Wednesday, September 06, 2006

Extended Class RTTI

As has been mentioned earlier, Delphi (since version 7) supports extended RTTI on the methods of a class - by compiling the class with $METHODINFO ON defined. This RTTI includes the signature information of the public and published methods. Delphi uses this to implement scripting of Delphi code in the WebSnap framework - see ObjAuto and friends for the details.

I have now been able to write my own types and routines to dig down and get hold of the extended class RTTI into a format that can easily be used for external consumption. As usual my sample app simply dumps out the source code declaration of a sample class.

While writing the HVMethodInfoClasses unit I refactored some of the earlier code and structures so that I could use as much common code with HVIntefaceMethods and HVMethodSignature.

We're getting used to this RTTI digging now, so lets go through the new code quickly. First there are the additional implementation-detail structures defining the approximate layout of the RTTI the compiler generates - gleaned from the "official" source in ObjAuto.

PReturnInfo = ^TReturnInfo;
TReturnInfo = packed record
Version: Byte;
CallingConvention: TCallConv;
ReturnType: PPTypeInfo;
ParamSize: Word;
PParamInfo = ^TParamInfo;
TParamInfo = packed record
Flags: TParamFlags;
ParamType: PPTypeInfo;
Access: Word;
Name: ShortString;

Exactly how to find where these structures start is a little subtle. Remember back to the Under the hood of published methods article? At the time I wrote the following (ignorant to the existence of the $MethodInfo directive and extended class RTTI):

"As you can see above the published method table now has the type PPmt. It points to a record that contains the number of published methods in this class followed by a packed array of TPublishedMethod records. Each record contains a size (used to find the start of the next record), a pointer to the address of the method and a packed shortstring containing the name of the method.
Notice that it appears that the Size field would have been unnecessary. In all my testing the value of Size has always been equal to the expression:

  Size :=  SizeOf(Size) + SizeOf(Address) + SizeOf(Name[0]) + Length(Name);
In other words, the next TPublishedMethod record starts just after the last byte of the method name. I’m not sure why Borland decided to add the Size field, but one possible reason might be to be able to extend the contents of the TPublishedMethod record in the future. One natural extension would be to include information about the parameters and calling convention of the method. Then Size would be adjusted accordingly and old code unaware of the new fields would still work fine"

It turns out that the Size field is indeed used to pack up the TReturnInfo and TParamInfo records just following the Name field of the TPublishedMethod record.

PPublishedMethod = ^TPublishedMethod;
TPublishedMethod = packed record
Size: word;
Address: Pointer;
Name: {packed} Shortstring;

To find and decode the method's signature we have to collect the number of extra bytes as indicated by the Size field. We'll see the code for that shortly.

First, here is the easy to use structure that will hold the decoded RTTI for a single class, including all public/published methods with all their parameters and return types.

// Easy-to-use fixed size structure
PClassInfo = ^TClassInfo;
TClassInfo = record
UnitName: string;
Name: string;
ClassType: TClass;
ParentClass: TClass;
MethodCount: Word;
Methods: array of TMethodSignature;

That should be largely self-documenting. As you can see have have reused the same TMethodSignature record as we used for interfaces. Who said you have to write OOP to do re-use ;)? All right, now we're more or less ready to write the actual code to convert a typeinfo of a class to the TClassInfo structure above. This implies getting our hands dirty by iterating over all public/published methods and over all extra RTTI info for each method containing signature info. After a little trial-and-error and a couple of peeks into ObjAuto, I ended up with the following code.

function ClassOfTypeInfo(P: PPTypeInfo): TClass;
Result := nil;
if Assigned(P) and (P^.Kind = tkClass) then
Result := GetTypeData(P^).ClassType;

