Let's say that I have:
type
TClassA = class
function prova: integer; virtual;
function provaSuA: integer; virtual;
end;
type
TClassB = class(TClassA)
function prova: integer; override;
function provaSuB: integer; virtual;
end;
Then I use this code:
procedure TForm1.Button1Click(Sender: TObject);
var a: TClassA;
b: TClassB;
begin
Memo1.Clear;
a := TClassB.Create;
try
b := ?? //dynamic_cast on C++
Memo1.Lines.Add(a.prova.ToString);
Memo1.Lines.Add(b.provaSuB.ToString);
finally
a.Free;
end;
end;
I am using polymorphism and the static type of a is TClassA but the dynamic type is TClassB. Of course I can only call on a methods that are declared on TClassA (or that are overridden in TClassB).
If I want to use a and have access to ALL methods in TClassB in C++ I'd use a dynamic_cast that is (together with typeid) in C++ RTTI. How can I use Delphi's RTTI to do that?
Delphi "RTTI" has a slightly different meaning from C++'s one. To replicate what dynamic_cast does, you need to follow one of the two patterns:
pattern 1: if the cast fails, I want an exception: this is achieved, by using the as operator which performs the proper type-checking and then casts the object when possible. In case of failure, a ClassCastException is thrown.
b:= a as TClassB;
pattern 2: if the cast fails, I want to handle it manually: this is achieved by using the is operator which strictly performs the test about the cast. Then you are need to manually cast (look comments in code) on success:
if (a is TClassB) then
begin
// this is a cast, however, in this context, you are simply interpreting
// the memory pointed by a as a class of type ClassB. You are not using any operator.
b:= TClassB(a);
end
else
begin
// here you know that a cannot be cast to TClassB, therefore you can gracefully take
// proper action here, without catching any exception
end;
Related
I have a logging class, which links to many modules. The main method of this class is a class method:
type
TSeverity = (seInfo, seWarning, seError);
TLogger = class
class procedure Log(AMessage: String; ASeverity: TSeverity);
end;
Somewhere else I have a function DoSomething() which does some things that I would like to log. However, I do not want to link all the modules of the logger to the module in which 'DoSomething()' is declared to use the logger. Instead I would like to pass an arbitrary logging method as a DoSomething's parameter and call it from its body.
The problem is that TLogger.Log requires parameter of TSeverity type which is defined in logger class. So I can't define a type:
type
TLogProcedure = procedure(AMessage: String; ASverity: TSeverity) of Object;
because I would have to include an unit in which TSeverity is declared.
I was trying to come up with some solution based on generic procedure but I am stuck.
uses
System.SysUtils;
type
TTest = class
public
class function DoSomething<T1, T2>(const ALogProcedure: TProc<T1,T2>): Boolean; overload;
end;
implementation
class function TTest.DoSomething<T1, T2>(const ALogProcedure: TProc<T1, T2>): Boolean;
var
LMessage: String;
LSeverity: Integer;
begin
//Pseudocode here I would like to invoke logging procedure here.
ALogProcedure(T1(LMessage), T2(LSeverity));
end;
Somewehere else in the code I would like to use DoSomething
begin
TTest.DoSomething<String, TSeverity>(Log);
end;
Thanks for help.
Update
Maybe I didn't make myself clear.
unit uDoer;
interface
type
TLogProcedure = procedure(AMessage: String; AErrorLevel: Integer) of Object;
// TDoer knows nothing about logging mechanisms that are used but it allows to pass ALogProcedure as a parameter.
// I thoight that I can somehow generalize this procedure using generics.
type
TDoer = class
public
class function DoSomething(const ALogProcedure: TLogProcedure): Boolean;
end;
implementation
class function TDoer.DoSomething(const ALogProcedure: TLogProcedure): Boolean;
begin
ALogProcedure('test', 1);
Result := True;
end;
end.
Separate unit with one of the logging mechanisms.
unit uLogger;
interface
type
TSeverity = (seInfo, seWarning, seError);
// I know that I could solve my problem by introducing an overloaded method but I don't want to
// do it like this. I thought I can use generics somehow.
