Re: Initializing a vector in a map

I use the following structure to store filenames for one or more
"sets" grouped together by a number:

map<int, map<string> > fileSets;

As arguments to the constructor I send a vector<vector<string> > where
each vector<string> contains the filenames of one set. The function
getNumber() calculates a number from the filename. The constructor
then looks like this:

Files::Files(vector<vector<string> > files&)
{
 // Add files to the sets
 // Loop throught all sets
 for (size_t i = 0; i < files.size(); ++i)
 {
   vector<string>::iterator iter = files[i].begin();
   vector<string>::iterator end = files[i].end();
   // Add a file to fileSets[number][set]
   for(; iter != end; ++iter)
   {
     fileSets[getNumber(*iter)][i] = *iter;
   }
 }
}

The reason that I need the first (outer) map is that the number
returned from getNumber() is very large but I don't have that many
files and there can be gaps in the numbers and so on. However I really
don't need the second (inner) map, a vector would work just as well
since I know the number of sets when the constructor is called
(files.size()).

So my question is: Is it possible to somehow "initialize" the vector
in the map to have a certain number of elements?

That is if I do
fileSets[######] and that element does not already exist then a new
vector<string> will be created with a specified number of elements
allocated, so that I can then index into the vector and assigns to the
specified element?

Something like this:

Files::Files(vector<vector<string> > files&) :
fileSets(map<vector<string>(files.size()) >)

or something like that?