Aug 14, 1998
The 'resource acquisition is initialization' Idiom
Many objects of various kinds share a similar characterization: they have to be acquired by means of initialization prior to their usage, then they can be used, and finally - they have to be released explicitly. Objects such as File, CommunicationSocket, DatabaseCursor, DeviceContext (on GUI systems), OperatingSystem and many others have to be opened, attached, initialized, constructed or booted respectively, before one can use them. When their job is done, they have to be flushed, detached, closed, released or logged out respectively. A common design mistake is to have the user request explicitly for the initialization and release operations to take place. A much better choice is to move all initialization action into the object's constructor, and all release actions - into the destructor. This technique is called 'resource acquisition is initialization.' The advantages are obvious: the user of such an object can start using the object right after it has been created without bothering whether the object is valid or whether further arbitrary initialization actions have to be done. Furthermore, since the object's destructor also releases all the resources it requires and resets it, the user is free from that bother too. Please note that this technique usually requires an appropriate exception handling code to cope with exceptions thrown during object's construction.