TLogger = class
class procedure Log(AMessage: String; ASeverity: TSeverity); {overload;}
{class procedure Log(AMessage: String; ASeverity: Integer); overload;}
end;
implementation
class procedure TLogger.Log(AMessage: String; ASeverity: TSeverity);
begin
//...logging here
end;
{class procedure TLogger.Log(AMessage: String; ASeverity: Integer);
begin
Log(AMessage, TSeverity(ASeverity));
end;}
end.
Sample usage of both units.
implementation
uses
uDoer, uLogger;
procedure TForm10.FormCreate(Sender: TObject);
begin
TDoer.DoSomething(TLogger.Log); //Incompatible types: Integer and TSeverity
end;
Introducing generics here does not help. The actual parameters that you have are not generic. They have fixed type, string and Integer. The function you are passing them to is not generic and receives parameters of type string and TSeverity. These types are mis-matched.
Generics won't help you here because your types are all known ahead of time. There is nothing generic here. What you need to do, somehow, is convert between Integer and TSeverity. Once you can do that then you can call your function.
In your case you should pass a procedure that accepts an Integer, since you don't have TSeverity available at the point where you call the procedure. Then in the implementation of that procedure, where you call the function that does accept a TSeverity, that's where you convert.
In scenarios involving generic procedural types, what you have encountered is quite common. You have a generic procedural type like this:
type
TMyGenericProcedure<T> = procedure(const Arg: T);
In order to call such a procedure you need an instance of T. If you are calling the procedure from a function that is generic on T, then your argument must also be generic. In your case that argument is not generic, it is fixed as Integer. At that point your attempt to use generics unravels.
Having said all of that, what you describe doesn't really hang together at all. How can you possibly come up with the severity argument if you don't know what TSeverity is at that point? That doesn't make any sense to me. How can you just conjure up an integer value and hope that it matches this enumerated type? Some mild re-design would enable you to do this quite simply without any type conversions.
As David Heffernan says, you cannot use generics in this way. Instead you should use a function to map the error level to a severity type, and use that to glue together the two. Based on your updated example, one could modify it like this:
unit uDoer;
interface
type
TLogProcedure = reference to procedure(const AMessage: String; AErrorLevel: Integer);
// TDoer knows nothing about logging mechanisms that are used but it allows to pass ALogProcedure as a parameter.
type
TDoer = class
public
class function DoSomething(const ALogProcedure: TLogProcedure): Boolean;
end;
implementation
class function TDoer.DoSomething(const ALogProcedure: TLogProcedure): Boolean;
begin
ALogProcedure('test', 1);
Result := True;
end;
end.
You can then provide the glue procedure which converts the error level to a severity:
implementation
uses
uDoer, uLogger;
function SeverityFromErrorLevel(const AErrorLevel: Integer): TSeverity;
begin
if (AErrorLevel <= 0) then
result := seInfo
else if (AErrorLevel = 1) then
result := seWarning
else
result := seError;
end;
procedure LogProc(const AMessage: String; AErrorLevel: Integer);
var
severity: TSeverity;
begin
severity := SeverityFromErrorLevel(AErrorLevel);
TLogger.Log(AMessage, severity);
end;
procedure TForm10.FormCreate(Sender: TObject);
begin
TDoer.DoSomething(LogProc);
end;
Note I didn't compile this, but the essence is there. I used a procedure reference (reference to procedure) as they're a lot more flexible, which may come in handy later.
I got strings in database like 'TGroupBox' or 'TEdit' ... now I need to check element against them... how do I enumerate string to type?
I mean something like this:
mystr := 'TGroupBox';
If (page.Controls[0] is mystr) then ...
Of course it won't work, as error appears:
E2015 Operator not applicable to this operand type
How do I do that correctly?
You can verify that
page.Controls[0].ClassName = mystr
using the ClassName property.
But notice that this doesn't do exactly the same thing as the is operator. To see the difference, suppose you have a class TFruit and a subclass TApple. If myFruit is an instance of a TApple, then both myFruit is TApple and myFruit is TFruit will yield true. But of course, the ClassName will still only be TApple.
