I'm writing a class with some virtual/abstract procedures which I expect to get overridden. However they might not be overridden as well, which is also fine. The problem is finding out whether one of these procedures was actually overridden or not. If they're not overridden, I shouldn't try to call those procedures, and need to go about something different instead. If I try to call one of them and isn't overridden, I get Abstract Error.
Is there a way I can detect whether these procedures were overridden or not?
Here's a sample of how they're declared:
type
TMyClass = class(TObject)
protected
procedure VProc; virtual;
procedure VAProc; virtual; abstract;
end;
You can do something like this. Note that I've removed "abstract". It may work with "abstract" in place, but I haven't tried that.
type
TMyClass = class(TObject)
protected
procedure VProc; virtual;
procedure VAProc; virtual; //abstract;
end;
function GetVAProcAddress(Instance: TMyClass): pointer;
var
p: procedure of object;
begin
p := Instance.VAProc;
result := TMethod(p).Code;
end;
//in one of the TMyClass methods you can now write:
if GetVAProcAddress(self) <> #TMyClass.VAProc then
I think this is the wrong approach, it smells really really bad.. Some suggestions:
Use non-abstract methods that do what you want to do when they are not overridden.
Use events instead of methods. It's easy to check whether they have been assigned.
A method that may or may not be overridden is not an abstract method, it is merely virtual.
An abstract method is one that has no base implementation and for which an implementation must be provided by a descendant.
In your case, simply declare them as virtual and provide them with the NO-OP (No operation) default implementation that your design dictates:
type
TMyBaseClass = class
protected
procedure SomeProc; virtual;
end;
procedure TMyBaseClass.SomeProc;
begin
// NO-OP
end;
Note - this illustrates my own personal convention of documenting a deliberate NO-OP, rather than just leaving an empty implementation.
Any shennanigans you go through to attempt to detect whether a declared abstract method has been overridden or not and to call it - or not - on the basis of that test is likely to cost more time than simply calling a NO-OP implementation. Plus, should you ever need to introduce an implementation in that base class you don't have to change the method from abstract to non-abstract (possibly disrupting those "detection" circuits you had w.r.t that method, and certainly rendering their cost as nothing but pure overhead).
Use RTTI to get the method address of the virtual method in question for the class in questions. Compare it to the method address for the same method of the base class. If it is not the same, then the method was overriden.
If you got a line code like this:
lObj := TMyClass.Create;
you will notice that the compiler will output a warning saying that you are constructing instance of 'TMyClass' containing abstract method TMyClass.VAProc.
Related
I have a thread library which has three constructors for three different method types. I want them to be merged into one and some kind of logic to tell them apart inside the constructor. Is it possible? As there is TValue for values and such, is there a similar thing for method types?
I have the following types supported for now;
TAgThreadMethod1 = procedure of object;
TAgThreadMethod2 = procedure;
TAgThreadMethod3 = procedure(const AThread: TAgThread) of object;
and the constructors are like so:
constructor Create(const AOnRun: TAgThreadMethod1); overload; virtual;
constructor Create(const AOnRun: TAgThreadMethod2); overload; virtual;
constructor Create(const AOnRun: TAgThreadMethod3); overload; virtual;
For reference, I don't want the user to have the ability to change the worker method at a later time after the construction at all. So if a solution exists which can do something like this in a single constructor, is also welcome;
constructor Create
(const AOnRun: [Some type which can hold arbitrary method types]);
begin
// code to identify the method contained in AOnRun.
// if supported, assign it the correct handler.
end;
There isn't any good way to do this, because the whole point of a method pointer is to be invoked at some later point, and you can't do that unless you know its signature. So losing the distinction between signatures is very counterproductive.
If you only want to have to store one type of call inside your object, you could make the three constructors each create an anonymous method with a unified signature that wraps calls to the three types, and just store that instead of having to deal with multiple distinct method types. But what you're asking for, specifically, won't work.
I think that the best that you can do is have a constructor that accepts the most general form of the procedural type. That is TProc<TAgThread>. This will be the master constructor, the only virtual constructor. You can then delegate the other constructors to the master constructors.
