#include <iostream>
#include <boost/function_output_iterator.hpp>
#include <boost/variant.hpp>
#include <string>
#include <map>
#include <algorithm>
 
typedef std::map<int,   std::string> FirstMap;
typedef std::map<short, double>      SecondMap;
 
typedef boost::variant<FirstMap::mapped_type, SecondMap::mapped_type> Both;
typedef std::map<long,  Both>        BothMap;
 
struct CustomOutput {
    CustomOutput(BothMap& ref) : _ref(ref) {}
 
    template <typename Pair>
    void operator()(const Pair& pair) const { 
        _ref.insert(BothMap::value_type { pair.first, pair.second }); 
    }
    private: BothMap& _ref;
};
 
struct CustomPred {
    template <typename P1, typename P2>
    bool operator()(const P1& p1, const P2& p2) const { return p1.first < p2.first; }
};
 
int main()
{
    FirstMap  first_map  { { 1, "aap" }, { 3, "noot" }, { 5, "mies" }  };
    SecondMap second_map { { 1, 1/3.  }, { 2, 2/3.   }, { 4, 4/3.}  };
 
    BothMap   both_map;
 
    // iterate over maps union
    std::set_union(
            first_map.begin(), first_map.end(),
            second_map.begin(), second_map.end(),
            boost::make_function_output_iterator(CustomOutput(both_map)),
            CustomPred()
            );
 
    for (auto& pair : both_map)
        std::cout << pair.first << ":\t" << pair.second << "\n";
}
 

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E:\mingw64>.\a.exe
1:      aap
2:      0.666667
3:      noot
4:      1.33333
5:      mies