What are some common pitfalls or best practices when using lock guards and unique locks in C++?

1 What are lock guards and unique locks?

Lock guards and unique locks are RAII wrappers for mutexes, which means they acquire a mutex when they are constructed and release it when they are destroyed. This way, you don't have to manually lock and unlock a mutex, and you avoid the risk of forgetting to do so or leaving a mutex locked in case of an exception. Lock guards and unique locks differ in how they handle the ownership and scope of the mutex. A lock guard always owns the mutex until it goes out of scope, while a unique lock can transfer, release, or reacquire the ownership of the mutex.

2 When to use lock guards and unique locks?

The general rule is to use lock guards when you need a simple and deterministic locking and unlocking of a mutex, and to use unique locks when you need more flexibility and control over the ownership and scope of the mutex. For example, you can use a lock guard when you want to protect a single critical section of code, such as modifying a shared variable or container. You can use a unique lock when you want to conditionally lock or unlock a mutex, such as waiting for a condition variable or using a try_lock method.

4 How to avoid performance issues with lock guards and unique locks?

Locking and unlocking mutexes can be expensive operations that affect the performance of your multithreaded program. To avoid unnecessary overhead, you should minimize the scope and duration of your lock guards and unique locks, and only lock the mutexes that are needed to protect the shared resources. You should also avoid locking mutexes in tight loops, or in functions that are called frequently or have high latency. Instead, you should use finer-grained locking, such as using separate mutexes for different parts of a shared data structure, or using lock-free algorithms or data structures that do not require mutexes at all.

5 How to avoid undefined behavior with lock guards and unique locks?

Undefined behavior is a term that describes the unpredictable and potentially dangerous consequences of violating the rules of the C++ language or the library. One common source of undefined behavior with lock guards and unique locks is accessing or modifying a shared resource without locking the corresponding mutex, which can lead to data races, memory corruption, or exceptions. To avoid this, you should always follow the synchronization rules of the C++ standard, which specify when and how you can access or modify a shared resource in different threads. You should also use the std::atomic class or the std::atomic_* functions to perform atomic operations on shared variables, which do not require locking a mutex.

6 How to use best practices with lock guards and unique locks?

To use lock guards and unique locks more effectively and safely in C++ code, there are some best practices to follow. For instance, it is better to use std::scoped_lock instead of std::lock_guard when locking multiple mutexes, as it is more concise and less error-prone. Additionally, when creating a unique lock without locking the mutex, it is advisable to use std::unique_lock::defer_lock, as it is more explicit and readable. In the case of creating a lock guard or a unique lock from an already locked mutex, using std::adopt_lock is recommended, as it avoids locking the mutex again. Furthermore, when creating a unique lock with a try_lock method, using std::try_to_lock should be considered, as it indicates that the locking may fail. Finally, if passing ownership of a locked mutex to another function, using std::lock_guard or std::unique_lock as function parameters is beneficial, as it avoids releasing the mutex prematurely.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?