procedure GetClassInfo(ClassTypeInfo: PTypeInfo;
var ClassInfo: TClassInfo);
// Converts from raw RTTI structures to user-friendly Info structures
TypeData: PTypeData;
i, j: integer;
MethodInfo: PMethodSignature;
PublishedMethod: PPublishedMethod;
MethodParam: PMethodParam;
ReturnRTTI: PReturnInfo;
ParameterRTTI: PParamInfo;
SignatureEnd: Pointer;
Assert(ClassTypeInfo.Kind = tkClass);
// Class
TypeData := GetTypeData(ClassTypeInfo);
ClassInfo.UnitName := TypeData.UnitName;
ClassInfo.ClassType := TypeData.ClassType;
ClassInfo.Name := TypeData.ClassType.ClassName;
ClassInfo.ParentClass := ClassOfTypeInfo(TypeData.ParentInfo);
ClassInfo.MethodCount := GetPublishedMethodCount(ClassInfo.ClassType);
SetLength(ClassInfo.Methods, ClassInfo.MethodCount);
// Methods
PublishedMethod := GetFirstPublishedMethod(ClassInfo.ClassType);
for i := Low(ClassInfo.Methods) to High(ClassInfo.Methods) do
// Method
MethodInfo := @ClassInfo.Methods[i];
MethodInfo.Name := PublishedMethod.Name;
MethodInfo.Address := PublishedMethod.Address;
MethodInfo.MethodKind := mkProcedure; // Assume procedure by default

// Return info and calling convention
ReturnRTTI := Skip(@PublishedMethod.Name);
SignatureEnd := Pointer(Cardinal(PublishedMethod)
+ PublishedMethod.Size);
if Cardinal(ReturnRTTI) >= Cardinal(SignatureEnd) then
MethodInfo.CallConv := ccReg; // Assume register calling convention
MethodInfo.HasSignatureRTTI := False;
MethodInfo.ResultTypeInfo := Dereference(ReturnRTTI.ReturnType);
if Assigned(MethodInfo.ResultTypeInfo) then
MethodInfo.MethodKind := mkFunction;
MethodInfo.ResultTypeName := MethodInfo.ResultTypeInfo.Name;
MethodInfo.MethodKind := mkProcedure;
MethodInfo.CallConv := ReturnRTTI.CallingConvention;
MethodInfo.HasSignatureRTTI := True;
// Count parameters
ParameterRTTI := Pointer(Cardinal(ReturnRTTI) + SizeOf(ReturnRTTI^));
MethodInfo.ParamCount := 0;
while Cardinal(ParameterRTTI) < Cardinal(SignatureEnd) do
Inc(MethodInfo.ParamCount); // Assume less than 255 parameters ;)!
ParameterRTTI := Skip(@ParameterRTTI.Name);
// Read parameter info
ParameterRTTI := Pointer(Cardinal(ReturnRTTI) + SizeOf(ReturnRTTI^));
SetLength(MethodInfo.Parameters, MethodInfo.ParamCount);
for j := Low(MethodInfo.Parameters) to High(MethodInfo.Parameters) do
MethodParam := @MethodInfo.Parameters[j];
MethodParam.Flags := ParameterRTTI.Flags;
if pfResult in MethodParam.Flags
then MethodParam.ParamName := 'Result'
else MethodParam.ParamName := ParameterRTTI.Name;
MethodParam.TypeInfo := Dereference(ParameterRTTI.ParamType);
if Assigned(MethodParam.TypeInfo) then
MethodParam.TypeName := MethodParam.TypeInfo.Name;
MethodParam.Location := TParamLocation(ParameterRTTI.Access);
ParameterRTTI := Skip(@ParameterRTTI.Name);
PublishedMethod := GetNextPublishedMethod(ClassInfo.ClassType,

As usual we test the code by defining some silly code and use RTTI to reconstruct the source of a class declaration. Here is the simplified test project.

program TestHVMethodInfoClasses;


HVMethodSignature in 'HVMethodSignature.pas',
HVMethodInfoClasses in 'HVMethodInfoClasses.pas';

procedure DumpClass(ClassTypeInfo: PTypeInfo);
ClassInfo: TClassInfo;
i: integer;
GetClassInfo(ClassTypeInfo, ClassInfo);
writeln('unit ', ClassInfo.UnitName, ';');
write(' ', ClassInfo.Name, ' = ');
if Assigned(ClassInfo.ParentClass) then
write(' (', ClassInfo.ParentClass.ClassName, ')');
for i := Low(ClassInfo.Methods) to High(ClassInfo.Methods) do
writeln(' ', MethodSignatureToString(ClassInfo.Methods[i]));
writeln(' end;');

