Is std::shared_ptr Truly Thread-Safe?

How Much Thread-Safety Does std::shared_ptr Provide?

std::shared_ptr is a smart pointer that provides reference counting and automatic memory management. While it ensures thread-safe reference counting platform-independently, some aspects of its thread-safety require clarification.

Core Thread-Safety Guarantees:

1. Reference Counting: The standard guarantees that reference counting operations within std::shared_ptr are thread-safe. This means that counting, incrementing, and decrementing references are handled atomically.
2. Destruction: When an object's reference count drops to zero, the standard ensures that only one thread will invoke its destructor. This is achieved through internal synchronization mechanisms.

Limitations of Thread-Safety:

1. Stored Object's Thread-Safety: std::shared_ptr does not guarantee any thread-safety for the object it stores. The stored object itself may have its own thread-safety concerns.

Example Demonstration:

Consider the following pseudo-code:

// Thread I
shared_ptr<A> a (new A (1));

// Thread II
shared_ptr<A> b (a);

// Thread III
shared_ptr<A> c (a);

// Thread IV
shared_ptr<A> d (a);

d.reset (new A (10));

It is incorrect to assume that after calling reset() in Thread IV, other threads will see only the new object. Threads II, III, and IV will still point to the original object, and only Thread IV will point to the new one.

Conclusion:

std::shared_ptr provides strong thread-safety guarantees for reference counting and destruction. However, it is important to remember that the stored object's thread-safety is not guaranteed and must be considered separately.

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