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Generics with Delphi 2009 Win32

With, as a bonus, anonymous routines and routine references

Date de publication : November 13th, 2008

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IX. RTTI and generics
IX-A. Changes to the pseudo-routine TypeInfo
IX-A-1. A more general TypeInfo function
IX-B. Do generic types have RTTI?

IX. RTTI and generics

In this last chapter, I am going to give some information about what generics changed to RTTI. If you never play with RTTI, you should skip this entire chapter. It is not intended at all to be an introduction to RTTI.

IX-A. Changes to the pseudo-routine TypeInfo

RTTI always begin with the pseudo-routine TypeInfo. You may know that certain types cannot be applied to this pseudo-routine; for example, pointer types. And that, because of that, it never returns nil.

So, may we call TypeInfo on a generic type T? The question is pertinent: T might as well be a pointer type (invalid for TypeInfo) or an integer type (valid).

The answer is yes, one can call TypeInfo on a generic type. But what happens then if T results in a type that has no RTTI? Well, in this only case, TypeInfo returns nil.

To illustrate the phenomenon, here is a small class method that prints the name and the kind of type, but via generics:


type
  TRTTI = class(TObject)
  public
    class procedure PrintType<T>; static;
  end;

class procedure TRTTI.PrintType<T>;
var
  Info: PTypeInfo;
begin
  Info := TypeInfo(T); // warning ! Info may be nil here

  if Info = nil then
    WriteLn('This type has no RTTI')
  else
    WriteLn(Info.Name, #9, GetEnumName(TypeInfo(TTypeKind), Byte(Info.Kind)));
end;

This method is quite special, in the sense that its real parameter is actually passed as a parameterized type.


begin
  TRTTI.PrintType<Integer>;
  TRTTI.PrintType<TObject>;
  TRTTI.PrintType<Pointer>;
end;

Whose execution yields:


Integer  tkInteger
TObject  tkClass
This type has no RTTI

IX-A-1. A more general TypeInfo function

I have already regretted that TypeInfo could not be applied to any type, even for a nil reply; maybe you did as well. Then, here is a small replacement method solving the method.


type
  TRTTI = class(TObject)
  public
    class procedure PrintType<T>; static;
    class function TypeInfo<T>: PTypeInfo; static;
  end;

class function TRTTI.TypeInfo<T>: PTypeInfo;
begin
  Result := System.TypeInfo(T);
end;

Which you may use as:


Info := TRTTI.TypeInfo<Pointer>; // Info = nil
// instead of :
Info := TypeInfo(Pointer); // compiler error, since Pointer has no RTTI

IX-B. Do generic types have RTTI?

There are two questions here: do non instaciated generic types (with a non defined T parameter) have RTTI? And do instanciated generic types (where T has been replaced with an actual type, like Integer) have RTTI?

The easiest way to know it is to try ;-) You may observe, with a simple test, that only instanciated generic types. Actually, that is quite logical, since non instanciated generic types are not actual types, but type templates, and do not exist once compilation is done.

One can ask himself what name would such a type have. So:


begin
  TRTTI.PrintType<TComparison<Integer>>;
  TRTTI.PrintType<TTreeNode<Integer>>;
end;

Which yields:


TComparison<System.Integer>  tkInterface
TTreeNode<System.Integer>    tkClass

This shows that the name includes, between angle brackets, the fully qualified name of the actual type replacing the type parameter.

You can also notice that the routine reference type TComparison<T> yields tkInterface, which proves that it is actually an interface.

That is all I wanted to say about RTTI and generics. I will not of course speak about other changes brought to RTTI in Delphi 2009 because of Unicode strings.

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