If you need the full functionality of the is operator, you can make use of the ClassParent property, as suggested by hvd:
function IsDerivedFrom(AClass: TClass; const AClassName: string): boolean;
begin
if not Assigned(AClass) then Exit(false);
result := SameText(AClass.ClassName, AClassName) or
IsDerivedFrom(AClass.ClassParent, AClassName);
end;
To get the class of an object, use the ClassType property:
IsDerivedFrom(page.Controls[0].ClassType, mystr);
The function you are looking for is GetClass located in System.Classes. Be aware that the class has to be registered.
System.Classes.GetClass
For the specific scenario in the question body the answer by Andreas Rejbrand (with assistance from hvd) is a good one. However, for the broader problem implied by the question title - how to I convert a string containing a class name to a class reference? - you can utilise extended RTTI in a new(ish) version of Delphi:
unit ClassLookupUtils;
interface
uses
System.SysUtils, System.Generics.Collections, System.Rtti;
type
RttiClassLookup = record
strict private
class var FMap: TDictionary<string, TClass>;
class destructor Destroy;
public
class function Find(const ClassName: string): TClass; static;
end;
implementation
class destructor RttiClassLookup.Destroy;
begin
FMap.Free;
end;
class function RttiClassLookup.Find(const ClassName: string): TClass;
var
RttiType: TRttiType;
RttiContext: TRttiContext;
begin
if FMap = nil then
begin
FMap := TDictionary<string, TClass>.Create;
for RttiType in RttiContext.GetTypes do
if RttiType is TRttiInstanceType then
FMap.AddOrSetValue(RttiType.Name.ToLowerInvariant, (RttiType as TRttiInstanceType).MetaclassType);
end;
if not FMap.TryGetValue(ClassName.ToLowerInvariant, Result) then
Result := nil;
end;
end.
In use:
var
MyStr: string;
MyStrClass: TClass;
begin
//...
MyStrClass := RttiClassLookup.Find(MyStr);
if MyStrClass <> nil then
for I := 0 to Page.ControlCount - 1 do
if Page.Controls[I].InheritsFrom(MyStrClass) then
begin
//...
end;
The background here is that SomeObj is SomeClass is implemented as (SomeObj <> nil) and SomeObj.InheritsFrom(SomeClass).
You have a good answer from #UweRaabe usingRTTIto getClassName.
A simple (and not very robust) hack without using RTTI would be to use the TComponent.Name property, which is a string, like this - without the is operator:
If (pos('GroupBox', page.Controls[0].name)>0 ) then ...
By default, a control gets the same name as the instance variable, so GroupBox1.name='GroupBox1'. You can either change your database entries to use the substr 'groupbox' or extract 'groupbox' from the type name string in your database.
That being said, if you've inherited this design approach of persisting type names as strings in a database and then using them at runtime to check the types of different components, then you're stuck with it, and so be it. But Delphi is a strongly typed, compiled language, so persisting type names as strings in a database and reading them at runtime and decoding them into Delphi types just doesn't "smell right" IMO. I would re-think this design if possible. Consider doing it all in Delphi using classOf type, enumerations, etc.
I ran into a strange situation while testing something today.
I have a number of interfaces and objects. The code looks like this:
IInterfaceZ = interface(IInterface)
['{DA003999-ADA2-47ED-A1E0-2572A00B6D75}']
procedure DoSomething;
end;
IInterfaceY = interface(IInterface)
['{55BF8A92-FCE4-447D-B58B-26CD9B344EA7}']
procedure DoNothing;
end;
TObjectB = class(TInterfacedObject, IInterfaceZ)
procedure DoSomething;
end;
TObjectC = class(TInterfacedObject, IInterfaceY)
public
FTest: string;
procedure DoNothing;
end;
TObjectA = class(TInterfacedObject, IInterfaceZ, IInterfaceY)
private
FInterfaceB: IInterfaceZ;
FObjectC: TObjectC;
function GetBB: IInterfaceZ;
public
procedure AfterConstruction; override;
procedure BeforeDestruction; override;
property BB: IInterfaceZ read GetBB implements IInterfaceZ;
property CC: TObjectC read FObjectC implements IInterfaceY;
end;
procedure TObjectB.DoSomething;
begin
Sleep(1000);
end;
procedure TObjectA.AfterConstruction;
begin
inherited;
FInterfaceB := TObjectB.Create;
FObjectC := TObjectC.Create;
FObjectC.FTest := 'Testing';
end;
procedure TObjectA.BeforeDestruction;
begin
FreeAndNil(FObjectC);
FInterfaceB := nil;
inherited;
end;
function TObjectA.GetBB: IInterfaceZ;
begin
Result := FInterfaceB;
end;
procedure TObjectC.DoNothing;
begin
ShowMessage(FTest);
end;
Now if I access the various implementations like this I get the following results:
procedure TestInterfaces;
var
AA: TObjectA;
YY: IInterfaceY;
ZZ: IInterfaceZ;
NewYY: IInterfaceY;
begin
AA := TObjectA.Create;
// Make sure that the Supports doesn't kill the object.
