What are some best practices for designing thread-safe classes in C++?

2 Use the copy-and-swap idiom

The copy-and-swap idiom is a common technique to implement the assignment operator for a class. It involves creating a copy of the right-hand side object, swapping its contents with the left-hand side object, and letting the copy go out of scope. This way, the assignment is exception-safe, as no resources are leaked or left in an inconsistent state. It is also thread-safe, as it does not modify the right-hand side object, and it only requires a single atomic operation to swap the pointers to the data. For example, you can implement the copy-and-swap idiom for a class

Foo        

as follows:

class Foo {
  // some data members
public:
  // copy constructor, move constructor, destructor, etc.
  Foo& operator=(Foo rhs) { // pass by value to create a copy
    swap(*this, rhs); // swap the contents with the copy
    return *this; // return the updated object
  }
  friend void swap(Foo& a, Foo& b) { // swap function
    using std::swap;
    // swap the data members of a and b
  }
};        

3 Use the immutable pattern

The immutable pattern is a design principle that states that an object's state cannot be changed after it is created. This means that the object does not have any setters, modifiers, or mutators, and that all its data members are either const or final. The immutable pattern simplifies thread safety, as it eliminates the need for synchronization and locking, since no thread can alter the object's state. It also enables safe sharing and copying of the object, as there is no risk of aliasing or stale data. For example, you can create an immutable class

Bar        

as follows:

class Bar {
  // some const or final data members
public:
  // constructor, getters, etc.
  Bar(const Bar& other) = default; // default copy constructor
  Bar& operator=(const Bar& other) = delete; // delete assignment operator
  // other methods that do not modify the object's state
};        

4 Use the thread-local storage pattern

The thread-local storage (TLS) pattern is a mechanism that allows each thread to have its own copy of a variable or an object. This way, the threads do not need to access or modify a shared resource, avoiding contention and synchronization overhead. TLS is useful for storing thread-specific data, such as configuration, context, or state. For example, you can declare a thread-local variable or object using the

thread_local        

keyword:

thread_local int x; // each thread has its own x
thread_local Foo foo; // each thread has its own foo        

5 Use the singleton pattern with caution

The singleton pattern is a design principle that ensures that only one instance of a class exists in the program, and it is often used to implement global or shared resources, such as configuration, logging, or database connections. However, this pattern can introduce thread safety issues like initialization order, lazy initialization, and double-checked locking. Therefore, you should use the singleton pattern with caution and follow some guidelines, such as preferring static initialization over dynamic initialization which is thread-safe by the C++ standard. Additionally, avoid lazy initialization as it can cause race conditions and performance degradation. You can also use the

std::call_once        

function or the

std::atomic        

template to implement thread-safe lazy initialization or double-checked locking. Furthermore, consider using dependency injection or factory methods instead of singleton accessors which improve testability and modularity.

6 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?