BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  how to specify an object as a parameter of a procedure          BorlandTalk.com Forum Index -> Delphi OO design View previous topic :: View next topic   Author Message Gerhard Venter Guest Posted: Tue Nov 18, 2003 6:17 pm    Post subject: how to specify an object as a parameter of a procedure When you pass an object as a parameter of a procedure, and in the procedure you will change a property of the object do you specify the Object parameter as var or as const or nothing? I know that even if it is specified as const you can still change the properties of the object but what I want to know is why or in what case would you specify an object parameter as const? Do you need to specify it at all? (because objects are dynamic pointers in Object Pascal and is it implied that they are var? Thanks Gerhard Back to top Eric Hill Guest Posted: Tue Nov 18, 2003 6:38 pm    Post subject: Re: how to specify an object as a parameter of a procedure Quote: When you pass an object as a parameter of a procedure, and in the procedure you will change a property of the object do you specify the Object parameter as var or as const or nothing? Typically nothing. An object variable is a special type of pointer under the hood. You're basically passing a pointer to the object into the routine. Quote: I know that even if it is specified as const you can still change the properties of the object but what I want to know is why or in what case would you specify an object parameter as const? "const" parameters are used so the compiler can do a few optimizations (like on string handling) and to ensure that you don't change the value passed into the routine. Using a "const" on an object pointer ensures that you can't change the pointer, but doesn't specify anything about the contents (the object) of the pointer. As a general rule, you shouldn't pass objects with the "const" parameter. Quote: Do you need to specify it at all? (because objects are dynamic pointers in Object Pascal and is it implied that they are var? Just realize that an object passed into a routine without the "var" keyword isn't a "var" because you're not changing the pointer, you're changing the values of the object that the pointer points to. <g> Eric Back to top Gerhard Venter Guest Posted: Tue Nov 18, 2003 7:03 pm    Post subject: Re: how to specify an object as a parameter of a procedure Thanks Eric (second time you have helped me to clarify my own interpretations, previously with RO ). It is like looking at a word sometimes and the spelling looks wrong but yet when you look it up it is correct. And then there is the English language - not a very descriptive language like many others :) One more question : So if an object has the "var" keyword does it mean that I can change the pointer? If that is true what will be an example when you want to do that for instance? By the way I decided to and have written an Indy framework with persistent connections for my messaging project. Gerhard "Eric Hill" <eric (AT) ijack (DOT) net> wrote Quote: When you pass an object as a parameter of a procedure, and in the procedure you will change a property of the object do you specify the Object parameter as var or as const or nothing? Typically nothing. An object variable is a special type of pointer under the hood. You're basically passing a pointer to the object into the routine. I know that even if it is specified as const you can still change the properties of the object but what I want to know is why or in what case would you specify an object parameter as const? "const" parameters are used so the compiler can do a few optimizations (like on string handling) and to ensure that you don't change the value passed into the routine. Using a "const" on an object pointer ensures that you can't change the pointer, but doesn't specify anything about the contents (the object) of the pointer. As a general rule, you shouldn't pass objects with the "const" parameter. Do you need to specify it at all? (because objects are dynamic pointers in Object Pascal and is it implied that they are var? Just realize that an object passed into a routine without the "var" keyword isn't a "var" because you're not changing the pointer, you're changing the values of the object that the pointer points to. Eric Back to top Mauricio Magni Guest Posted: Tue Nov 18, 2003 7:04 pm    Post subject: Re: how to specify an object as a parameter of a procedure Hello Eric "Eric Hill" <eric (AT) ijack (DOT) net> wrote Quote: As a general rule, you shouldn't pass objects with the "const" parameter. I usually pass objects as const, in order to avoid confuse my self and assign a new object reference to the parameter passed. I think there is no cost associated with const parameters. Can you explain me why I shouldn't do this? Thanks a lot. Mauricio. Back to top Eric Hill Guest Posted: Tue Nov 18, 2003 8:25 pm    Post subject: Re: how to specify an object as a parameter of a procedure Quote: One more question : So if an object has the "var" keyword does it mean that I can change the pointer? If that is true what will be an example when you want to do that for instance? The use of var to pass an object back is not very common, but is useful in some situations. If you need to have a routine instantiate many variables at once, this makes more sense: procedure MakeInstances(var ObjLeft, ObjRight, ObjTop, ObjBottom: TSide); It's not a great example though as you would more commonly have a TBox object that has those four sides as properties :) Quote: By the way I decided to and have written an Indy framework with persistent connections for my messaging project. Cool. I just started using the NexusDB transport system for just such a need - it has been designed around messaging from the ground up so it lends itself rather well to that functionality :) Eric Back to top Eric Hill Guest Posted: Tue Nov 18, 2003 8:27 pm    Post subject: Re: how to specify an object as a parameter of a procedure Quote: I usually pass objects as const, in order to avoid confuse my self and assign a new object reference to the parameter passed. I think there is no cost associated with const parameters. Can you explain me why I shouldn't do this? I can't tell you you /shouldn't/ do this since it's not hurting you. <g> I can only tell you that it's /more common/ to use the (AObject: TObject) syntax over (const AObject: TObject). 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