To recap, the declaration in SysUtils is:
type
TProc<T> = reference to procedure(Arg1: T);
So your master constructor is:
constructor Create(AOnRun: TProc<TAgThread>); overload; virtual;
The other constructors are not virtual and might look like this:
constructor Create(AOnRun: TAgThreadMethod1); overload;
Implemented as:
constructor TMyClass.Create(AOnRun: TAgThreadMethod1);
begin
Create(
procedure(Thread: TAgThread)
begin
AOnRun();
end;
);
end;
The master constructor, the virtual one, does all the work. The other constructors, the non-virtual ones, adapt their arguments to fit that of the master constructor.
It is possible, but not with only a TValue.
Instead of passing the method itself, you could pass it as a TRttiMethod like this: MyRegisterMethod(TRttiContext.GetType(TMyClassType).GetMethod('MyMethodName'));
If you want to pass all constructors you will need to use a loop like this:
for MyMethod in TRttiContext.GetType(TMyClassType).GetMethods do
if MyMethod.IsConstructor then // As in your example you only want constructors
MyRegisterMethod(MyMethod);
Later, you can call this method like this: MyRegisteredMethod.Invoke(nil, MyMethodParams). MyMethodParams is an array of TValue, which you will need to provide. This can be stored in a TValue itself (although it might be easier to just store them in a TArray). Note that you will only want to use nil in the invoke in case of a class method.
If you want to make sure the parameters are OK, you can verify them by looking them up in MyRttiMethod.GetParameters and compare their actual types to the actual types of the TValues in the array of TValue. This can also be used to find the correct method as well.
The one with virtual/dynamic
// declare in Child Class
constructor Create; virtual;
constructor TChildClass.Create;
begin
inherited;
end;
The one with override.
// declare in Child Class
constructor Create; override;
constructor TChildClass.Create;
begin
inherited;
end;
The one with nothing
// declare in Child Class
constructor Create;
constructor TChildClass.Create;
begin
inherited;
end;
Are these the same thing? It looks confusing.
Yes, there is a difference, but let's deal with the virtual keyword in more basic OOP terms first, yet still how it applies to Delphi methods.
When you declare a derived (child) class, and implement a method as "override", it means that you're overriding (surprise) the matching method of the base class.
This means that you can write code like this:
var child : TBaseClass;
begin
child := TChildClass.Create; // note that it is stored in TBaseClass variable
child.VirtualMethodDefinedInBaseClassThatHasBeenOverriddenInChildClass;
This will call the method in the child class, even if the variable is defined to be of the base class type. This is the whole purpose of virtual methods, you can access the object through a reference of a more general type, and still call methods that have been written for the particular type of object you're dealing with.
If you have a virtual method in the base class that you chose not to override in the child class, but instead reintroduce, you're effectively replacing it in some cases. Note that in most cases you need to tell the compiler that you really meant to do this, though I'm unsure about what Delphi requires here.
Basically, if your variable is of type TBaseClass, and you call a virtual method on it, that has been reintroduced in the TChildClass, it will still call the method in the base class.
However, if your variable is of type TChildClass, and you call that method on it, you will get the new method instead.
Now, for constructors, in Delphi, it is slightly different.
The point of virtual constructors is to be able to virtually construct objects, and to do that, Delphi also has "class types".
You can say this:
type TClassToUse = class of TBaseClass;
var cls : TClassToUse;
obj : TBaseClass;
begin
cls := TChildClass;
obj := cls.Create;
(note that my Delphi knowledge is a bit rusty here, if anyone spots bugs or glaring problems in the above code, please let me know or just fix it)
Here we store a "class" in a variable, and then ask the class to please construct an object for us. This allows us to switch out which class to create, but we also need to declare the constructors we want to use virtual, otherwise we will have problems.
So in the above code, if you declared the constructor as virtual in TBaseClass, and then override it in TChildClass (which the code is actually using in cls), the overridden constructor is the one that will be used.
If, on the other hand, you don't declare the constructor as virtual, we're back to the base class constructor. Virtual basically means to figure out the right method to execute at runtime, whereas non-virtual will figure it out at compile time.
Reintroduction as described for normal methods above, also works this way.
However, virtual constructors are only used as virtual when used through a class type.
No, static and virtual methods are not the same thing.
