Using unique_ptr as an example, the presenter talks briefly about the empty object problem, ie when classes have non-zero size despite being empty, and then presents two ways to deal with that:

• Empty Base Class Optimization (EBCO): exploits the fact that if a base class is empty, it won't increase the size of the derived class. The presenter uses this trick through the cool compressed_pair to have a unique_ptr implementation where the deleter doesn't take an extra byte;
• [[no_unique_address]] which is a C++20 feature.