Mutley2003
asked on
Using Interfaces to simplify my life
Hi all
I am still struggling to see how I can use Interfaces and their reference counting to simplify my life.
Suppose I have a class ThandyDate
The traditional object way requires me typing
var myHandyDate : ThandyDate;
begin
myHandyDate := ThandyDate.create;
.. do something with myHandyDate
myHandyDate.Free;
end;
whereas (if I understand it correctly), converting THandyDate to an interface IHandyDate will allow me to simplify this to
var myHandyDate : IhandyDate;
begin
.. do something with myHandyDate
end;
no constructor and no destructor calls, because of reference counting.
Is it that simple? and how can I convert my ThandyDate class below to an interface ?
(************************* ********** ********** ********** ********)
(* TdsHandyDate *)
(* *)
(* Easy to use date with Year,Month,Day properties *)
(************************* ********** ********** ********** ********)
type
TdsHandyDate = class(Tobject)
private
fYear,fMonth,fDay : Word;
fDate : TdateTime;
procedure _setDateTime(newDateTime:T dateTime);
public
procedure Clear;
procedure SetDate(const Year,Month,Day:word);
property asDateTime : TdateTime read fDate write _setDateTime;
property Year : Word read fYear;
property Month : Word read fMonth;
property Day : Word read fDay;
end;
thanks
I am still struggling to see how I can use Interfaces and their reference counting to simplify my life.
Suppose I have a class ThandyDate
The traditional object way requires me typing
var myHandyDate : ThandyDate;
begin
myHandyDate := ThandyDate.create;
.. do something with myHandyDate
myHandyDate.Free;
end;
whereas (if I understand it correctly), converting THandyDate to an interface IHandyDate will allow me to simplify this to
var myHandyDate : IhandyDate;
begin
.. do something with myHandyDate
end;
no constructor and no destructor calls, because of reference counting.
Is it that simple? and how can I convert my ThandyDate class below to an interface ?
(*************************
(* TdsHandyDate *)
(* *)
(* Easy to use date with Year,Month,Day properties *)
(*************************
type
TdsHandyDate = class(Tobject)
private
fYear,fMonth,fDay : Word;
fDate : TdateTime;
procedure _setDateTime(newDateTime:T
public
procedure Clear;
procedure SetDate(const Year,Month,Day:word);
property asDateTime : TdateTime read fDate write _setDateTime;
property Year : Word read fYear;
property Month : Word read fMonth;
property Day : Word read fDay;
end;
thanks
Actually, it can be a bit easier...
var
Date:IHandyDate;
begin
IDate:=THandyDate.Create; // Create the interface object.
IDate.Clear; // Use it...
IDate := nil; // Free it...
end;
If you have defined IDate as a local variable (Part of a function or procedure) then Delphi's garbage collection will free it for you. However, don't always count on it since nasty errors could occur that way. For global interface variables, you will have to free them though. And also in some other cases the garbage collection won't notice certain interfaces that need to be freed.
Interfaces don't free you from the create/destroy actions. However, interfaces are more aware of when they should be freed. As long as at least one variable is looking at the object, it will not be freed. Furthermore, you cannot mix interfaces and classes that easily. If you have:
ADate:THandyDate;
IDate:IHandyDate;
Then some people are tempted to write:
IDate := ADate;
But then if you assign Nil to IDate, the object is freed because the reference counter is out of sync. Thus the use of ADate after this will generate an exception. Also, if you use this:
ADate.GetInterface(IHandyD ate, IDate);
ADate.Free;
Then you're in trouble again. IDate just doesn't know that the object has been destroyed thus if you assign nil to it, another exception will occur because it tries to decrease a non-existing reference counter...
Still, I love the use of interfaces. I often work with lists, dynamic arrays of interface and add/delete interfaces to this list. And I could remove items from this list even while some other function was still using the object. They would just be removed when they are not in use anymore. Objects would just be removed, passed around, added again, handled some more until nothing is using them anymore. Then it's freed.
And this special freeing mechanism does make life a lot simpler...
var
Date:IHandyDate;
begin
IDate:=THandyDate.Create; // Create the interface object.
IDate.Clear; // Use it...
IDate := nil; // Free it...
end;
If you have defined IDate as a local variable (Part of a function or procedure) then Delphi's garbage collection will free it for you. However, don't always count on it since nasty errors could occur that way. For global interface variables, you will have to free them though. And also in some other cases the garbage collection won't notice certain interfaces that need to be freed.
Interfaces don't free you from the create/destroy actions. However, interfaces are more aware of when they should be freed. As long as at least one variable is looking at the object, it will not be freed. Furthermore, you cannot mix interfaces and classes that easily. If you have:
ADate:THandyDate;
IDate:IHandyDate;
Then some people are tempted to write:
IDate := ADate;
But then if you assign Nil to IDate, the object is freed because the reference counter is out of sync. Thus the use of ADate after this will generate an exception. Also, if you use this:
ADate.GetInterface(IHandyD
ADate.Free;
Then you're in trouble again. IDate just doesn't know that the object has been destroyed thus if you assign nil to it, another exception will occur because it tries to decrease a non-existing reference counter...
Still, I love the use of interfaces. I often work with lists, dynamic arrays of interface and add/delete interfaces to this list. And I could remove items from this list even while some other function was still using the object. They would just be removed when they are not in use anymore. Objects would just be removed, passed around, added again, handled some more until nothing is using them anymore. Then it's freed.
And this special freeing mechanism does make life a lot simpler...
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I just gotta give the points to Madshi on this .. that is SO COOL!!!!
"
procedure bla;
begin
NewHandyDate.Whatever;
end;
....... but interfaces are nicer to use, because you don't need to care about freeing them. And construction can be easily wrapped up in a function like "NewHandyDate".
"
YEAH. So NO var declaration., NO CREATE, no FREE
So I can do
procedure bla2;
begin
with NewHandyDate do begin
something;
somethingelse;
end; // end with
// NewHandyDate goes out of scope here and is freed (I hope)
end;
This is nice clean minimalist code.
GREAT!
that's wrong, because you've to Create the object prior to interface querying.
some of the advantages of using interfaces:
1- implementing multiple inheritance.
2- procedure calling across modules.
3- object interactions in a distributed object model.
usually you've to declare the interface then implement it in other objects., objects that implements an interface should implement all of it's methods(interface methods).
however, only methods and properties are allowed in interface decalaration.
interface methods are abstract,. thus they're useless unless implemented by some objects.
in your case you may declare IhandyDate as follows(using only 2 methods as an example):
however you can add your other properties to IHandyDate but you hv to declare their Get and Set procedures then implement them in THandyDate following the same way.
type
IHandyDate = interface
procedure Clear;
procedure SetDate(const Year,Month,Day:word);
end;
//as you can see there's no fields
//also visibility specifiers and storage specifiers are not allowed.
//then implement IHandyDate in THandyDate
//TInterfacedObject is base class for Delphi classes that support interfaces
THandyDate = class(TInterfacedObject, IHandyDate)
public
procedure Clear;
procedure SetDate(const Year,Month,Day:word);
end;
procedure THandyDate.Clear;
begin
end;
procedure SetDate(const Year,Month,Day:word);
begin
end;
//now how could we use that interface;
//1st Create a THandyDate object then query it for IHandyDate
var
ADate:THandyDate;
IDate:IHandyDate;
ADate:=THandyDate.Create;
ADate.GetInterface(IHandyD
then you can use IDate to call methods implemented by THandyDate.