What is the difference in casting using Supports vs as keyword, besides that with as I first need to check if cast is possible with is keyword.
Supports will provide an interface if that interface is supported.
is determines if a class/interface derives from another class/interface.
as does the same thing as is, but also returns a type-checked cast.
If you have already confirmed with is that a cast will succeed, you don't need to use as, you can just do a direct cast, which is more efficient:
if(pSomeObjectRef is TMyObject) then
TMyObject(pSomeObjectRef).MyUsefulMethod(...);
As Delphi does not support multiple inheritance, using interfaces is the only way to implement that kind of behavior. An object which can be more than one thing, not just itself or its ancestors.
If you're not using interfaces, you shouldn't need to use Supports().
Using as to cast allows you to cast an object reference to an interface, as well as to a reference to a different class of object. Personally, I don't use as, and I rarely see it in the code I'm looking at. Since as can raise an exception, you ought to take steps to avoid the exceptions, and catch them if they are raised. As you can check these anyway, there should never be a need to use as.
When casting to an interface, rather than relying on the exception to catch the interface not being there, you can use the result of Supports():
if (SysUtils.Supports(pSomeObjectRef, IMyWantedInterface, diInterfaceRef)) then
begin
diInterfaceRef._AddRef(); // removed when diInterface falls out of scope
...
end
else
begin // it doesn't support the interface!
...
end;
Whether you want to catch exceptions (which some people like, some people don't - it does make the code less linear), or code for if..else, is usually a matter of preference. I prefer not to rely on exceptions (but I still have try..finally or try..except blocks), but of course other opinions are available!
Related
How can i make a typecast using the object classType parameter?
(obj as obj.classType).items[i]...//obj.classType = TList<myType>
This code is rejected by the compiler. But, moreover, I need to get access to the properties of the object, no matter what class (TList) it possesses:
system.classes.TList
system.generics.collections.TList<T>
In my program there is an object that actually belongs to the class TList<T>, and I am afraid that casting to a system.classes.TList type may cause some errors in my program.
It's impossible to typecast like this. That is because Delphi is a statically typed language.
Imagibe you could write code like this:
(obj as obj.classType).items
Since the value if obj.classType is unknown at compile time, the compiler cannot, at compile time, know whether items even exists, never mind how to access it and so on.
As for your goal regarding list classes, the generic and non-generic list types do not share a common ancestor beyond TObject. What you are attempting is simply impossible.
Whatever your problem is, you'll need to find a different solution.
Destructors in Delphi are usually named "Destroy", however as far as i understand you can also
name destructors differently
have multiple destructors
Is there any reason why this was implemented this way? What are the possible use cases for differently named / multiple destructors?
In theory you can manually call different destructors to free different external resources, like breaking ref-counting loops, deleting or just closing file, etc.
Also, since the Object Pascal language does not have those magical new/delete operations, there just should be some identifier to call for disposing of the object.
I'd prefer to look at that in retrospect.
"Turbo Pascal with Objects" style objects have both - you call a "magical" Dispose procedure but explicitly specify a destructor to call, since language itself did not knew what to choose. Similarly "magic" procedure New had to be supplied with a manually selected constructor.
http://www.freepascal.org/docs-html/rtl/system/dispose.html
http://putka.acm.si/langref/turboPascal/0547.html
http://www.freepascal.org/docs-html/rtl/system/new.html
http://putka.acm.si/langref/turboPascal/04A4.html
This however violates DRY principle: compiler knows that we are calling d-tor or c-tor, but yet we have to additionally call those "New" and "Dispose" functions. In theory that probably provided to decouple memory allocation and information feeding and combine them anyway we'd like. But i don't think this feature was actually used anything wide.
Interesting that the same design is used in Apple Objective C. You 1st allocate memory for the object and after that you call a constructor for that new instance: http://en.wikipedia.org/wiki/Objective-C#Instantiation
When that model was streamlined for Delphi few decisions was made to make things more simplified (and unified). Memory [de]allocation strategy was shifted to the class level, rather than call-site. That made the redundancy of both calling "New" and named constructor very contrast. One had to be dropped.
C++/C#/Java chosen to retain a special language-level keywords for it, using overloaded functions to provide different c-tors. Perhaps that corresponds to USA style of computer languages.
However Pascal at its core has two ideas: verbosity and small vocabulary. Arguably they can be tracked in other European-school languages like Scala. If possible, the keywords should be removed from language itself and moved to external modules - libraries that you can add or remove from project. And overloaded functions were introduced much later to the language and early preference was to surely have two differently named (self-documenting) function names.
This both ideas probably caused Delphi to remove "magic" procedures and to deduce object creation/destruction at the call-site just by used function names. If you call MyVar.Destroy then compiler looks at the declaration of .Destroy and knows we are deleting the object. Similarly it knows TMyType.CreateXXX(YYY,ZZZ) is an object instanbtiation due to the way CreateXXX was declared.
