Question

I am following http://www.cplusplus.com/reference/map/map/map/.

I am trying to build an invert index structure, which is a map that has a 64 bits integer as a key and each key will hold a pointer to a sub-map. A sub-map will contain int int pair. So I got myself writing some samples:

map<unsigned long long int, map<int, int>*> invertID;

int main(int argc, char *argv[]){

    map<int,int>* m = new map<int,int>();

    m[10] = 1;

    invertID[1] = m;

    return 0;

}

But here is the trouble, normally, for a non-pointer type map like

std::map<char,int> first;

as described in http://www.cplusplus.com/reference/map/map/map/, I see that we can do

first['a']= 10;

But if we have a pointer type for map, how can we do that? My code above will generate an error complaining

error C2679: binary '=' : no operator found which takes a right-hand operand of type 'int' (or there is no acceptable conversion)

Answer 1

You can de-reference the pointer:

 (*m)[10] = 1;

or

m->operator[](10) = 1;

On the other hand, do you really need all these pointers? Would your program suffer greatly from using this form?

map<unsigned long long int, map<int, int>> invertID;
int main()
{
    map<int,int> m;
    invertID[1][10] = 1;
}