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GiantMatrix

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Delphi records question

Hi All,

I have the following block of code which I am having difficulties understanding it, so I will appreciate it so much if someone can please help me understand it

======================================
type TFileStatus = record
  Name: PChar;
  Category: PChar;    
  Files_Entres: DWORD;
  Size: INT64;
  Size_NA: PChar;
  Status: boolean;
  Link: DWORD;
end;
type TPtrFileStatus = ^TFileStatus;
type PtrFileStatus = array[1..65536] of ^TFileStatus;
======================================

A. Why (  type TPtrFileStatus = ^TFileStatus;  ) was written before (   type PtrFileStatus = array[1..65536] of ^TFileStatus;  )? In other words why it wasn't wrote as:

type TFileStatus = record
  Name: PChar;
  Category: PChar;    
  Files_Entres: DWORD;
  Size: INT64;
  Size_NA: PChar;
  Status: boolean;
  Link: DWORD;
end;
type PtrFileStatus = array[1..65536] of ^TFileStatus;
?


B. What is the value of writing:
type PtrFileStatus = array[1..65536] of ^TFileStatus;
instead of
type PtrFileStatus = array[1..65536] of TFileStatus;

I mean why a pointer to TFileStatus was used in this line?

Thanks in advance
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AmigoJack

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GiantMatrix

ASKER

AmigoJack, D-Master,

Thanks you for your help. I've read AmigoJack reply in addition to the article posted by D-Master. Although I am a bit confused about two issues:

A. Now for the line:

type PtrFileStatus = array[1..65536] of ^TFileStatus;

If I understood this line correctly here the programmer is trying to declare a new array type (PtrFileStatus) which is basically of type (^TFileStatus) pointer to TFileStatus....so each cell in PtrFileStatus will represent a pointer (^TFileStatus)....is this correct?


B. Also I noticed in the source code that the programmer wrote after that in the implementation of the unit:

var
FileStatusV: PtrFileStatus;

begin
New(FileStatusV [i]);
FileStatusV[i].Files_Entres := 0;
FileStatusV[i].Size := 0;
FileStatusV[i].Status := false;
FileStatusV[i].Link := i;

So is this normal taking in consideration that PtrFileStatus is just a pointer array?

Thanks in advance for your help
A: ya it's correct >>each cell in PtrFileStatus will represent a pointer (^TFileStatus)....

B: ya it is normal to... also you must now that you are dealing with the (value "not address") of which the pointer in FileStatusV is pointing to...
D-Master,

Thanks again for your reply and help. Well there is something I am missing here as I am still a bit confused so I am sorry if I am going to ask again

Based on the tutorial link you provided earlier. A pointer is defined and used as follows:

Var
   pIntValue : ^integer; //Define a pointer of type integer
begin
   pIntValue := @iValue; //Assign the memory address of iValue to pIntValue
   j:= pIntValue^; //Assign the value inside the pointer memory address to J



Now in the code I provided above

type
    PtrFileStatus = array[1..65536] of ^TFileStatus;  //Defined a new array of pointers to TFileStatus record

var
    FileStatusV: PtrFileStatus; //Define a new var of " type pointer array"

begin
New(FileStatusV [i]);
FileStatusV[i].Files_Entres := 0;
FileStatusV[i].Size := 0;
FileStatusV[i].Status := false;
FileStatusV[i].Link := i;

So since FileStatusV is simply a new var of type pointer array, how we can use it directly (assign values to it)?

Thanks in advance
Hi,

It is Delphi compiler which makes your life easier and understands that you should have actually written:

FileStatusV[i]^.Files_Entres := 0;

Regards, Geo
as Geo said...
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FileStatusV itself is an array, so we use array syntax:

FileStatusV[1]   to access element number 1. and so on...

the elements themselves are pointers, so when we are accessing the element, we have to touch it like that

FileStatusV[1]^.Size    to access the actual content where the pointer refers to.

what youve written is correct. however, understanding pointers is not the easiest part of programming languages - it takes enough practice to fully know how to handle them. in delphi, this is even simpler than in c++.

when you say

type PInteger = ^Integer;

you create a pointer-type to an integer. if youre defining your own datatype/record, the compiler allows you to define ^MyNewType even before you define your MyNewType. this is needed because otherwise both definitions would require each other - a circular problem.

any other questions about pointers?
Hi guys,

Thank you all for your detailed explanation and outstanding help.

Well, the last thing I could say here to close this question:

A. Where I can find a detailed / good tutorial about pointers in Delphi? Beside the delphi.about.com one.

B. Where i can find a reference to all of those things Delphi compiler makes for me to make my life easier? Like accepting FileStatusV[i].Files_Entres := 0; instead of FileStatusV[i]^.Files_Entres := 0;

Thank you all for your time and help
?? not sure about tutoral or info about delphi pointer rules and use, in an overall programming instruction, , seems like some I've seen are about specific issues with certain pointer methods. . . . there are some helpful stuff in the Delphi Help, you may look at index  "Overview of pointers", "About pointer types", "pointer operators" , "pointer and pointer types", "Data types and variables" and others. . . .
you may look at the delphizeus about PChar pointers at
http://www.angelfire.com/hi5/delphizeus/useapi.html
>>
B. Where i can find a reference to all of those things Delphi compiler makes for me to make my life easier? Like accepting FileStatusV[i].Files_Entres := 0; instead of FileStatusV[i]^.Files_Entres := 0;
<<

this is dangerous! when it comes down to advanced projects, i had memory errors and couldnt resolve them. then i turned off compiler options that seem to be "user friendly" - and guess what: got about 10 compiler errors. but that were good ones, they forced me to explicitly write code right. and after that i had no more problems with my program. (i use delphi 5; also found out that "code optimization" is rather hazadous)
I could answer something similar as above but there is no point. If you want to make your code easier to use, and with less memory errors then STOP using arrays and pointers directly. Especially if you are not comfortable with the difficulties.

Delphi is an object oriented language so make use of it.

Use a TList (read the help) not an array.
Use TObject, not Records. Then the memory is managed cleanly for you including initialization of data and finalization of data with Create & Destroy destructors.

use strings, not pchars where you can.

If this is all existing code you are maintaining then this wont help much, but if you are writing new code then make use of delphi as an oo language not a Ansi C alternative.

regards