How Thread-Safe is std::shared_ptr's Object Management?

To What Extent Does std::shared_ptr Guarantee Thread-Safety?

Background:

The std::shared_ptr class in C provides a means of managing shared ownership of objects. Thread-safety is a crucial consideration when working with shared resources, and it is important to understand how std::shared_ptr handles thread-safety.

Question 1:

• Does the standard guarantee that reference counting in std::shared_ptr is handled thread-safe and platform-independent?

Answer:

Yes, according to the standard, the reference counting mechanism within std::shared_ptr is implemented in a thread-safe and platform-independent manner.

Question 2:

• Does the standard ensure that only one thread (holding the last reference) will call delete on the shared object?

Answer:

Yes, the standard ensures that the deletion of the shared object will be performed by only one thread, which is the thread that holds the last reference to the object.

Question 3:

• Does std::shared_ptr guarantee any thread safety for the object stored within it?

Answer:

No, std::shared_ptr does not guarantee thread safety for the object it manages. The responsibility of ensuring the thread-safety of the contained object lies with the developer.

Example:

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);

// Thread IV then calls reset to replace the object
d.reset(new A(10));

In this example, only thread IV's d will point to the new A(10) object, while a, b, and c will continue to point to the original A(1) object. This demonstrates that std::shared_ptr does not enforce thread safety for the managed object.

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