http://en.cppreference.com/w/cpp/container/map/operator_at

Returns a reference to the value that is mapped to a key equivalent to key, performing an insertion if such key does not already exist.
If an insertion is performed, the mapped value is value-initialized (default-constructed for class types, zero-initialized otherwise) and a reference to it is returned.

Default value for pointers in C++11 is the value "nullptr", which I'm checking in the following lines.