Thread Safety of Unique Pointers

Using std::unique_ptr in multithreaded applications is generally safe, but with some important caveats. Let's break this down:

Thread Safety of the Pointer Itself

The std::unique_ptr itself is not thread-safe. This means that if you have multiple threads trying to modify the same unique_ptr (e.g., resetting it, moving it, or destroying it), you need to provide your own synchronization mechanisms, such as mutexes.

1#include <memory>
2#include <mutex>
3
4std::unique_ptr<Character> GlobalCharacter;
5std::mutex CharacterMutex;
6
7void UpdateCharacter() {
8  std::lock_guard<std::mutex> Lock{CharacterMutex};
9  GlobalCharacter = std::make_unique<Character>("Aragorn");
10}

Thread Safety of the Pointed-to Object

The thread safety of the object that the unique_ptr points to depends on the object itself, not on the unique_ptr. If multiple threads are accessing the pointed-to object concurrently, you need to ensure that the object itself is thread-safe or provide appropriate synchronization.

1#include <memory>
2#include <mutex>
3
4class ThreadSafeCharacter {
5  mutable std::mutex Mtx;
6  std::string Name;
7
8public:
9  void SetName(const std::string& NewName) {
10    std::lock_guard<std::mutex> Lock{Mtx};
11    Name = NewName;
12  }
13};
14
15std::unique_ptr<ThreadSafeCharacter> GlobalCharacter;

Unique Ownership and Threading

The unique ownership model of std::unique_ptr can actually be beneficial in multithreaded scenarios. Since only one unique_ptr can own an object at a time, it can help prevent issues like multiple threads trying to delete the same object.

However, be cautious when transferring ownership between threads:

1#include <memory>
2#include <thread>
3
4void ProcessCharacter(std::unique_ptr<Character> Char) {
5  // Process the character
6}
7
8int main() {
9  auto Frodo{std::make_unique<Character>("Frodo")};
10  std::thread T{ProcessCharacter, std::move(Frodo)}; 
11  T.join();
12  // After this, Frodo is nullptr
13}

In conclusion, while std::unique_ptr itself isn't inherently thread-safe, its unique ownership model can be valuable in multithreaded applications. Just remember to handle synchronization for the pointer itself and the pointed-to object when necessary.