Thursday, January 07, 2010

Delphi 2010 RTTI Contexts: how they work, and a usage note

Delphi 2010 includes extended support for RTTI, also known as run-time type info or reflection. Many design approaches that have previously only been possible with managed languages like C# and Java because of the code annotation and introspection they required should now be possible in the Delphi world.

Something somewhat interesting about how the RTTI works is its approach to object pools. Delphi isn't a garbage collected language, so users need to be careful to free objects when they're no longer needed, either explicitly, or by designing or using some kind of ownership scheme, such as that used by TComponent, where the Owner takes care of destruction.

Type information usage scenarios don't mesh particularly well with a TComponent-style of ownership. Typically, when working with RTTI, you want to do some kind of search for interesting objects, do something with them, and then go on your way. That means that many objects may get allocated for inspection, but not actually be used. Managing those objects' lifetimes independently would be tedious, so a different approach is used: there is a single global RTTI object pool. While there is at least one RTTI context active in the application, this object pool keeps all its objects alive. When the last context goes out of scope, the objects get freed up.

The pool management works by using a Delphi record that contains an interface reference. The first time any given RTTI context is used, it fills in this interface reference. It can't fill it in any later than first use, because Delphi records don't support default constructors, which besides have their own problems. For example, how do you handle exceptions in default constructors, in all the places they can occur? Allocating arrays, thread-local variables, global variables, global variables in packages, temporary objects created in expressions, etc. It can get ugly, and in C++ it sometimes does.

So, this first use allocates an interface, called a pool token. It acts as a reference-counted handle to the global object pool. For so long as this interface is alive, the the global object pool should stay alive. Even if the RTTI context is copied around, Delphi's built-in interface handling logic, designed along COM principles, will ensure that the interface doesn't gets disposed of prematurely or get its reference count muddled up. And when an RTTI context goes out of scope, either by being a local variable in a function that is exited, or a field in an object that is freed, the reference count is reduced. When it hits zero, the pool is emptied.

The biggest upside of this approach is that RTTI usage should feel reasonably cheap, conceptually speaking. Code need only declare a variable of the appropriate type, and start using it:

procedure Foo;
  ctx: TRttiContext;
  t: TRttiType;
  t := ctx.GetType(TypeInfo(Integer));

A downside, however, is that lazy initialization can create a gotcha. Imagine this scenario:

  1. Library A declares an RTTI context A.C
  2. User code B declares an RTTI context B.C
  3. B pulls some RTTI objects O out of B.C, in order to hand them to library A
  4. B.C goes out of scope
  5. Library A now tries to work with O, but discovers much to its surprise, that the objects have been prematurely disposed, even though A already has an RTTI context, A.C

The problem is that A never used A.C, so it never allocated a pool token. When B.C used its context, the pool came into being, and objects O were assigned to it; but after B.C went out of scope, the objects got freed.

The solution to this problem is for Library A, knowing that it has a long-lived RTTI context and it expects to communicate with third-party code which allocates objects from its own RTTI context and hands them back, it should ensure that the long-lived context's pool token is allocated. A trivial way to do this is like this:

  TFooManager = class
    FCtx: TRttiContext;
    // ...
    constructor Create;
    // ...

constructor TFooManager.Create;
  // ...

This will allocate only a bare minimum of RTTI objects, those needed to represent the type System.Integer, but more importantly, will ensure that FCtx has a pool token and will keep the global RTTI pool alive.

In future releases of Delphi, the static method TRttiContext.Create will make sure that its return value has a pool token allocated; currently, it does not. TRttiContext.Create was originally defined to make the TRttiContext record feel more like a class for people unfamiliar with the idiom of using interfaces for automated deterministic lifetime management. The corresponding TRttiContext.Free method disposes of the internal pool token, and should remain the same.

1 comment:

Henri Gourvest said...

why there isn't RTTI on unnamed data types ?
for example:
T = array[0..1, 0..1] of Integer;
It is possible to know the total size of the array but it is impossible to know the size of each dimension.