In the previous chapter, we discussed how std::shared_ptr<T> implements reference-counting memory management, so that an object's lifetime can be cooperatively controlled by stakeholders who might be otherwise unaware of each other--for example, the stakeholders might live in different threads. In C++ before C++11, this would have posed a stumbling block right away: if one stakeholder decrements the reference count while, simultaneously, a stakeholder in another thread is in the process of decrementing the reference count, then don't we have a data race and therefore undefined behavior?
In C++ before C++11, the answer was generally "yes." (In fact, C++ before C++11 didn't have a standard concept of "threads," so another reasonable answer might have been that the question itself was irrelevant.) In C++ as of 2011, though, we...
