STL map to generic vector C++ - c++

Question

I want to implement a map, which maps a string to a generic vector.
I want to do this:
std::map<std::string, std::vector<class T> > myMap;
Assuming the proposed myMap had the following inserted into it, it could be used as such:
vector<int> intVec = myMap["ListOfInts"]; // Works because "ListOfInts" maps to a vector<int>
vector<string> stringVec = myMap["ListOfStrings"]; // Works because "ListOfInts" maps to a vector<string>
When I declare the map with the above syntax the compiler has a heart attack.
Can anybody make any suggestions? Or a better associate array option in C++ (suggest non-boost before boost).

Answer 1

Since you know the type you want when you are writing your code, I propose this approach (untested):
// base class for any kind of map
class BaseMap {
public:
virtual ~BaseMap() {}
};
// actual map of vector<T>
template<typename T>
class MapT : public BaseMap, public std::map<std::string, std::vector<T>> {};
class MultiMap {
public:
template<typename T>
std::vector<T>& get(const std::string& key) {
std::unique_ptr<BaseMap>& ptr = maps_[std::type_index(typeid(T))];
if (!ptr) ptr.reset(new MapT<T>());
return ptr->second[key];
}
private:
std::map<std::type_index, std::unique_ptr<BaseMap>> maps_;
}
int main() {
MultiMap map;
std::vector<int>& intVec = map.get<int>("ListOfInts");
std::vector<std::string>& stringVec = map.get<std::string>("ListOfStrings");
}

Answer 2

Maybe this could work for you:
template <typename T>
class MyMap {
std::map<std::string, std::vector<typename T> > map;
public:
/*...*/
};

Answer 3

As mattnewport said boost::variant is one option.
Or to support any types, use boost::any by explicit using any_cast.
Considering boost might be heavy weight, maybe you can reinvent the wheel and simplify it, so that is non boost any more? lol

Answer 4

With C++11, you can use using, it does exactly what you want:
#include <vector>
#include <map>
#include <string>
template<typename T> using mymap = std::map<std::string, std::vector<T>>;
int main()
{
mymap<int> intmap;
mymap<std::string> stringmap;
std::vector<int> intvec = intmap["test"];
std::vector<std::string> stringvec = stringmap["test"];
return 0;
}
Live Demo

Answer 5

Template arguments must be decided compile time. If you want each key of your map to correspond to a vector of a different type, it won't really work. You can get around it with a polymorphic container that returns a void* that you cast to the correct type, but I would suggest trying to find another way to do whatever you want to do with your map first.