// This line of code is necessary in XE2 but not in XE4
AA._AddRef;
// This will add one to the refcount for AA despite the fact
// that AA has delegated the implementation of IInterfaceY to
// to FObjectC.
Supports(AA, IInterfaceY, YY);
YY.DoNothing;
// This will add one to the refcount for FInterfaceB.
// This is also allowing a supports from a delegated interface
// to another delegated interface.
Supports(YY, IInterfaceZ, ZZ);
ZZ.DoSomething;
// This will fail because the underlying object is actually
// the object referenced by FInterfaceB.
Supports(ZZ, IInterfaceY, NewYY);
NewYY.DoNothing;
end;
The first Supports call, which uses the variable in the implements, returns YY which is actually a reference to TObjectA. My AA variable is reference counted. Because the underlying reference counted object is a TObjectA, the second supports, which uses the interface in the supports call, works and returns me an interface. The underlying object is actually now a TObjectB. The internal object behind FInterfaceB is the object being reference counted. This part makes sense because GetBB is actually FInterfaceB. As expected here, the last call to Supports returns a null for NewYY and the call at the end fails.
My question is this, is the reference counting on TObjectA with the first supports call by design? In other words, when the property that implements the interface is returning an object and not an interface does this mean that the owner object will be the one doing the reference counting? I was always under the impression that implements would also result in the internal delegated object being reference counted instead of the main object.
The declarations are as follows:
property BB: IInterfaceZ read GetBB implements IInterfaceZ;
With this option above, the internal object behind FInterfaceB is the one that is reference counted.
property CC: TObjectC read FObjectC implements IInterfaceY;
With this second option above, TObjectA is the one that is being reference counted and not the delegated object FObjectC.
Is this by design?
Edit
I just tested this in XE2 and the behavior is different. The second Supports statement returns nil for ZZ. The debugger in XE4 tells me that the YY is referring to (TObjectA as IInterfaceY). In XE2 it tells me that its a (Pointer as IInterfaceY). Also, in XE2, the AA is not ref counted on the first support statement but the internal FObjectC is reference counted.
Additional Information after the question answered
There is one caveat to this. You can chain the Interface version but not the object version. That means that something like this will work:
TObjectBase = class(TInterfacedObject, IMyInterface)
…
end;
TObjectA = class(TInterfacedObject, IMyInterface)
FMyInterfaceBase: IMyInterface;
property MyDelegate: IMyInterface read GetMyInterface implements IMyInterface;
end;
function TObjectA.GetMyInterface: IMyInterface;
begin
result := FMyInterfaceBase;
end;
TObjectB = class(TInterfacedObject, IMyInterface)
FMyInterfaceA: IMyInterface;
function GetMyInterface2: IMyInterface;
property MyDelegate2: IMyInterface read GetMyInterface2 implements IMyInterface;
end;
function TObjectB.GetMyInterface2: IMyInterface;
begin
result := FMyInterfaceA;
end;
But the object version gives a compiler error with this saying that TObjectB doesn't implement the methods for the interface.