And override is a case of virtual method.
http://en.wikipedia.org/wiki/Virtual_function
http://docwiki.embarcadero.com/RADStudio/XE4/en/Methods#Method_Binding
Constructors bring nothing special here - they conform to the same rules as other methods for the question
Question about OO design.
Suppose I have a base object vehicle. And two descendants: truck and automobile.
Further, suppose the base object has a base method:
Procedure FixFlatTire(); abstract;
When the truck and automobile override the base object's, they require different information from the caller.
Am I better off overloading FixFlatTire like this in the two descendant objects:
Procedure Truck.FixFlatTire( OfficePhoneNumber: String;
NumberOfAxles: Integer): Override; Overload;
Procedure Automobile.FixFlatTire( WifesPhoneNumber: String;
AAAMembershipID: String): Override; Overload;
Or introducing new properties in each of the descendants and then setting them before calling FixFlatTire, like this:
Truck.OfficePhoneNumber := '555-555-1212';
Truck.NumberOfAxles := 18;
Truck.FixFlatTire();
Automobile.WifesPhoneNumber := '555-555-2323';
Automobile.AAAMembershipID := 'ABC';
Automobile.FixFlatTire();
If the data in question are logically part of the descendant class (if you might use them in other contexts) it makes sense to add the properties directly to the descendant class.
If they apply strictly to the FixFlatTire method, it makes more sense to specify them as arguments to that function.
If you supply a different method signature in a descendant class, that is not (to my knowledge) a case of overloading. You are simply overriding the original function with a new function with different arguments. Overloading is when two identically named functions are available, distinguished by different signatures.
[Edit]
Looking at your arguments a bit closer, here are some more thoughts:
NumberOfAxles pretty clearly seems to be a property of TTruck.
PhoneNumber and AAAMembershipID seem to me to be properties of a class you don't have yet, TOwner (with possible descendant classes TCorporateOwner and TIndividualOwner). Then, a TOwner reference can become a property of TVehicle or an argument to FixFlatTire() (although I imagine it would better belong as a property of the TVehicle).
The real error is
FixFlatTire(); abstract;
It certainly should be
FixFlatTire(); virtual; abstract;
to enable polymorphic behavior.
You should not overload virtual methods, only override preserving method's arguments.
Update
You should not and can not overload virtual methods. The following code does not even compile:
type
TVehicle = class
procedure FixFlatTire(); virtual; abstract;
end;
TTruck = class(TVehicle)
Procedure FixFlatTire( OfficePhoneNumber: String;
NumberOfAxles: Integer); Override; Overload;
end;
TAutomobile = class(TVehicle)
Procedure FixFlatTire( WifesPhoneNumber: String;
AAAMembershipID: String); Override; Overload;
end;
You should preserve virtual method's arguments while overriding the virtual method.
So the answers is - the first option (overloading) is just impossible.
I wouldn't use the latter, as it's easy for someone to set some properties and forget to later call the method. The first is a problem too though, because you don't have a matching signature for each method in the base class.
Most problems in OOP can be solved with another layer of abstraction. You could have something like this in the base class, shared by both Automobile and Truck.
procedure FixFlatTire(TireProvider: ITireProvider); abstract; virtual;
ITireProvider can be subclassed with an OfficeTireProvider and AAATireProvider, which contain the respective properties you need to set for each, and they can override any members of ITireProvider needed to do the actual tyre fixing.
The approach with properties is pointless. You can't use polymorphism properly since the functions take different parameters. You should just have distinct methods in the subclasses with the appropriate parameters.
Now, if you come back with a more complex version in which you only hold a reference to the base class then a different design would be called for, but it would be neither of the ones you have offered so far.
Rant: Should I call "inherited" in the constructor of a class derived from TObject or TPersistent?
constructor TMyObject.Create;
begin
inherited Create; // Delphi doc: Do not create instances of TPersistent. Use TPersistent as a base class when declaring objects that are not components, but that need to be saved to a stream or have their properties assigned to other objects.