To make c-tor and d-tor no-named like in C++, Delphi would have to introduce two more keywords to the language level, like those C++ new and delete. And there seems to be no clear advantage in that. At least personally i better like Delphi way.
PS. I had to add there one assumption: we are talking about real C++ and Delphi languages as they were around 1995. They only featured manual memory control for heap-allocated objects, no garbage collection and no automatic ref-counting. You could not trigger object destruction by assigning variable with nil/NULL pointer.
I'm fairly new to Delphi and have been doing all my memory management manually, but have heard references to Delphi being able to use interfaces to do reference counting and providing some memory management that way. I want to get started with that, but have a few questions.
Just generally, how do I use it. Create the interface and the class implementing it. Then anytime I need that object, have the variable actually be of the Interface type, but instantiate the object and presto? No nee to think about freeing it? No more try-finallys?
It seems very cumbersome to create a bunch of interfaces for classes that really don't need them. Any tips on auto generating those? How do I best organize that? Interface and class in the same file?
What are common pitfalls that might cause me grief? Ex: Does casting the interfaced object to the an object of its class break my reference counting? Or are there any non-obvious ways Delphi would create reference loops? (meaning besides A uses B uses C uses A)
If there are tutorials that cover any of this, that would be great, but I didn't come up with anything in my searches. Thanks.
I am currently working with a very large project that takes advantage of the "side affect" of interface reference counting for the purpose of memory management.
My own personal conclusion is that you end up with a lot of code that is overly complex for no better reason than, "I don't have to worry about calling free"
I would strongly advise against this course of action for some very basic reasons:
1) You are using a side affect that exists for the purpose of COM compatibility.
2) You are making your object footprint and efficiency heavier. Interfaces are pointers to lists of pointers.. or something along those lines.
3) Like you stated... you now have to make piles of interfaces for the sole purpose of avoiding freeing memory yourself... this causes more trouble than it's worth in my opinion.
4) Most common bug that will be a HUGE pain to debug will become when an object gets freed, before it's reference. We have special code in our own reference counting to try and test for this problem before software goes out the door.
Now to answer your questions.
1) Given TFoo and interface IFoo you can have a method like the following
function GetFoo: IFoo;
begin
Result := (TFoo.Create as IFoo);
end;
...and presto, you don't need the finally to free it.
2) Yes like I said, you think it's a great idea, but it turns into a huge pain in the bupkis
3) 2 problems.
A) you have Object1.Interface2 and Object2.Interface1... these objects will never be freed due to the circular reference
B) Freeing the object before all the references are released, I cannot stress how dificult these bugs are to track down...
The most common complaint leading to the desire for "automatic garbage collection" in Delphi is the way that even short-lived temporary objects have to be disposed of manually and that you have to write a fair amount of "boiler-plate" code to ensure that this takes place when exceptions occur.
For example, creating a TStringList for some temporary sorting or other algorithmic purpose within a procedure:
procedure SomeStringsOperation(const aStrings: TStrings);
var
list: TStringList;
begin
list := TStringList.Create;
try
:
// do some work with "list"
:
finally
list.Free;
end;
end;
As you mentioned, objects that implement the COM protocol of reference counted lifetime management avoid this by cleaning themselves up when all references to them have been released.
But since TStringList isn't a COM object, you cannot enjoy the convenience this offers.
Fortunately there is a way to use COM reference counting to take care of these things without have to create all new, COM versions of the classes you wish to use. You don't even need to switch to an entirely COM based model.
I created a very simple utility class to allow me to "wrap" ANY object inside a lightweight COM container specifically for the purpose of getting this automatic cleanup behaiour. Using this technique you can replace the above example with:
procedure SomeStringsOperation(const aStrings: TStrings);
var
list: TStringList;
begin
AutoFree(#list);
list := TStringList.Create;
:
// do some work with "list"
:
end;
The AutoFree() function call creates an "anonymous" interfaced object that is Release()'d in the exit code generated by the compiler for the procedure. This autofree object is passed a pointer to the variable that references the object you wish to be free'd. Among other things this allows us to use the AutoFree() function as a pseudo-"declaration", placing any and ALL AutoFree() calls at the top of the method, as close as possible to the variable declarations that they reference, before we have even created any objects.
Full details of the implementation, including source code and further examples, are on my blog in this post.
The memory management of interfaces is done through implementation of _AddRef and _Release which are implemented by TInterfacedObject.
