Resource acquisition is initialization

Resource acquisition is initialization (RAII) is a programming idiom used in several object-oriented, statically typed programming languages to describe a particular language behavior. In RAII, holding a resource is a class invariant, and is tied to object lifetime. Resource allocation (or acquisition) is done during object creation (specifically initialization), by the constructor, while resource deallocation (release) is done during object destruction (specifically finalization), by the destructor. In other words, resource acquisition must succeed for initialization to succeed. Thus, the resource is guaranteed to be held between when initialization finishes and finalization starts (holding the resources is a class invariant), and to be held only when the object is alive. Thus, if there are no object leaks, there are no resource leaks.

RAII is associated most prominently with C++, where it originated, but also Ada, Vala, and Rust. The technique was developed for exception-safe resource management in C++ during 1984–1989, primarily by Bjarne Stroustrup and Andrew Koenig, and the term itself was coined by Stroustrup.

Other names for this idiom include Constructor Acquires, Destructor Releases (CADRe) and one particular style of use is called Scope-based Resource Management (SBRM). This latter term is for the special case of automatic variables. RAII ties resources to object lifetime, which may not coincide with entry and exit of a scope. (Notably variables allocated on the free store have lifetimes unrelated to any given scope.) However, using RAII for automatic variables (SBRM) is the most common use case.

The following C++23 example demonstrates usage of RAII for file access and mutex locking:

This code is exception-safe because C++ guarantees that all objects with automatic storage duration (local variables) are destroyed at the end of the enclosing scope in the reverse order of their construction. The destructors of both the lock and file objects are therefore guaranteed to be called when returning from the function, whether an exception has been thrown or not.

Local variables allow easy management of multiple resources within a single function: they are destroyed in the reverse order of their construction, and an object is destroyed only if fully constructed—that is, if no exception propagates from its constructor.

Using RAII greatly simplifies resource management, reduces overall code size and helps ensure program correctness. RAII is therefore recommended by industry-standard guidelines, and most of the C++ standard library follows the idiom.

The advantages of RAII as a resource management technique are that it provides encapsulation, exception safety (for stack resources), and locality (it allows acquisition and release logic to be written next to each other).