Question

I have nested containers std::map<int, std::map<T, U> > and want to populate them properly, either inserting a new sub map or appending to the sub map if the integer key exists. So I came up with something like the following example:

int n = ...;
int m = ...;
obj get_some_random_obj(int i, int j);        // returns some object 

std::map<int, std::map<int, obj> > container; // prepopulated container

// insert some new obj's. Create a new sub map if key i is not found in container, 
// append to existing sub map otherwise
for(int i = 0; i < n; ++i) {
    for(int j = 0; j < m; ++j) {
        std::map<int, std::map<int, obj> >::iterator found = container.find(i);
        obj newobj = get_some_random_obj(i,j);
        std::pair<int, obj> newpair(j, newobj);
        if(found != container.end()) {
            found->second.insert(newpair);
        } else {
            std::map<int, obj> newmap;
            newmap.insert(newpair);
            container.insert(std::make_pair(i, newmap));
        }
    }
}

Two questions:

• Is there a more elegant (more efficient?) way to write this?
• How can one make the above code more abstract, so that it becomes possible to populate containers with type std::map<int, std::map<U,T> with U and T arbitrary types? I have tried to come up with a template function, but couldn’t get it to work at all.

Thank you for your help!

Answer 1

container[i][j] = get_some_random_obj(i,j);

map’s operator[] inserts if the element isn’t present.