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The RAII (Resource Acquisition Is Initialization) idiom

12/04/07

Permalink 09:42:06 pm, by lano1106, 566 words, 1552 views   English (CA)
Categories: C++

The RAII (Resource Acquisition Is Initialization) idiom

The name has been coined by Bjarne Stroustrup and is using the property that an object constructor and destructor will always be called at the beginning of the object existence and when it will goes out of scope. This is guaranteed by the language itself and the object will be destructed no matter how it goes out of scope. It could be by exiting a function or when the stack is unrolled by an exception been propagated out of the stack.

The RAII consists of encapsulating a resource (be it a file handle, a lock or a pointer on an object allocated on the heap) in a class. The resource acquisition will be performed in the constructor and then the C++ rules governing the lifetime of an object will take care of releasing the resource by calling the class destructor. It is to support that technique that the class template auto_ptr has been included in the standard library.

The technique has 2 purposes:

1- Simplifying code

Traditionally, it has been very error-prone to perform resource management from a function having many exit points. Suppose the following function:

void foo()
{
  // Acquire resource A
  // Do some processing
  if( error )
  {
    // release resource A
    return;
  }
  // Acquire resource B
  if( !B )
  {
    // release resource A
    return;
  }
  // Do some processing
  // Release resource A
  // Do some more processing
  // And finally, release resource B
  return;
}

Now imagine that this small game of resources tracking spans over more than a hundred lines of code and that you have dozen of resources. This is a waste of energy and chances are that there will be a resource leak in this type of code. If it is not today, it can be during the next maintenance of the function in few months or years where you will need to add another exit point in the middle of that type of function. The best way to handle this is with RAII. Once you acquire the resources, you can forget about them as they will be automatically released when the code will exit the function. As an added bonus, it will simplify the code as all the statements for releasing the resources are not needed anymore.

2- Make it easier to achieve exception safety in functions and particularly in constructors.

When an exception is thrown from a constructor of a class acquiring resources, the hard and error prone way to make sure that no resource are leaking is to place the constructor code in a try block and to manually release the acquired resource in a catch all block before rethrowing the intercepted exception. The C++ language specify that in the event of an exception during the construction, the compiler will generete code to destroy constructed base classes and data members that are objects (raw pointers on dynamically allocated memory does not count) before letting the exception escape from the constructor. Hence again, if you used RAII to encapsulate resource acquired during construction in objects, you will not need to worry about releasing them as the compiler will handle this for you.

One detail worth mentioning is the RAII is not an applicable idiom in some other OO programming languages. For instance, in Java because of its garbage collector, there is no guarantee from the language when objects will be destructed. It will just happen eventually after objects goes out of scope once the garbage collector recuperates the unused objects.

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