VectorNames := TStringList.Create;
Clear;
end;
Yes. It does nothing, true, but it's harmless. I think there is value in being consistent about always calling the inherited constructor, without checking to see if there is, in fact, an implementation. Some will say that it's worth calling inherited Create because Embarcadero might add an implementation for TObject.Create in the future, but I doubt this is true; it would break existing code which does not call inherited Create. Still, I think it is a good idea to call it for the reason of consistency alone.
I always do this.
If you are refactoring and move code to a common ancestor, calling the inherited Create has the following advantages:
If the common ancestor has a constructor, you can't forget to call it.
If the common ancestor has a constructor with different parameters, the compiler warns you for this.
You can also override "procedure AfterConstruction". This procedure is always called, no matter what kind of constructor.
public
procedure AfterConstruction; override;
end;
procedure TfrmListBase.AfterConstruction;
begin
inherited;
//your stuff, always initialized, no matter what kind of constructor!
end;
For example: if you want to create an object with a different constructor than the normal TObject.Create, such as TComponent.Create(AOwner) or a custom (overloaded) constructor, you can get problems because your override is not called and (in this case) your "VectorNames" variable will be nil.
I call it, except when i need a very optimized constructor.
What was the motivation for having the reintroduce keyword in Delphi?
If you have a child class that contains a function with the same name as a virtual function in the parent class and it is not declared with the override modifier then it is a compile error. Adding the reintroduce modifier in such situations fixes the error, but I have never grasped the reasoning for the compile error.
If you declare a method in a descendant class that has the same name as a method in an ancestor class then you are hiding that ancestor method — meaning if you have an instance of that descendant class (that is referenced as that class) then you will not get the behavior of the ancestor. When the ancestor's method is virtual or dynamic, the compiler will give you a warning.
Now you have one of two choices to suppress that warning message:
Adding the keyword reintroduce just tells the compiler you know you are hiding that method and it suppresses the warning. You can still use the inherited keyword within your implementation of that descended method to call the ancestor method.
If the ancestor's method was virtual or dynamic then you can use override. It has the added behavior that if this descendant object is accessed through an expression of the ancestor type, then the call to that method will still be to the descendant method (which then may optionally call the ancestor through inherited).
So difference between override and reintroduce is in polymorphism. With reintroduce, if you cast the descendant object as the parent type, then call that method you will get the ancestor method, but if you access it the descendant type then you will get the behavior of the descendant. With override you always get the descendant. If the ancestor method was neither virtual nor dynamic, then reintroduce does not apply because that behavior is implicit. (Actually you could use a class helper, but we won't go there now.)
In spite of what Malach said, you can still call inherited in a reintroduced method, even if the parent was neither virtual nor dynamic.
Essentially reintroduce is just like override, but it works with non-dynamic and non-virtual methods, and it does not replace the behavior if the object instance is accessed via an expression of the ancestor type.
Further Explanation:
Reintroduce is a way of communicating intent to the compiler that you did not make an error. We override a method in an ancestor with the override keyword, but it requires that the ancestor method be virtual or dynamic, and that you want the behavior to change when the object is accessed as the ancestor class. Now enter reintroduce. It lets you tell the compiler that you did not accidentally create a method with the same name as a virtual or dynamic ancestor method (which would be annoying if the compiler didn't warn you about).
There are lots of answers here about why a compiler that lets you hide a member function silently is a bad idea. But no modern compiler silently hides member functions. Even in C++, where it's allowed to do so, there's always a warning about it, and that ought to be enough.
So why require "reintroduce"? The main reason is that this is the sort of bug that can actually appear by accident, when you're not looking at compiler warnings anymore. For example, let's say you're inheriting from TComponent, and the Delphi designers add a new virtual function to TComponent. The bad news is your derived component, which you wrote five years ago and distributed to others, already has a function with that name.
If the compiler just accepted that situation, some end user might recompile your component, ignore the warning. Strange things would happen, and you would get blamed. This requires them to explicitly accept that the function is not the same function.
The RTL uses reintroduce to hide inherited constructors. For example, TComponent has a constructor which takes one argument. But, TObject has a parameterless constructor. The RTL would like you to use only TComponent's one-argument constructor, and not the parameterless constructor inherited from TObject when instantiating a new TComponent. So it uses reintroduce to hide the inherited constructor. In this way, reintroduce is a little bit like declaring a parameterless constructor as private in C#.
