This is going to sound so basic as to make one think I made zero effort to find the answer myself, but I swear I did search for about 20 minutes and found no answer.
The value will be uninitialised so yes you do need to explicitly initialise it to nullptr.
nullptr
Using smart pointers (std::unique_ptr, std::shared_ptr, boost::shared_ptr, etc.) would mean that you don't need to do this explicitly.
std::unique_ptr
std::shared_ptr
boost::shared_ptr