.size() << std::endl; std::cout << "The winner is " << results.right.at(1) << std::endl << std::endl; std::cout << "Countries names ordered by their final position:" << std::endl; // results.right works like a std::map< int, std::string > print_map( results.right, ") ", std::cout ); std::cout << std::endl << "Countries names ordered alphabetically along with" "their final position:" << std::endl; // results.left works like a std::map< std::string, int > print_map( results.left, " ends in position ", std::cout ); return 0; }
The output of this program will be the following:
The number of countries is 4
The winner is Argentina
Countries names ordered by their final position:
1) Argentina
2) Spain
3) Germany
4) France
Countries names ordered alphabetically along with their final position:
Argentina ends in position 1
France ends in position 4
Germany ends in position 3
Spain ends in position 2
For information on function signatures, see any standard library documentation or read the reference section of this documentation.
Caution | |
---|---|
Be aware that a bidirectional map is only signature-compatible with standard containers. Some functions may give different results, such as in the case of inserting a pair into the left map where the second value conflicts with a stored relation in the container. The functions may be slower in a bimap because of the duplicated constraints. It is strongly recommended that you read The full tutorial if you intend to use a bimap in a serious project. |
[1] A type is signature-compatible with other type if it has the same signature for functions and metadata. Preconditions, postconditions and the order of operations need not be the same.