Writing your own (naïve) shared_ptr
A shared_ptr<T> type is a difficult beast to implement and a harder beast yet to optimize. The invitation to use the standard version of existing smart pointers is stronger in this case than it was for unique_ptr<T>: this type is hard to get right, and the standard version benefits from years of experience and testing. Only use the naïve version in this section for experimentation (it works and does the job for simple cases, but writing an industrial-strength implementation is major-league work).
The main difficulty when writing a shared_ptr is that it’s a type with two responsibilities: it co-owns both the pointee and the usage counter, requiring some measure of care, especially with respect to exception safety. The single responsibility principle of classical object-oriented programming is a sound principle: a type with a single responsibility is exceedingly simpler to get right than a type with two or more responsibilities...
