RAII

"Resource Acquisition Is Initialization" or RAII, is a C++ programming technique^[1] which binds the life cycle of a resource (allocated memory, open socket, open file, mutex, database connection - anything that exists in limited supply) to the lifetime of an object with automatic storage duration.

RAII guarantees that the resource is available to any function that may access the object (resource availability is a class invariant). It also guarantees that all resources are released when their controlling objects go out of scope, in reverse order of acquisition. Likewise, if resource acquisition fails (the constructor exits with an exception), all resources acquired by every fully-constructed member and base subobject are released in reverse order of initialization. This leverages the core language features (object lifetime, scope exit, order of initialization and stack unwinding) to eliminate resource leaks and guarantee exception safety. Another name for this technique is "Scope-bound resource management" (SBRM).

RAII can be summarized as follows:

• encapsulate each resource into a class, where
  • the constructor acquires the resource and establishes all class invariants or throws an exception if that cannot be done
  • the destructor releases the resource and never throws exceptions
• always use the resource via an instance of a RAII-class that either
  • has automatic storage duration
  • is a non-static member of a class whose instance has automatic storage duration

Classes with open()/close(), lock()/unlock(), or init()/copyFrom()/destroy() member functions are typical examples of non-RAII classes:

std::mutex m;
void bad() 
{
    m.lock(); // acquire the mutex
    f();      // if f() throws an exception, the mutex is never released
    if(!everything_ok()) return; // early return, the mutex is never released
    m.unlock(); // only if execution reaches this statement, the mutex is released
}
void good()
{
    std::lock_guard<std::mutex> lk(m); // RAII class: mutex acquisition is initialization
    f(); // if f() throws an exception, the mutex is released
    if(!everything_ok()) return; // early return, the mutex is released
} // if the function returns normally, the mutex is released

The standard library

The C++ library classes that manage their own resources follow RAII: std::string, std::vector, std::thread, and many others allocate their resources in constructors (which throw exceptions on errors), release them in their destructors (which never throw), and don't require explicit cleanup.

In addition, the standard library offers several RAII wrappers to manage user-provided resources:

• std::unique_ptr and std::shared_ptr to manage dynamically-allocated memory or, with a user-provided deleter, any resource represented by a plain pointer
• std::lock_guard, std::unique_lock, std::shared_lock to manage mutexes

References

1. RAII in Stroustrup's C++ FAQ