Question

I am in front of following problem:

My Main programming Language is C++ with the Qt4 Library, but now I have to write a Pascal Wrapper, which should give the possibility to use the functions of a C DLL in Pascal.
Now I want to make it possible to invoke a method from any Pointer. But I can’t find a Pascal method to invoke a method. I want something like the QMetaObject::invokeMethod method in QT. I got following code:

unit CgPConnect;

//{$mode objfpc}{$H+}
{$mode delphi}

interface

    uses
        Classes, SysUtils, dynlibs;

    type

        Callback = Record
          var callbackObject: Pointer;
          var objectFunction: string;
        end;

        CallbackObject = Record
          var objectName: string;
          var callback: Callback;
        end;

        MutableObject = Object
          var name: string;
          var state: string;
          var properties: array of VariantMap;
          var annotations: array of VariantMap;
        end;



        PConnect = Class
            constructor create(connectorPath: string);
            destructor destroy;

            private
                var hostactionCallbacks: array of CallbackObject;
                var mConnectorPath: string;
                var mConnectorLibrary: TLibHandle;
                function loadConnectorLibrary: Boolean;

            public
                procedure registerCallbackForHostaction(objectName, objectFunction: string; callbackObject: pointer);
                procedure callHostactionCallback(receivedObject :MutableObject);

                var mLibraryLoaded: Boolean;
        end;

    implementation

    constructor PConnect.create(connectorPath: string);
    begin
        mConnectorPath:= connectorPath;
        mLibraryLoaded:= false;
        //Eventuell noch slash hinzufügen
        mLibraryLoaded:= loadConnectorLibrary;
    end;

    destructor PConnect.destroy;
    begin
        UnloadLibrary(mConnectorLibrary);
    end;

    procedure PConnect.registerCallbackForHostaction(objectName, objectFunction: string; callbackObject: pointer);
    var c: Callback;
    var callbackCount: integer;
    begin
        if mLibraryLoaded = true then
        begin
            c.callbackObject:= callbackObject;
            c.objectFunction:= objectFunction;
            callbackCount:= Length(hostactionCallbacks)+1;
            SetLength(hostactionCallbacks, callbackCount);
            hostactionCallbacks[callbackCount].objectName:= objectName;
            hostactionCallbacks[callbackCount].callback:= c;
        end;
    end;

    procedure PConnect.callHostactionCallback(receivedObject :MutableObject);
    var receivedObjectName, objectFunction: string;
    var i, count: integer;
    var callbackObject: pointer;
    begin
        if mLibraryLoaded = true then
        begin
            receivedObjectName:= receivedObject.name;
            count:= Length(hostactionCallbacks);
            for i:=0 to count do
            begin
                if hostactionCallbacks[i].objectName = receivedObjectName
                begin
                    objectFunction:= hostactionCallbacks[i].callback.objectFunction;
                    callbackObject:= hostactionCallbacks[i].callback.callbackObject;
                    if callbackObject <> 0 then
                        //INVOKE METHOD (objectFunction) OF OBJECT (callbackObject)
                end;
            end;
        end;
    end;
end.

I would be happy about a fast answer 🙂

Answer 1

You cannot directly and portably call a C++ method from Pascal. If your callbackfunction is a C++ object, forget it.

Otherwise fill a TMethod object and cast that to the proper “procedure of object” declaration. Don’t forget the calling convention.

For more bizarre solutions you might want to have a look at (RemObjects’) Pascalscript.

P.s. this is the same that you can’t even call a C++ method reliably from another C++ compiler. It is not Pascal vs C++ per se.