TNormalClass = class
TSetOfByte = set of byte;
TEnum = (enOne, enTwo, enThree);
TMyClass = class
function Test1(const A: string): string;
function Test2(const A: string): byte;
procedure Test3(R: integer);
procedure Test4(R: TObject);
procedure Test5(R: TNormalClass);
procedure Test6(R: TSetOfByte);
procedure Test7(R: shortstring);
procedure Test8(R: openstring);
procedure Test9(R: TEnum);
function Test10: TNormalClass;
function Test11: integer;
function Test18: shortstring;
function Test19: TObject;
function Test20: IInterface;
function Test21: TSetOfByte;
function Test22: TEnum;

//... Dummy implementations of TMyClass methods left out...

procedure Test;

on E:Exception do

 And the output of the program is:

unit TestHVMethodInfoClasses;
TMyClass = class (TObject)
function Test1(A: String): String;
function Test2(A: String): Byte;
procedure Test3(R: Integer);
procedure Test4(R: TObject);
procedure Test5(R: TNormalClass);
procedure Test6(R: TSetOfByte);
procedure Test7(R: ShortString);
procedure Test8(R: ShortString);
procedure Test9(R: TEnum);
function Test10(): TNormalClass;
function Test11(): Integer;
function Test18(): ShortString;
function Test19(): TObject;
function Test20(): IInterface;
function Test21(): TSetOfByte;
function Test22(): TEnum;

The full code is available at CodeCentral.

As my diligent reader, Ralf, pointed out the output of this program is not a verbatim copy of the source code. Aside from my sloppiness of not omitting the empty parens in the functions, the A: string parameters are not declared const. This is because the RTTI for those parameters does not include pfConst (duh!). I think the reason is that the method and parameter RTTI is optimized to achieve dynamic run-time calling of methods and a const modifier does not affect the caller - it only influences the code the compiler generates in the implementation of the method (omitting string ref-counting and a try-finally clause).

In fact, I've been (unsuccessfully so far) lobbying Borland DevCo CodeGear to ease up the compiler and allow having const in the implementation section and non-const in the interface. This might sound like a sloppy request, but it would allow changing the const-ness of a parameter without affecting the interface. Oh well, a story for some other time, perhaps.

[PS. This blog post was edited in Windows Live Writer - I like it, although I goofed up and posted this article too early, probably by accidentally pressing Ctrl+P = publish post ;)].

Updated (27. Oct 2007): $METHODINFO was first available in Delphi 7, not Delphi 6.


Anonymous said...

sounds cool! I guess it is on CodeCentral again? If yes a link would be nice!

best regards and thanks for your work!

Ralf Grenzing

Anonymous said...


It seems to be that the Code is not on CodeCentral yet?

Looking at your output of
Test1(const A: string): string;
The "const" word is missing in output dump.

Best regards and thanks for your work! Much appreciated

Ralf Grenzing

Hallvards New Blog said...

Hi Ralf,

I was a little quick on the trigger - CodeCEntral has not been updated yet. I will upload to CC and update the article shortly.

Anonymous said...

So here is you diligent reader again :-)

while playing with the source of David Glassborow I realized that only the flag for var is set and I hoped you bugged that out!

BTW: when I did my first comment post at 8:43 in you BLOG entry only the first parargaph was published (all above the first code)! It read like the full article! Strange behaviour?


Hallvards New Blog said...

Hi Dave,

I'm not up to par with the inner details of the C++ Builder compiler, so I don't know if it supports this extended class RTTI or not.

The Websnap technology depends on the presence of this RTTI, but has the stock Websnap objects are written in Delphi, it works out of the box, even in C++. If the C++ compiler does not support generating this extra RTTI, you cannot extend Websnap in advanced ways, adding your own classes with methods that can be invoked from the Websnap serverside script templates.

In short; I don't know - maybe someone else does.

Unknown said...

This doesn't work on Delphi 6?

$METHODINFO is undefined?

Anonymous said...

METHODINFO directive is not supported in Delphi 6. It first appears in Delphi 7, I think

Theo Bebekis

Hallvards New Blog said...

> METHODINFO directive is not supported in Delphi 6. It first appears in Delphi 7, I think

Thanks, Theo.

I don't have D6 here to test now, but I do think you are right. I will update the article.

Anonymous said...

Sorry, I can't download files.
Would you be so kind as to send them to my email:

Anonymous said...

Very usefull article and very needed to, but CodeCentral does not respond :(

Would you be so kind as to send them to my email:

Anonymous said...

good info and great blog ;)

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