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c++raii

Does RAII support resource ownership transfer?

I mainly used to think about RAII as being about using object lifetime to avoid leaking resources and that served me well enough in practice. But I recently had some discussions about what exactly constitutes an RAII pattern and what does not, which made me search for more definitions and commentaries online, which ended up adding more confusion than clarity.

The standard definition of an RAII class seems to require two properties:

  1. The constructor of an RAII class should acquire the resources or throw an exception if it fails in that process.
  2. The destructor of an RAII class should release the resources.

But then I've also seen mentioned in some RAII definitions that resource ownership can be "safely transferred" between instances of such RAII classes. So resource ownership transfer seems to be accepted as part of the RAII pattern.

But then it seems that resource ownership transfer also leads to breaking those very 2 properties that seem to define RAII.

Let's say that I have two instances of an RAII class - Instance_Source and Instance_Destination - and that I transfer ownership of the underlying resource(s) from Instance_Source to Instance_Destination. Then we have:

  • Conflict with Property 2:
    • The destructor of Instance_Source will not release any resource, so we now broke the requirement that destructors should release resources.
  • Conflicts with Property 1:
    • One advantage of doing the resource acquiring in the initialization is that I can use instances knowing that they contain valid resources (otherwise they would not have been constructed). But with the ownership transfer, I am now left with instances that no longer contain valid resources, which eliminates the principal advantage that acquiring resources in the constructor was supposed to bring.
    • There are also scenarios in which I will not want Instance_Destination to acquire any resource during its construction because I want it to only get ownership of what Instance_Source acquired, under special conditions. To support such scenarios, I would have to break the requirement that constructors acquire resources, to allow initializing Instance_Destination without acquiring any resource.

So in scenarios in which I need to allow resource ownership transfer, I find that I have to "play loose" with the 2 RAII requirements about acquiring resources in the constructors and releasing them in destructors. And this works well enough in practice, but does it still constitute an RAII pattern in theory?

This is what led me to my question: Does RAII support resource ownership transfer?

If the answer is yes, then it looks like most RAII definitions should be reworked to not rely on what the constructors and the destructors are supposed to do with resources.

If the answer is no, then this should be highlighted as an important limitation of RAII.

Solution

  • Does RAII support resource ownership transfer?

    It can, yes.

    But then it seems that resource ownership transfer also leads to breaking those very 2 properties that seem to define RAII.

    Depends a bit on details of how one defines RAII.

    The solution is to extend the definition of RAII shown in the question to allow the representation of empty state. If where there is a representation for empty state, then moving ownership of the resource is possible by leaving the source RAII object in such empty state.

    The definitions given in the question for construction and destruction are trivial to adjust for this:

    1. Construction either acquires a resource or initialises to empty state. Technically that isn't required, but if empty state is allowed, it's convenient to allow default construction.
    2. Destructor releases resource if and only if it owns any.
    3. Additional definition for moving as described in earlier paragraph.

    Most RAII classes in standard library have representation for empty state, and those support transferring their resource. Typical examples of such RAII classes and their empty state:

    • Any dynamic container - a container that contains no elements (and has empty capacity)
    • Any smart pointer - the null value
    • std::fstream - a stream that isn't associated with a file
    • std::thread - a wrapper that isn't associated with a thread

    The standard library does also have RAII classes that don't have representation for empty state, and thus cannot support transfer of the resource. An example of such class is std::lock_guard.

    I wish someone could also provide a historical perspective

    Oldest source for the definition that I have is Stroustrup's book "C++ programming language 3rd ed.". According to wikipedia's estimation RAII was developed around 1984–89, so it would have been a 8-13 year old idea by the time this book was published. Here are most relevant bits that are hopefully not too much to violate copy right:

    14.4.1 Using Constructors and Destructors

    The technique for managing resources using local objects is usually referred to as "resource acquisition is initialization." This is a general technique that relies on the properties of constructors and destructors and their interaction with exception handling.

    ...

    A constructor tries to ensure that its object is completely and correctly constructed. When that cannot be achieved, a well-written constructor restores - as far as possible - the state of the system to what it was before creation.

    14.4.2 Auto_ptr

    ... auto_ptr, which supports the "resource acquisition is initialization" technique.

    Given that std::auto_ptr doesn't necessarily own a resource, and consequently its destructor doesn't in that case release a resource, and it can transfer resource to another instance, and the author who coined RAII considers that std::auto_ptr "supports RAII", I feel confident to say that conflicting the properties described in the question does not disqualify from RAII.

    Note that std::auto_ptr was obsoleted by introduction of move semantics in C++11 and has since been removed from the language.

    E.3.5.3 Delaying resource acquisition

    ... resources should be acquired in constructors whenever delayed resource acquisition isn't mandated by the semantics of a class.

    I found no explicit description of how RAII relates to ability to transfer ownership of the resource. I suspect that it may be discussed more in later editions written for a language that has move semantics.