How to add to a collection which is already stored in a collection?

I have the following:

class SpritesheetManager{

        std::unordered_map<std::string,std::unordered_set<std::string>> _loadedFiles;

        void addFileToFileListForSheet(std::string sheetprefix,std::string filename);
}

When adding files I do this:

void SpritesheetManager::addFileToFileListForSheet(std::string sheetprefix,std::string filename){
    bool containsKey = (_loadedFiles.find(sheetprefix)!= _loadedFiles.end());

    std::unordered_set<std::string> values;
    if(!containsKey){
        _loadedFiles[sheetprefix] = values;
    }

    _loadedFiles[sheetprefix].insert(filename);
}

Is there a better way to do this?