In general using interfaces to make memory management less cumbersome can be a nice idea, but you need to take care of these things:
Make sure the classes that implement interfaces are derived from TInterfacedObject or roll your own ancestor class that provides good implementations for _AddRef and _Release
Use either/or: so either user interfaces references, or use object instance references, don't mix them. That can be problematic when implementing interfaces in components (as those derive from TComponent, not TInterfacedObject)
Don't go the TInterfacedComponent way as that mixes Owner based memory management and _AddRef/_Release based memory management
Watch circular interface references (you can go around implementing "weak interface references" mentioned here and implemented here)
You need to maintain extra code as you need to define interfaces for the parts your classes that you want to expose, and keep those two in sync (you could Model Maker Code Explorer for this; it allows you to extract interfaces and in general boost your development because it manages the interface/implementation parts of code in single-actions)
You need some extra plumbing to create instances of the underlying classes. You can use the factory pattern for that.
That is not always effectively, but does answer a few of your underlying questions.
Shortest possible answer: The default delphi memory model is that owners free the objects they own. All other references are weak references and must let go before the owner does. "Sharing" an object that has a lifetime shorter than the entire lifetime of the app is rarely done. Reference counting is rarely done, and when it is done, it is only done by experts, or else it adds more bugs and crashes than it solves.
Learn idiomatic delphi style and try to imitate it, don't fight the grain. Sadly, people think that "program against interfaces, not implementations" means "Use IUnknown everywhere". That's not true. I recommend you don't use COM IUnknown interfaces, and use abstract base classes instead. The only thing you can't do is implement two abstract base classes in a single class, and the need for that is rare.
Update: I've recently found it helpful to use COM Interfaces (IUnknown based) to help me separate out my model and controller implementations from my UI classes. So I do find using IUnknown based interfaces useful. But there is not a lot of documentation and prior art out there to base your efforts on. I'd like to see a "cookbook" style recipe that lays all this out for people, so they can work without the usual problem of combining interface and non-interface based lifetime management, and all the trouble that comes while you get used to that extra complexity.
Switching to interfaces only for avoiding manual Free's is senseless. Little economy in Free/try-finally lines will hardly compensate the necessity of declaring both g/setters and properties in the interface not mentioning the necessity of keeping the intf/class declarations in sync. Interfaces also bring performance loss due to implicit finalize code and reference counting. If performance is not the main point and all you want to achieve is autofreeing, I'd recommend using some universal interface wrappers like the one Deltics suggested.
I have been reading up on the c++ auto_ptr and unique_ptr and stuff and thought to try and use them in a class I am playing with... but I was having trouble getting it to work...
How would I convert these pointers to auto pointers or some equivalent so the deletion of the pointers is handled automatically?
Header - http://ideone.com/Z9bc5
Body - http://ideone.com/WfwBY
At the moment it is working using normal pointers but I sometimes get a access violation error. I am pretty sure I know what it causing it.. but the "best" way might be to use the automatic deletion stuff recently added to c++11?
Thanks in advance.
Don't use auto_ptr. Try one of unique_ptr or shared_ptr. Here's Sutter explaining when to use which:
When in doubt, prefer unique_ptr by default, and you can always later move-convert to shared_ptr if you need it. If you know from the start you need shared ownership, however, go directly to shared_ptr via make_shared (see #2 below).
Also from his blog-post:
3. What’s the deal with auto_ptr?
auto_ptr is most charitably characterized as a valiant attempt to
create a unique_ptr before C++ had move semantics.
auto_ptr is now deprecated, and should not be used in new code. When
you get a chance, try doing a global search-and-replace of auto_ptr to
unique_ptr in your code base; the vast majority of uses will work the
same, and it might expose (as a compile-time error) or fix (silently)
a bug or two you didn’t know you had.
So, your member declarations change from:
sf::Texture * tSpriteSheet;
to:
std::unique_ptr<sf::Texture> tSpriteSheet;
As for member functions which return a raw pointer you have but the obvious choice: You cannot return a unique_ptr if the class is not movable. So, you can either:
Keep the signature as-is
Return a const& unique_ptr<T>
Return a reference to the object
Choose the one that suits your needs the best.
I have used Delphi classes for a while now but never really got into using interfaces. I already have read a bit about them but want to learn more.
I would like to hear which pros and cons you have encountered when using interfaces in Delphi regarding coding, performance, maintainability, code clearness, layer separation and generally speaking any regard you can think of.
All I can think of for now:
Pros:
Clear separation between interface and implementation
Reduced unit dependencies
Multiple inheritance
Reference counting (if desired, can be disabled)
Cons:
Class and interface references cannot be mixed (at least with reference counting)
Getter and setter functions required for all properties
Reference counting does not work with circular references
Debugging difficulties (thanks to gabr and Warren for pointing that out)
Adding to the answers few more advantages:
Use interfaces to represent the behavior and each implementation of a behavior will implement the interface.
API Publishing: Interfaces are great to use when publishing APIs. You can publishing an interface without giving out the actual implementation. So you are free to make internal structural changes without causing any problems to the clients.
