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Delphi records question

Posted on 2006-05-08
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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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Question by:GiantMatrix
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13 Comments
 
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Accepted Solution

by:
AmigoJack earned 672 total points
ID: 16630116
A.) can be omitted. its never used. you can write it like you suggested yourself
B.) mostly to avoid allocating too much unneeded memory. a pointer has only 4 bytes in size; the type itself has a size summed up by all sizes of its members. a good reason is that there will not always be 65536 TFileStatus which must be present. instead the memory for each one will be allocated dynamically (when its needed) - and the pointer to that memory is then put into one of the arrays position
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Assisted Solution

by:D-Master
D-Master earned 664 total points
ID: 16630212
hi,

you can check this website to learn more about pointers and their uses

http://delphi.about.com/od/beginners/l/aa011601a.htm
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Author Comment

by:GiantMatrix
ID: 16630863
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
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Expert Comment

by:D-Master
ID: 16631010
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...
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Author Comment

by:GiantMatrix
ID: 16631251
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
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Expert Comment

by:geobul
ID: 16631384
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
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Expert Comment

by:D-Master
ID: 16631647
as Geo said...
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Assisted Solution

by:Slick812
Slick812 earned 664 total points
ID: 16632969
if you are new to using pointers and pointer types, it can be confusing, and unclear, because you are used to variables that already have memory assigned to them. . .

below is how I might try and assign these Types -->

type // only need one type declaration
  PFileStatus = ^TFileStatus;
{ In the "Usual" code format style of the Delphi Pascal in the Borland created Delphi files,
  you should always declare a "Pointer" type for any type you declare, it has a "P"
  as the first Char, to desinate a pointer type}
  TFileStatus = record {the usuall name for TYPEs has a capital "T" as the first Char to designate a TYPE}
    pName: PChar; // small p indicates a pointer type
    pCategory: PChar;    
    Files_Entres: Cardinal;
    Size: Int64;
    pSize_NA: PChar;
    Status: Boolean;
    Link: Cardinal;
    end;

  PAryPFileStatus = ^TAryPFileStatus; { even if not nessary in your code, you may as well declare a pointer type}
  TAryPFileStatus = Array[1..65536] of PFileStatus; { using a PFileStatus (not a ^TFileStatus) can help in IDE code completion
                       (and maybe some function parameter delarations), even though it amounts to the same thing for using this}
  { the designation  PtrFileStatus  seems confusing to me, if I saw it I would think it is a Pointer type, but it is not, ,
     and it has NO indication that it is an Array }

var
FileStatusV: TAryPFileStatus;
{ the above is an Array of Pointers (PFileStatus), there is NO memory allocated for each TFileStatus record that the PFileStatus points to }

aryFileStatus: array[1..65536] of TFileStatus; // not Pointer type here
{ the above is an Array records, and ALL of the memory for ALL of the records has been allocated, there are memory blocks to recieve data}

begin
{IMPORTANT, for PointerTypes like ^TFileStatus (PFileStatus) you MUST allocate memory inorder to use them!
 you can call New or GetMem or AllocMem to set a memory block for the pointer type - OR - you can set the pointer address
 to a valid record (as pIntValue := @iValue; ), but this is tricky for an array of pointer types}

New(FileStatusV [i]); // get memory, now you can use the array member
FileStatusV[i].Files_Entres := 0;
FileStatusV[i].Size := 0;
FileStatusV[i].Status := false;
FileStatusV[i].Link := i;
{ WARNING if you use the memory allocation functions - New or GetMem or AllocMem -
  you should ALWAYS make sure that you Free the memory somewhere, yea, kind of a hassel}


aryFileStatus[i].Files_Entres := 0;
// you DO not need to assign memory with the aryFileStatus (call New), since all memory is there already
aryFileStatus[i].Size := 0;
aryFileStatus[i].Status := false;
aryFileStatus[i].Link := i;
// you do NOT need to ever free and memory alocations with the aryFileStatus



 = = = = = = = = = = = = = = = = = = = = =
one of the main reasons for using Pointer Types is to get a memory address from a function and then assign it to your Pointer Type variable
OR
to try and use less memory, which is what what seems the reason in your code, , the FileStatusV variable gets 262,144 bytes of memory
  the aryFileStatus variable gets 2,097,152 bytes of memory, which is much more, , however, if you want to save memory I would use a different type -

  PAryFileStat = ^TAryFileStat; {this Pointer type will use only 4 bytes of memory}
  TAryFileStat = Array[1..65536] of TFileStatus; { this will use 2097152 bytes of memory}


global variable
var
aryMemSize: Integer = 0;


var
pFileStat: PAryFileStat;

begin
Inc(aryMemSize, SizeOf(TFileStatus));
ReallocMem(pFileStat, aryMemSize); // get memory, now you can use the new array member

pFileStat[i].Files_Entres := 0;
pFileStat[i].Size := 0;
pFileStat[i].Status := false;
pFileStat[i].Link := i;
// you should Free any memory you allocate for pFileStat somewhere

but this is not so easy since you will manually have to manage the memory use,
I would think delphi Dynamic arrays are your best bet for this kind of thing?

var
aryFileStat: Array of TFileStatus;
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Expert Comment

by:AmigoJack
ID: 16633570
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?
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Author Comment

by:GiantMatrix
ID: 16634932
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
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Expert Comment

by:Slick812
ID: 16636336
?? 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
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Expert Comment

by:AmigoJack
ID: 16636958
>>
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)
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by:KyleyHarris
ID: 16658397
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
 
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