First of all, "reintroduce" breaks the inheritance chain and should not be used, and I mean never ever. In my entire time I worked with Delphi (ca 10 years) I've stumbled upon a number of places that do use this keyword and it has always been a mistake in the design.
With that in mind here's the simplest way it works:
You have like a virtual method in a base class
Now you wanna have a method that has the exact same name, but maybe a different signature. So you write your method in the derived class with the same name and it will not compile because the contract is not fulfilled.
You put the reintroduce keyword in there and your base class does not know about your brand new implementation and you can use it only when accessing your object from a directly specified instance type. What that means is toy can't just assign the object to a variable of base type and call that method because it's not there with the broken contract.
Like I said it's pure evil and must be avoided at all cost (well, that's my opinion at least). It's like using goto - just a terrible style :D
The purpose of the reintroduce modifier is to prevent against a common logical error.
I will assume that it is common knowledge how the reintroduce keyword fixes the warning and will explain why the warning is generated and why the keyword is included in the language. Consider the delphi code below;
TParent = Class
Public
Procedure Procedure1(I : Integer); Virtual;
Procedure Procedure2(I : Integer);
Procedure Procedure3(I : Integer); Virtual;
End;
TChild = Class(TParent)
Public
Procedure Procedure1(I : Integer);
Procedure Procedure2(I : Integer);
Procedure Procedure3(I : Integer); Override;
Procedure Setup(I : Integer);
End;
procedure TParent.Procedure1(I: Integer);
begin
WriteLn('TParent.Procedure1');
end;
procedure TParent.Procedure2(I: Integer);
begin
WriteLn('TParent.Procedure2');
end;
procedure TChild.Procedure1(I: Integer);
begin
WriteLn('TChild.Procedure1');
end;
procedure TChild.Procedure2(I: Integer);
begin
WriteLn('TChild.Procedure2');
end;
procedure TChild.Setup(I : Integer);
begin
WriteLn('TChild.Setup');
end;
Procedure Test;
Var
Child : TChild;
Parent : TParent;
Begin
Child := TChild.Create;
Child.Procedure1(1); // outputs TChild.Procedure1
Child.Procedure2(1); // outputs TChild.Procedure2
Parent := Child;
Parent.Procedure1(1); // outputs TParent.Procedure1
Parent.Procedure2(1); // outputs TParent.Procedure2
End;
Given the above code both of the procedures in TParent are hidden. To say they are hidden means that the procedures can not be called through the TChild pointer. Compiling the code sample produces a single warning;
[DCC Warning] Project9.dpr(19): W1010 Method 'Procedure1' hides virtual method of base type 'TParent'
Why only a warning for the virtual function and not the other? Both are hidden.
A virtue of Delphi is that library designers are able to release new versions without fear of breaking the logic of existing client code. This contrasts to Java where adding new functions to a parent class in a library is fraught with danger because classes are implicitly virtual. Lets say that TParent from above lives in a 3rd party library, and the library manufacture releases the new version below.
// version 2.0
TParent = Class
Public
Procedure Procedure1(I : Integer); Virtual;
Procedure Procedure2(I : Integer);
Procedure Procedure3(I : Integer); Virtual;
Procedure Setup(I : Integer); Virtual;
End;
procedure TParent.Setup(I: Integer);
begin
// important code
end;
Imagine we had the following code in our client code
Procedure TestClient;
Var
Child : TChild;
Begin
Child := TChild.Create;
Child.Setup;
End;
For the client it does not matter if the code is compiled against version 2 or 1 of the library, in both cases TChild.Setup is called as the user intends. And in the library;
// library version 2.0
Procedure TestLibrary(Parent : TParent);
Begin
Parent.Setup;
End;
If TestLibrary is called with a TChild parameter, everything works as intended. The library designer have no knowledge of the TChild.Setup, and in Delphi this does not cause them any harm. The call above correctly resolves to TParent.Setup.
What would happen in a equivalent situation in Java? TestClient would work correctly as intended. TestLibrary would not. In Java all functions are assumed virtual. The Parent.Setup would resolve to TChild.Setup, but remember when TChild.Setup was written they had no knowledge of the future TParent.Setup, so they are certainly not going to ever call inherited. So if the library designer intended TParent.Setup to be called it will not be, no matter what they do. And certainly this could be catasrophic.