All I say is that interfaces WITHOUT reference counting are VERY HIGH on my wishlist for delphi!!!
--> The real use of interfaces is the declaration of an interface. Not the ability for reference counting!
There are some SUBTLE downsides to interfaces that I don't know if people consider when using them:
Debugging becomes more difficult. I have seen a lot of strange difficulties stepping into interfaced method calls, in the debugger.
Interfaces in Delphi come with IUnknown semantics, if you like it or not, you'r stuck with reference counting being a supported interface. And, thus, with any interfaces created in Delphi's world, you have to be sure you handle reference counting correctly, and if you don't, you'll end up with leaks. When you want to avoid reference counting, your only choice is to override addref/decref and don't actually free anything, but this is not without its own problems. I find that the more heavily interface-laden codebases have some of the hardest-to-find access violations, and memory leaks, and this is, I think because it is very difficult to combine the refcount semantics, and the default delphi semantics (owner frees objects, and nobody else does, and most objects live for the entire life of their parents.).
Badly-done implementations using Interfaces can contribute some nasty code-smells. For example, Interfaces defined in the same unit that defines the initial concrete implementation of a class, add all the weight of interfaces, without really providing proper separation between the users of the interfaces and the implementors. I know this isn't a problem with interfaces themselves, but more of a quibble with those who write interface-based code. Please put your interface declarations in units that only have those interface declarations in them, and avoid unit-to-unit dependency hell caused by glomming your interface declarations into the same units as your implementor classes.
I mostly use interfaces when I want objects with different ancestry to offer a common service. The best example I can think of from my own experience is an interface called IClipboard:
IClipboard = interface
function CopyAvailable: Boolean;
function PasteAvailable(const Value: string): Boolean;
function CutAvailable: Boolean;
function SelectAllAvailable: Boolean;
procedure Copy;
procedure Paste(const Value: string);
procedure Cut;
procedure SelectAll;
end;
I have a bunch of custom controls derived from standard VCL controls. They each implement this interface. When a clipboard operation reaches one of my forms it looks to see if the active control supports this interface and, if so, dispatches the appropriate method.
For a very simple interface you can do this with an of object event handler, but once it gets sufficiently complex an interface works well. In fact I think that is a very good analogue. Use an interface where you a single of object event won't fit the functionality.
Interfaces solves a certain kind of issues. The primary function is to... well, ...define interfaces. To distinguish between definition and implementation.
When you want to specify or check if a class supports a set of methods - use interfaces.
You cannot do that in any other way.
(If all classes inherits from the same base class, then an abstract class will define the interface. But when you are dealing with different class hierarchies, you need interfaces to define the methods thy have in common...)
Extra note on
Cons: Performance
I think many people are too blithely dismissing the performance penalty of interfaces. (Not that I don't like and use interfaces but you should be aware of what you are getting into). Interfaces can be expensive not just for the _AddRef / _Release hit (even if you are just returning -1) but also that properties are REQUIRED to have a Get method. In my experience, most properties in a class have direct access for the read accessor (e.g., propery Prop1: Integer read FProp1 write SetProp1). Changing that direct, no penalty access to a function call can be significant hit on your speed (especially when you start adding 10s of property calls inside a loop.
For example, a simple loop using a class
for i := 0 to 99 do
begin
j := (MyClass.Prop1 + MyClass.Prop2 + MyClass.Prop3) / MyClass.Prop4;
MyClass.Update;
// do something with j
end;
goes from 0 function calls to 400 function calls when the class becomes an interface. Add more properties in that loop and it quickly gets worse.
The _AddRef / _Release penalty you can ameliorate with some tips (I am sure there are other tips. This is off the top of my head):
Use WITH or assign to a temp variable to only incur the penalty of one _AddRef / _Release per code block
Always pass interfaces using const keyword into a function (otherwise, you get an extra _AddRef / _Release occurs every time that function is called.
The only case when we had to use interfaces (besides COM/ActiveX stuff) was when we needed multiple inheritance and interfaces were the only way to get it. In several other cases when we attempted to use interfaces, we had various kinds of problems, mainly with reference counting (when the object was accessed both as a class instance and via interface).
So my advice would be to use them only when you know that you need them, not when you think that it can make your life easier in some aspect.
Update: As David reminded, with interfaces you get multiple inheritance of interfaces only, not of implementation. But that was fine for our needs.
Beyond what others already listed, a big pro of interfaces is the ability of aggregating them.
I wrote a blog post on that topic a while ago which can be found here: http://www.nexusdb.com/support/index.php?q=intf-aggregation (tl;dr: you can have multiple objects each implementing an interface and then assemble them into an aggregate which to the outside world looks like a single object implementing all these interfaces)
You might also want to have a look at the "Interface Fundamentals" and "Advanced Interface Usage and Patterns" posts linked there.