Is std::shared_ptr Thread-Safe When Modifying Shared Objects?

Understanding std::shared_ptr Thread Safety

The documentation refers to the thread safety of the control block, which manages the shared ownership of the object. Multiple threads can concurrently access different shared_ptr objects, as they share ownership of the underlying resource without conflicts. However, this does not guarantee the safety of modifying the shared object itself.

Shared_ptr and Object Modifications

In your code example, thread 1 creates a private copy of global using shared_ptr, while thread 2 modifies global itself. The following explains the behavior:

• Thread 1's private is initially set to the original value of global.
• When thread 2 modifies global, it does not affect private's pointer, as private points to a different control block.
• However, private will eventually be destructed, decrementing the reference count in the control block that was modified by thread 2.
• While private will remain a valid shared_ptr, its lifetime is independent of the modified global.

Thread Safety Considerations

To safely modify an object shared by multiple threads, you must use a synchronization mechanism such as std::mutex. The following example demonstrates a thread-safe configuration update using a mutex:

// Shared pointer to the configuration object
std::shared_ptr<Configuration> global_config = make_shared<Configuration>();

// Mutex to protect the configuration object
std::mutex config_mutex;

void thread_fcn() {
    // Lock the mutex before accessing the configuration
    std::lock_guard<std::mutex> lock(config_mutex);

    // Update the configuration from global_config
    // ...

    // Unlock the mutex after finishing the update
}

By acquiring the mutex before modifying global_config, you prevent any interference from other threads. This ensures that the configuration is always updated in a consistent and thread-safe manner.

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