So the object model in Delphi requires explicit declaration of virtual functions down the chain of child classes. A side effect of this is that it is easy to forget to add the override modifier on child methods. The existence of the Reintroduce keyword is a convenience to the programmer. Delphi was designed so that the programmer is gently persuaded, by the generation of a warning, to explicitly state their intentions in such situations.
tl;dr: Trying to override a non-virtual method makes no sense. Add the keyword reintroduce to acknowledge that you're making a mistake.
Reintroduce tells the compiler you want to call the code defined in this method as an entry point for this class and its descendants, regardless of other methods with the same name in the ancestors’ chain.
Creating a TDescendant.MyMethod would create a potential confusion for the TDescendants in adding another method with the same name, which the compiler warns you about.
Reintroduce disambiguates that and tells the compiler you know which one to use.
ADescendant.MyMethod calls the TDescendant one, (ADescendant as TAncestor).MyMethod calls the TAncestor one. Always! No confusion…. Compiler happy!
This is true whether you want the descendant method to be virtual or not: in both cases you want to break the natural linkage of the virtual chain.
And it does not prevent you from calling the inherited code from within the new method.
TDescendant.MyMethod is virtual: ...but you cannot or don’t want to use the linkage.
You cannot because the method signature is different. You have no other choice as overriding is impossible in this case with return type or parameters not exactly the same.
You want to restart an inheritance tree from this class.
TDescendant.MyMethod is not virtual: You turn MyMethod into a static one at the TDescendant level and prevent further overriding. All classes inheriting from TDescendant will use the TDescendant implementation.
When the ancestor class also has a method with the same name, and it is not necessarily declared virtual, you would see a compiler warning (as you would hide this method).
In other words: You tell the compiler that you know that you hide the ancestor function and replace it with this new function and do so deliberately.
And why would you do this? If the method is virtual in the parent class, the only reason is to prevent polymorphism. Other then that just override and do not call inherited. But if the parent method is not declared virtual (and you cannot change that, because you do not own the code for example), you can inherit from that class and let people inherit from your class without seeing a compiler warning.
This has been introduced to the language because of Framework versions (including the VCL).
If you have an existing code base, and an update to a Framework (for instance because you bought a newer Delphi version) introduced a virtual method with the same name as a method in an ancestor of your code base, then reintroduce will allow you to get rid of the W1010 warning.
This is the only place where you should use reintroduce.
First, as it was said above, you should never ever deliberately reintroduce virtual method. The only sane use of reintroduce is when the author of the ancestor (not you) added a method that goes into conflict with your descendant and renaming your descendant method is not an option. Second, you can easily call the original version of the virtual method even in classes where you reintroduced it with different parameters:
type
tMyFooClass = class of tMyFoo;
tMyFoo = class
constructor Create; virtual;
end;
tMyFooDescendant = class(tMyFoo)
constructor Create(a: Integer); reintroduce;
end;
procedure .......
var
tmp: tMyFooClass;
begin
// Create tMyFooDescendant instance one way
tmp := tMyFooDescendant;
with tmp.Create do // please note no a: integer argument needed here
try
{ do something }
finally
free;
end;
// Create tMyFooDescendant instance the other way
with tMyFooDescendant.Create(20) do // a: integer argument IS needed here
try
{ do something }
finally
free;
end;
so what should be the purpose of reintroducing virtual method other than make things harder to read?
reintroduce allows you to declare a method with the same name as the ancestor, but with different parameters. It has nothing to do with bugs or mistakes!!!
For example, I often use it for constructors...
constructor Create (AOwner : TComponent; AParent : TComponent); reintroduce;
This allows me to create the internal classes in a cleaner fashion for complex controls such as toolbars or calendars. I normally have more parameters than that. Sometimes it is almost impossible or very messy to create a class without passing some parameters.
For visual controls, Application.Processmessages can get called after Create, which can be too late to use these parameters.
constructor TClassname.Create (AOwner : TComponent; AParent : TComponent);
begin
inherited Create (AOwner);
Parent := AParent;
..
end;