What is meant by Resource Acquisition is Initialization (RAII)?

What is meant by Resource Acquisition is Initialization (RAII)?

It's a really terrible name for an incredibly powerful concept, and perhaps one of the number 1 things that C++ developers miss when they switch to other languages. There has been a bit of a movement to try to rename this concept as Scope-Bound Resource Management, though it doesn't seem to have caught on just yet.

When we say 'Resource' we don't just mean memory - it could be file handles, network sockets, database handles, GDI objects... In short, things that we have a finite supply of and so we need to be able to control their usage. The 'Scope-bound' aspect means that the lifetime of the object is bound to the scope of a variable, so when the variable goes out of scope then the destructor will release the resource. A very useful property of this is that it makes for greater exception-safety. For instance, compare this:

RawResourceHandle* handle=createNewResource();
handle->performInvalidOperation();  // Oops, throws exception
...
deleteResource(handle); // oh dear, never gets called so the resource leaks

With the RAII one

class ManagedResourceHandle {
public:
   ManagedResourceHandle(RawResourceHandle* rawHandle_) : rawHandle(rawHandle_) {};
   ~ManagedResourceHandle() {delete rawHandle; }
   ... // omitted operator*, etc
private:
   RawResourceHandle* rawHandle;
};

ManagedResourceHandle handle(createNewResource());
handle->performInvalidOperation();

In this latter case, when the exception is thrown and the stack is unwound, the local variables are destroyed which ensures that our resource is cleaned up and doesn't leak.

This is a programming idiom which briefly means that you

• encapsulate a resource into a class (whose constructor usually - but not necessarily** - acquires the resource, and its destructor always releases it)
• use the resource via a local instance of the class*
• the resource is automatically freed when the object gets out of scope

This guarantees that whatever happens while the resource is in use, it will eventually get freed (whether due to normal return, destruction of the containing object, or an exception thrown).

It is a widely used good practice in C++, because apart from being a safe way to deal with resources, it also makes your code much cleaner as you don't need to mix error handling code with the main functionality.

* Update: "local" may mean a local variable, or a nonstatic member variable of a class. In the latter case the member variable is initialized and destroyed with its owner object.

** Update2: as @sbi pointed out, the resource - although often is allocated inside the constructor - may also be allocated outside and passed in as a parameter.

"RAII" stands for "Resource Acquisition is Initialization" and is actually quite a misnomer, since it isn't resource acquisition (and the initialization of an object) it is concerned with, but releasing the resource (by means of destruction of an object).
But RAII is the name we got and it sticks.

At its very heart, the idiom features encapsulating resources (chunks of memory, open files, unlocked mutexes, you-name-it) in local, automatic objects, and having the destructor of that object releasing the resource when the object is destroyed at the end of the scope it belongs to:

{
  raii obj(acquire_resource());
  // ...
} // obj's dtor will call release_resource()

Of course, objects aren't always local, automatic objects. They could be members of a class, too:

class something {
private:
  raii obj_;  // will live and die with instances of the class
  // ... 
};

If such objects manage memory, they are often called "smart pointers".

There are many variations of this. For example, in the first code snippets the question arises what would happen if someone wanted to copy obj. The easiest way out would be to simply disallow copying. std::unique_ptr<>, a smart pointer to be part of the standard library as featured by the next C++ standard, does this.
Another such smart pointer, std::shared_ptr features "shared ownership" of the resource (a dynamically allocated object) it holds. That is, it can freely be copied and all copies refer to the same object. The smart pointer keeps track of how many copies refer to the same object and will delete it when the last one is being destroyed.
A third variant is featured by std::auto_ptr which implements a kind of move-semantics: An object is owned by only one pointer, and attempting to copy an object will result (through syntax hackery) in transferring ownership of the object to the target of the copy operation.

An object's lifetime is determined by its scope. However, sometimes we need, or it is useful, to create an object that lives independently of the scope where it was created. In C++, the operator new is used to create such an object. And to destroy the object, the operator delete can be used. Objects created by the operator new are dynamically allocated, i.e. allocated in dynamic memory (also called heap or free store). So, an object that was created by new will continue to exist until it's explicitly destroyed using delete.

Some mistakes that can occur when using new and delete are:

• Leaked object (or memory): using new to allocate an object and forget to delete the object.
• Premature delete (or dangling reference): holding another pointer to an object, delete the object, and then use the other pointer.
• Double delete: trying to delete an object twice.

Generally, scoped variables are preferred. However, RAII can be used as an alternative to new and delete to make an object live independently of its scope. Such a technique consists of taking the pointer to the object that was allocated on the heap and placing it in a handle/manager object. The latter has a destructor that will take care of destroying the object. This will guarantee that the object is available to any function that wants access to it, and that the object is destroyed when the lifetime of the handle object ends, without the need for explicit cleanup.

Examples from the C++ standard library that use RAII are std::string and std::vector.

Consider this piece of code:

void fn(const std::string& str)
{
    std::vector<char> vec;
    for (auto c : str)
        vec.push_back(c);
    // do something
}

when you create a vector and you push elements to it, you don't care about allocating and deallocating such elements. The vector uses new to allocate space for its elements on the heap, and delete to free that space. You as a user of vector you don't care about the implementation details and will trust vector not to leak. In this case, the vector is the handle object of its elements.

Other examples from the standard library that use RAII are std::shared_ptr, std::unique_ptr, and std::lock_guard.

Another name for this technique is SBRM, short for Scope-Bound Resource Management.

The book C++ Programming with Design Patterns Revealed describes RAII as:

1. Acquiring all resources
2. Using resources
3. Releasing resources

Where

• Resources are implemented as classes, and all pointers have class wrappers around them (making them smart pointers).

• Resources are acquired by invoking their constructors and released implicitly (in reverse order of acquiring) by invoking their destructors.