ISO compilation issue on C++ template declaration

It's been a long while since I've written C++ but I have no clue why this is not compiling. It's VERY straightforward.

using namespace std;

template<class IDX, class TYPE>
class ActiveMap
{
    public:
        ActiveMap(){}

    private:
        map<IDX, TYPE> idMap;
};

I am getting a

ISO C++ forbids declaration of 'ActiveMap' with no type

even when I change the map declaration to a simple map<int, int> I get the same thing. I know there are many questions regarding this but this is too much of a simple example.

I am attempting to compile with g++ -c ActiveMap.cpp and so will not compile against my main.cpp. BUT....

main.cpp

#include "ActiveMap.h"

int (int argc, char* argv[]) {
    ActiveMap<int, float> map;

    map[2] = 3.2;

    return 0;
}

That's it.

How can I compile ActiveMap.cpp with g++?