TObjectBase = class(TInterfacedObject, IMyInterface)
…
end;
TObjectA = class(TInterfacedObject, IMyInterface)
FMyObjectBase: TMyObjectBase;
property MyDelegate: TMyObjectBase read FMyObjectBase implements IMyInterface;
end;
TObjectB = class(TInterfacedObject, IMyInterface)
FMyObjectA: TObjectA;
property MyDelegate2: TObjectA read FMyObjectA implements IMyInterface;
end;
So if you want to start chaining the delegation then you need to stick to interfaces or work around it another way.
tl;dr This is all by design – it's just that the design changes between XE2 and XE3.
XE3 and later
There is quite a difference between delegation to an interface type property and delegation to a class type property. Indeed the documentation calls out this difference explicitly with different sections for the two delegation variants.
The difference from your perspective is as follows:
When TObjectA implements IInterfaceY by delegating to class type property CC, the implementing object is the instance of TObjectA.
When TObjectA implements IInterfaceZ by delegating to interface type property BB, the implementing object is the object that implements FInterfaceB.
One key thing to realise in all this is that when you delegate to a class type property, the class that is delegated to need not implement any interfaces. So it need not implement IInterface and so need not have _AddRef and _Release methods.
To see this, modify your code's definition of TObjectC to be like so:
TObjectC = class
public
procedure DoNothing;
end;
You will see that this code compiles, runs, and behaves exactly the same way as does your version.
In fact this is ideally how you would declare a class to which an interface is delegated as a class type property. Doing it this way avoids the lifetime issues with mixing interface and class type variables.
So, let's look at your three calls to Supports:
Supports(AA, IInterfaceY, YY);
Here the implementing object is AA and so the reference count of AA is incremented.
Supports(YY, IInterfaceZ, ZZ);
Here the implementing object is the instance of TObjectB so its reference count is incremented.
Supports(ZZ, IInterfaceY, NewYY);
Here, ZZ is an interface implemented by the instance of TObjectB which does not implement IInterfaceY. Hence Supports returns False and NewYY is nil.
XE2 and earlier
The design changes between XE2 and XE3 coincide with the introduction of the mobile ARM compiler and there were many low-level changes to support ARC. Clearly some of these changes apply to the desktop compilers too.
The behavioural difference that I can find concerns delegation of interface implementation to class type properties. And specifically when the class type in question supports IInterface. In that scenario, in XE2, the reference counting is performed by the inner object. That differs from XE3 which has the reference counting performed by the outer object.
Note that for a class type that does not support IInterface, the reference counting is performed by the outer object in all versions. That makes sense since there's no way for the inner object to do it.
Here's my example code to demonstrate the difference:
{$APPTYPE CONSOLE}
uses
SysUtils;
type
Intf1 = interface
['{56FF4B9A-6296-4366-AF82-9901A5287BDC}']
procedure Foo;
end;
Intf2 = interface
['{71B0431C-DB83-49F0-B084-0095C535AFC3}']
procedure Bar;
end;
TInnerClass1 = class(TObject, Intf1)
function QueryInterface(const IID: TGUID; out Obj): HResult; stdcall;
function _AddRef: Integer; stdcall;
function _Release: Integer; stdcall;
procedure Foo;
end;
TInnerClass2 = class
procedure Bar;
end;
TOuterClass = class(TObject, Intf1, Intf2)
private
FInnerObj1: TInnerClass1;
FInnerObj2: TInnerClass2;
public
constructor Create;
function QueryInterface(const IID: TGUID; out Obj): HResult; stdcall;
function _AddRef: Integer; stdcall;
function _Release: Integer; stdcall;
property InnerObj1: TInnerClass1 read FInnerObj1 implements Intf1;
property InnerObj2: TInnerClass2 read FInnerObj2 implements Intf2;
end;
function TInnerClass1.QueryInterface(const IID: TGUID; out Obj): HResult;
begin
if GetInterface(IID, Obj) then
Result := 0
else
Result := E_NOINTERFACE;
end;
function TInnerClass1._AddRef: Integer;
begin
Writeln('TInnerClass1._AddRef');
Result := -1;
end;
function TInnerClass1._Release: Integer;
begin
Writeln('TInnerClass1._Release');
Result := -1;
end;
procedure TInnerClass1.Foo;
begin
Writeln('Foo');
end;
procedure TInnerClass2.Bar;
begin
Writeln('Bar');
end;
constructor TOuterClass.Create;
begin
inherited;
FInnerObj1 := TInnerClass1.Create;
end;
function TOuterClass.QueryInterface(const IID: TGUID; out Obj): HResult;
begin
if GetInterface(IID, Obj) then
Result := 0
else
Result := E_NOINTERFACE;
end;
function TOuterClass._AddRef: Integer;
begin
Writeln('TOuterClass._AddRef');
Result := -1;
end;
function TOuterClass._Release: Integer;
begin
Writeln('TOuterClass._Release');
Result := -1;
end;
var
OuterObj: TOuterClass;
I1: Intf1;
I2: Intf2;
begin
OuterObj := TOuterClass.Create;
Supports(OuterObj, Intf1, I1);
Supports(OuterObj, Intf2, I2);
I1.Foo;
I2.Bar;
I1 := nil;
I2 := nil;
Readln;
end.
