Storing map's first element to a vector in the most generic form. Best solution

Question

My goal is to store all the keys of a map (first item) to a vector and I'm doing the following.

    template < class vecDet>
class storeInto
{
 public:
  storeInto(vecDet& source) : VectorInfo(source) { }
  ~storeInto();
  template <class pairdet>
  void operator()(pairdet& pairinfo) 
  {
   VectorInfo.push_back(pairinfo.first);
  }

 private:
  vecDet& VectorInfo;
};
template<class mapDet, class vecDet>
void storeMapToVector(const mapDet& mapContA,  vecDet& vecContA)
{
 for_each(mapContA.begin(), mapContA.end() , storeInto<vecDet>(vecContA));
}

Finally, from my main program, I'm calling the storeMapToVector() as follows

storeMapToVector<mapinfo,vector<char> >(mapContents, vecContents);

where mapinfo is declared as follows

typedef map<char,int> mapinfo;

Is there a better way to do this? Is there anything wrong with this?

Answer 1

The code seems fine. Some nitpicks though:

~storeInto();

What do you need the destructor for? The functor could just live without it.

storeMapToVector<mapinfo,vector<char> >(mapContents, vecContents);

The template argument list seems unnecessary here, it could just be

storeMapToVector(mapContents, vecContents);

The template parameters would be inferred in this context.

Answer 2

Your code looks like it would work at first glance. However, there's a much simpler way to do this:

I haven't evaluated your code, but there is certainly a much easier way to do what you want built into most STL implementations:

vecContents.resize(mapContents.size());
std::transform(mapContents.begin(), mapContents.end(), vecContents.begin(),
    select1st<pair<const char, int> >());

Alternatively:

vecContents.resize(mapContents.size());
std::transform(mapContents.begin(), mapContents.end(), vecContents.begin(),
    select1st<mapinfo::value_type>());

There is 1 wrinkle though - select1st is a SGI extension. It's in almost every STL implementation but where varies. For GNU, you have to do:

#include <ext/functional>

using __gnu_cxx::select1st; // or just using namespace __gnu_cxx;

See this answer for using it in Visual Studio.