Is std::shared_ptr Truly Thread-Safe for Object Access?

Shared Pointers and Thread Safety Reconsidered

The thread safety of std::shared_ptr has been a subject of much discussion among C programmers. Let's delve into the finer details to clarify its true nature.

Caution: What MS Could Mean

The statement from MSDN you mentioned may be misleading. Yes, multiple threads can read and write different shared_ptr objects without issue. However, this only applies to the control block that manages the reference count of the object being shared. It does not mean that modifying the contents of the shared object is safe.

The Control Block's Privilege

The control block itself, which coordinates thread-safe reference counting, is indeed thread-safe. This allows you to create and destroy shared_ptr instances concurrently without worrying about data corruption. However, the object they point to is a different story.

Accessing the Shared Object: Proceed with Caution

In the example you provided:

shared_ptr<myClass> global = make_shared<myClass>();

This is perfectly fine for thread-safe reference counting management. However, the following lines:

shared_ptr<myClass> private = global;
global = make_shared<myClass>();

are hazardous if multiple threads access these shared_ptr instances concurrently. Modifying global can potentially destabilize private. The value it references may become invalid or corrupt.

The Need for Synchronization

To ensure thread-safe access to the actual object pointed to by shared_ptr, you need a separate synchronization mechanism. A common approach is to use a mutex or lock to guard any operations that modify the shared object. This way, only one thread can modify it at a time, preventing data corruption.

Conclusion

Remember, std::shared_ptr is primarily for coordinating reference counting among multiple copies. It does not magically make the underlying object thread-safe. Therefore, when accessing and modifying shared objects, always employ proper thread synchronization techniques to avoid potential issues.

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