The output on XE2 is:
TInnerClass1._AddRef
TOuterClass._AddRef
Foo
Bar
TInnerClass1._Release
TOuterClass._Release
The output on XE3 is:
TOuterClass._AddRef
TOuterClass._AddRef
Foo
Bar
TOuterClass._Release
TOuterClass._Release
Discussion
Why did the design change? I cannot answer that definitively, not being privy to the decision making. However, the behaviour in XE3 feels better to me. If you declare a class type variable you would expect its lifetime to be managed as any other class type variable would be. That is, by explicit calls to destructor on the desktop compilers, and by ARC on the mobile compilers.
The behaviour of XE2 on the other hand feels inconsistent. Why should the fact that a property is used for interface implementation delegation change the way its lifetime is managed?
So, my instincts tell me that this was a design flaw, at best, in the original implementation of interface implementation delegation. The design flaw has led to confusion and lifetime management troubles over the years. The introduction to ARC forced Embarcadero to review this issue and they changed the design. My belief is that the introduction of ARC required a design change because Embarcadero have a track record of not changing behaviour unless absolutely necessary.
The paragraphs above are clearly speculation on my part, but that's the best I have to offer!
You are mixing object pointers and interface pointers, which is always a recipe for disaster. TObjectA is not incrementing the reference count of its inner objects to ensure they stay alive for its entire lifetime, and TestInterfaces() is not incrementing the reference count of AA to ensure it survives through the entire set of tests. Object pointers DO NOT participate in reference counting! You have to manage it manually, eg:
procedure TObjectA.AfterConstruction;
begin
inherited;
FObjectB := TObjectB.Create;
FObjectB._AddRef;
FObjectC := TObjectC.Create;
FObjectC._AddRef;
FObjectC.FTest := 'Testing';
end;
procedure TObjectA.BeforeDestruction;
begin
FObjectC._Release;
FObjectB._Release;
inherited;
end;
AA := TObjectA.Create;
AA._AddRef;
Needless to say, manual reference counting undermines the use of interfaces.
When dealing with interfaces, you need to either:
Disable reference counting completely to avoid premature destructions. TComponent, for instance, does exactly that.
Do EVERYTHING using interface pointers, NEVER with object pointers. This ensures proper reference counting across the board. This is generally the preferred solution.
I need a class implementing interface with no reference counting. I did the following:
IMyInterface = interface(IInterface)
['{B84904DF-9E8A-46E0-98E4-498BF03C2819}']
procedure InterfaceMethod;
end;
TMyClass = class(TObject, IMyInterface)
protected
function _AddRef: Integer;stdcall;
function _Release: Integer;stdcall;
function QueryInterface(const IID: TGUID; out Obj): HResult;stdcall;
public
procedure InterfaceMethod;
end;
procedure TMyClass.InterfaceMethod;
begin
ShowMessage('The Method');
end;
function TMyClass.QueryInterface(const IID: TGUID; out Obj): HResult;
begin
if GetInterface(IID, Obj) then
Result := 0
else
Result := E_NOINTERFACE;
end;
function TMyClass._AddRef: Integer;
begin
Result := -1;
end;
function TMyClass._Release: Integer;
begin
Result := -1;
end;
Lack of reference counting works fine. But my concern is that I cannot cast TMyClass to IMyInterface using as operator:
var
MyI: IMyInterface;
begin
MyI := TMyClass.Create as IMyInterface;
I am given
[DCC Error] E2015 Operator not applicable to this operand type
The problem disappears when TMyClass derives from TInterfacedObject - i.e. I can do such casting without compiler error. Obviously I do not want to use TInterfacedObject as a base class as it would make my class reference counted. Why is such casting disallowed and how one would workaround it?
