How to construct Boost bimap from static list?

Question

I have a bimap like this:

using MyBimap = boost::bimaps::bimap<
    boost::bimaps::unordered_set_of<A>,
    boost::bimaps::unordered_set_of<B>>;

I want to construct it from a static initializer list, as it can be done for std::map:

MyBimap map{{a1, b1}, {a2, b2}, {a3, b3}};

Unfortunately, it doesn't work because bimap doesn't support initializer lists, so I tried a workaround. Boost's documentation lists the following constructors:

 bimap();

 template< class InputIterator >
 bimap(InputIterator first,InputIterator last);

 bimap(const bimap &);

So I tried the second one, like this:

std::vector<std::pair<A,B>> v{{a1, b1}, {a2, b2}, {a3, b3}};
MyBimap map(v.begin(), v.end());

It also didn't work. The documentation isn't exactly clear what kind of iterators this constructor expects, but apparently it's not simply an iterator of std::pair<A, B> objects. Then what does this constructor expect for this kind of bimap?

Answer 1

The iterator begin/end should be for a sequence of bimap values.

boost::bimap< A, B>::value_type

A bimap value is a lot like a std::pair and can be initialized with {a1, b1}syntax. A vector of them seems to work too, which provides usable iterators for the constructor.

Ok, here is an example that compiles and runs for me (gcc 4.8.2 --std=c++11)

#include <vector>
#include <boost/bimap.hpp>

using namespace std;
int main() {
    typedef boost::bimap< int, int > MyBimap;

    std::vector<MyBimap::value_type > v{{1, 2}, {3, 4}, {5, 6}};

    MyBimap M(v.begin(),v.end());

    std::cout << "The size is " << M.size()
              << std::endl;

    std::cout << "An entry is 1:" << M.left.at(1)
              << std::endl;
}

Answer 2

I use the following "factory function" that takes a braced initializer list and returns a boost::bimap:

template <typename L, typename R>
boost::bimap<L, R>
makeBimap(std::initializer_list<typename boost::bimap<L, R>::value_type> list)
{
    return boost::bimap<L, R>(list.begin(), list.end());
}

Usage:

auto myBimap = makeBimap<int, int>({{1, 2}, {3, 4}, {5, 6}});

Answer 3

C++ beginner here: You can use boost::assign to generate the initialization. I found this solution here.

Example:

#include <boost/bimap.hpp>
#include <boost/assign.hpp>

//declare the type of bimap we want
typedef boost::bimap<int, std::string> bimapType;
//init our bimap
bimapType bimap = boost::assign::list_of< bimapType::relation >
( 1, "one"   )
( 2, "two"   )
( 3, "three" );

//test if everything works
int main(int argc, char **argv)
{
    std::cout << bimap.left.find(1)->second << std::endl;
    std::cout << bimap.left.find(2)->second << std::endl;
    std::cout << bimap.left.find(3)->second << std::endl;
    std::cout << bimap.right.find("one")->second << std::endl;
    std::cout << bimap.right.find("two")->second << std::endl;
    std::cout << bimap.right.find("three")->second << std::endl;

    /* Output:
     * one
     * two
     * three
     * 1
     * 2
     * 3
     */
}

Answer 4

This leaves a vector to be cleaned up, which might in some cases be an issue. Here a short helper class that might solve your problem as well. As the class instance is a temporary, it gets cleaned up immediately wherever it is used. This is based on https://stackoverflow.com/a/1730798/3103767

// helper for bimap init (simple, lightweight version of boost::assign)
template <typename T, typename U>
class create_bimap
{
    typedef boost::bimap< T, U > bimap_type;
    typedef typename bimap_type::value_type value_type;
private:
    boost::bimap<T, U> m_map;
public:
    create_bimap(const T& left, const U& right)
    {
        m_map.insert( value_type(left, right) );
    }

    create_bimap<T, U>& operator()(const T& left, const U& right)
    {
        m_map.insert( value_type(left, right) );
        return *this;
    }

    operator boost::bimap<T, U>()
    {
        return m_map;
    }
};

Use as follows:

boost::bimap<string,int> myMap = create_bimap<string,int>
    ("c",1)
    ("b",2)
    ("a",3);