Boost C++ Libraries

As has already been said, in STL maps, you can only control the constraints from one of the collections, namely the one that you are viewing. In Boost.Bimap, you can control both and it is as easy as using the STL.

The idea is to use the same constraint names that are used in the standard. If you don't specify the collection type, Boost.Bimap assumes that the collection is a set. The instantiation of a bimap with custom collection types looks like this:

typedef bimap< CollectionType_of<A>, CollectionType_of<B> > bm_type;

The following is the list of all supported collection types.

list_of and vector_of map views are not associated with any existing STL associative containers. They are two examples of unsorted associative containers. unconstrained_set_of allow the user to ignore a view. This will be explained later.

The selection of the collection type affects the possible operations that you can perform with each side of the bimap and the time it takes to do each. If we have:

typedef bimap< CollectionType_of<A>, CollectionType_of<B> > bm_type;
bm_type bm;

The following now describes the resulting map views of the bidirectional map.

Each collection type template has different parameters to control its behaviour. For example, in set_of specification, you can pass a Functor type that compares two types. All of these parameters are exactly the same as those of the standard library container, except for the allocator type. You will learn later how to change the allocator for a bimap.

The following table lists the meanings of each collection type's parameters.

Countries Populations

We want to store countries populations. The requirements are:

Lets create the appropriate bimap.

typedef bimap<

    unordered_set_of< std::string >,
    multiset_of< long, std::greater<long> >

> populations_bimap;

First of all countries names are unique identifiers, while two countries may have the same population. This is why we choose multiset_of for populations.

Using a multiset_of for population allow us to iterate over the data. Since listing countries ordered by their names is not a requisite, we can use an unordered_set_of that allows constant order look up.

And now lets use it in a complete example

Go to source code

typedef bimap<

    unordered_set_of< std::string >,
    multiset_of< long, std::greater<long> >

> population_bimap;

typedef population_bimap::value_type population;

population_bimap pop;
pop.insert( population("China",          1321000000) );
pop.insert( population("India",          1129000000) );
pop.insert( population("United States",   301950000) );
pop.insert( population("Indonesia",       234950000) );
pop.insert( population("Brazil",          186500000) );
pop.insert( population("Pakistan",        163630000) );

std::cout << "Countries by their population:" << std::endl;

// First requirement
for( population_bimap::right_const_iterator
        i = pop.right.begin(), iend = pop.right.end();
        i != iend ; ++i )
{
    std::cout << i->second << " with " << i->first << std::endl;
}

// Second requirement
std::cout << "Population of China: " << pop.left.at("China") << std::endl;

Repetitions counter

We want to count the repetitions for each word in a text and print them in order of appearance.

Go to source code

typedef bimap
<
    unordered_set_of< std::string >,
    list_of< counter > 

> word_counter;

typedef boost::tokenizer<boost::char_separator<char> > text_tokenizer;

std::string text=
    "Relations between data in the STL are represented with maps."
    "A map is a directed relation, by using it you are representing "
    "a mapping. In this directed relation, the first type is related to "
    "the second type but it is not true that the inverse relationship "
    "holds. This is useful in a lot of situations, but there are some "
    "relationships that are bidirectional by nature.";

// feed the text into the container
word_counter   wc;
text_tokenizer tok(text,boost::char_separator<char>(" \t\n.,;:!?'\"-"));

for( text_tokenizer::const_iterator it = tok.begin(), it_end = tok.end();
     it != it_end ; ++it )
{
    ++ wc.left[*it];
}

// list words with counters by order of appearance
for( word_counter::right_const_iterator
        wit = wc.right.begin(), wit_end = wc.right.end();

     wit != wit_end; ++wit )
{
    std::cout << wit->second << ": " << wit->first;
}