The reason you cannot use as in your code is that your class does not explicitly list IInterface in its list of supported interfaces. Even though your interface derives from IInterface, unless you actually list that interface, your class does not support it.
So, the trivial fix is to declare your class like this:
TMyClass = class(TObject, IInterface, IMyInterface)
The reason that your class needs to implement IInterface is that is what the compiler is relying on in order to implement the as cast.
The other point I would like to make is that you should, in general, avoid using interface inheritance. By and large it serves little purpose. One of the benefits of using interfaces is that you are free from the single inheritance constraint that comes with implementation inheritance.
But in any case, all Delphi interfaces automatically inherit from IInterface so in your case there's no point specifying that. I would declare your interface like this:
IMyInterface = interface
['{B84904DF-9E8A-46E0-98E4-498BF03C2819}']
procedure InterfaceMethod;
end;
More broadly you should endeavour not to use inheritance with your interfaces. By taking that approach you will encourage less coupling and that leads to greater flexibility.
for a framework I wrote a wrapper which takes any object, interface or record type to explore its properties or fields. The class declaration is as follows:
TWrapper<T> = class
private
FType : TRttiType;
FInstance : Pointer;
{...}
public
constructor Create (var Data : T);
end;
In the constructor I try to get the type information for further processing steps.
constructor TWrapper<T>.Create (var Data : T);
begin
FType := RttiCtx.GetType (TypeInfo (T));
if FType.TypeKind = tkClass then
FInstance := TObject (Data)
else if FType.TypeKind = tkRecord then
FInstance := #Data
else if FType.TypeKind = tkInterface then
begin
FType := RttiCtx.GetType (TObject (Data).ClassInfo); //<---access violation
FInstance := TObject (Data);
end
else
raise Exception.Create ('Unsupported type');
end;
I wonder if this access violation is a bug in delphi compiler (I'm using XE).
After further investigation I wrote a simple test function, which shows, that asking for the class name produces this exception as well:
procedure TestForm.FormShow (Sender : TObject);
var
TestIntf : IInterface;
begin
TestIntf := TInterfacedObject.Create;
OutputDebugString(PChar (TObject (TestIntf).ClassName)); //Output: TInterfacedObject
Test <IInterface> (TestIntf);
end;
procedure TestForm.Test <T> (var Data : T);
begin
OutputDebugString(PChar (TObject (Data).ClassName)); //access violation
end;
Can someone explain me, what is wrong? I also tried the procedure without a var parameter which did not work either. When using a non generic procedure everything works fine, but to simplify the use of the wrapper the generic solution would be nice, because it works for objects and records the same way.
Kind regards,
Christian
Your code contains two wrong assumptions:
That you can obtain meaningful RTTI from Interfaces. Oops, you can get RTTI from interface types.
That a Interface is always implemented by a Delphi object (hence your attempt to extract the RTTI from the backing Delphi object).
Both assumptions are wrong. Interfaces are very simple VIRTUAL METHOD tables, very little magic to them. Since an interface is so narrowly defined, it can't possibly have RTTI. Unless of course you implement your own variant of RTTI, and you shouldn't. LE: The interface itself can't carry type information the way an TObject does, but the TypeOf() operator can get TypeInfo if provided with a IInterface
Your second assumption is also wrong, but less so. In the Delphi world most interfaces will be implemented by Delphi objects, unless of course you obtain the interface from a DLL written in an other programming language: Delphi's interfaces are COM-compatible, so it's implementations can be consumed from any other COM-compatible language and vice versa. But since we're talking Delphi XE here, you can use this syntax to cast an interface to it's implementing object in an intuitive and readable way:
TObject := IInterface as TObject;
that is, use the as operator. Delphi XE will at times automagically convert a hard cast of this type:
TObject := TObject(IInterface);
to the mentioned "as" syntax, but I don't like this magic because it looks very counter-intuitive and behaves differently in older versions of Delphi.
