C++ Primer

The level of copying determines object relationships

§12.1.1 shared_ptr and StrBlob; §12.1.6 weak_ptr; §13.1.4 Rule of Three/Five; §13.2 Copy Control and Resource Management

After reading about dynamic memory and then looking at copy control, one discovers that the two topics cannot be learned separately. Callers who copy a vector usually expect an independent value; copying a shared_ptr only adds another owner of the same object. The sharing behavior of StrBlob is well suited to making this difference visible. Default memberwise copying is not always wrong; the key is whether the member types already express the desired responsibilities. If storing a container directly meets the need, adding a handwritten destructor may instead interrupt generation of the default move operations. A modern synthesis therefore first writes down independent, shared, or exclusive semantics, then chooses the member representation, and only then discusses optimization. weak_ptr is an observing relationship that acknowledges the object may disappear; lock is the action that obtains a lifetime guarantee for the duration of use, not a count check that can replace all synchronization.