Casting the Interface back to it's implementing object is also wrong from an other perspective: It would show all the properties of the implementing object, not only those related to the interface, and that's very wrong, because you're using Interfaces to hide those implementation details in the first place!
Example: Interface implementation not backed by Delphi object
Just for fun, here's a quick demo of an interface that's not backed by an Delphi object. Since an Interface is nothing but an pointer to a virtual method table, I'll construct the virtual method table, create a pointer to it and cast the the pointer to the desired Interface type. All method pointers in my fake Virtual Method table are implemented using global functions and procedures. Just imagine trying to extract RTTI from my i2 interface!
program Project26;
{$APPTYPE CONSOLE}
uses
SysUtils;
type
// This is the interface I will implement without using TObject
ITestInterface = interface
['{CFC4942D-D8A3-4C81-BB5C-6127B569433A}']
procedure WriteYourName;
end;
// This is a sample, sane implementation of the interface using an
// TInterfacedObject method
TSaneImplementation = class(TInterfacedObject, ITestInterface)
public
procedure WriteYourName;
end;
// I'll use this record to construct the Virtual Method Table. I could use a simple
// array, but selected to use the record to make it easier to see. In other words,
// the record is only used for grouping.
TAbnormalImplementation_VMT = record
QueryInterface: Pointer;
AddRef: Pointer;
ReleaseRef: Pointer;
WriteYourName: Pointer;
end;
// This is the object-based implementation of WriteYourName
procedure TSaneImplementation.WriteYourName;
begin
Writeln('I am the sane interface implementation');
end;
// This will implement QueryInterfce for my fake IInterface implementation. All the code does
// is say the requested interface is not supported!
function FakeQueryInterface(const Self:Pointer; const IID: TGUID; out Obj): HResult; stdcall;
begin
Result := S_FALSE;
end;
// This will handle reference counting for my interface. I am not using true reference counting
// since there is no memory to be freed, si I am simply returning -1
function DummyRefCounting(const Self:Pointer): Integer; stdcall;
begin
Result := -1;
end;
// This is the implementation of WriteYourName for my fake interface.
procedure FakeWriteYourName(const Self:Pointer);
begin
WriteLn('I am the very FAKE interface implementation');
end;
var i1, i2: ITestInterface;
R: TAbnormalImplementation_VMT;
PR: Pointer;
begin
// Instantiate the sane implementation
i1 := TSaneImplementation.Create;
// Instantiate the very wrong implementation
R.QueryInterface := #FakeQueryInterface;
R.AddRef := #DummyRefCounting;
R.ReleaseRef := #DummyRefCounting;
R.WriteYourName := #FakeWriteYourName;
PR := #R;
i2 := ITestInterface(#PR);
// As far as all the code using ITestInterface is concerned, there is no difference
// between "i1" and "i2": they are just two interface implementations.
i1.WriteYourName; // Calls the sane implementation
i2.WriteYourName; // Calls my special implementation of the interface
WriteLn('Press ENTER to EXIT');
ReadLn;
end.
Two possible answers.
If this always happens, even when T is an object, then it's a compiler error and you ought to file a QC report about it. (With Interfaces, the cast-an-interface-to-an-object thing requires some black magic from the compiler, and it's possible that the generics subsystem doesn't implement it properly.)
If you're taking a T that's not an object, though, such as a record type, and getting this error, then everything's working as designed; you're just using typecasts improperly.
Either way, there's a way to get RTTI information out of any arbitrary type. You know how TRttiContext.GetType has two overloads? Use the other one. Instead of calling GetType (TObject (Data).ClassInfo), try calling GetType(TypeInfo(Data)).
Oh, and declare FInstance as a T instead of a pointer. It'll save you a lot of hassle.