Generic Pointers

[John Strasser]

Hi Folks,

I'm writing a program where I'me working with several random access files (a mini database using Btree type keys).

What I'd like to do is create a generic UDT for most of what I need (filename, filenumber, etc) and then have another UDT for the specific data that would go into each one.

Ideally I would have a generic pointer in the "meta' UDT where I could store the address of the "data" udt. I know that I can do this by having a dword store the actual value and use a union with "options" for every "data" udt I need.

But that's a PITA.

If I remember my C this would be typecasting the pointer (been about 10 years...)

I'd like it to look like this:

 
Type data01
   a as long
   b as string * 10
End Type
 
Type data02
   c as quad
   d as string * 30
End Type
 
Type meta
   filename as string
   filenumber as long
   GPTR as generic pointer
End Type
 
dim meta01 as meta
dim meta02 as meta
dim d1 as data01
dim d2 as data02
 
meta01.filename="file01.dat"
meta01.filenumber=freefile
meta01.GPTR=varptr(d1)
 
meta02.filename="file02.dat"
meta02.filenumber=freefile
meta02.GPTR=varptr(d2)

Is that do-able? Or am I missing something?

JS

[Scott Hauser]

Hi John,

You can't access data files, or UDT members in the explicit manner you are attempting in code. Read the PB help file on Types and Unions. In your samples directory under DDT, you will find address.bas. It is a good starting point and shows record based file access as well. Then study up on pointers.

As to btree, here is a random sample:

http://www.powerbasic.com/support/pb...ighlight=btree

Search the forum for "BTREE" you wil find quite a few hits.

If you would prefer not to roll your own, I recommend Paul Squires Cheetah dll that works on xbase - dbase file formats. It is very fast, small and problem free. It also has numerous code samples of the various functions and a very well written help file. Recently, Paul released it as freeware. I bought my copy when it was commercial and it was worth every penny!

Get it at:

http://www.planetsquires.com/

HTH,
Scott

[Kev Peel]

This is an ideal candidate for a UNION - you can define a pointer to any structure and reference that structure from the UNION member:

 
Type data01
   a As Long
   b As String * 10
End Type
Type data02
   c As Quad
   d As String * 30
End Type
Union data_union
      pAny As Dword
      pd1 As data01 Ptr
      pd2 As data02 Ptr
End Union
Type meta
   filename As String * 260
   filenumber As Long
   data_union
End Type
Function PBMain
 
  Dim meta01 As meta
  Dim meta02 As meta
  Dim d1 As data01
  Dim d2 As data02
  meta01.filename="file01.dat"
  meta01.filenumber=FreeFile
  meta01.pAny = VarPtr(d1)
  meta02.filename="file02.dat"
  meta02.filenumber=FreeFile
  meta02.pAny = VarPtr(d2)
 
  [email protected] = "test_meta1"
  [email protected] = "test_meta2"
  ? [email protected]
  ? [email protected]
 
End Function

The above code still involves a degree of "separation", however, as the data01 and data02 members are not aligned the same.
Even in C, using a typecast, it would still work the same as PB.

[John Strasser]

Thanks guys

I appreciate the quick response. Hadn't considered Cheetah - I'll take a look.

Scott:
That's a good reference thread you pointed me to. Thanks. The idea was to put together a set of "pseudo-classes" where then the only thing I'd have to do is de-reference the pointer when it was time to get or put the udt into the random access file. I don't know if I explained what I wanted clear enough.

Kev:
That's pretty much what I figured I'd have to do - I was just hoping I didn't have to take the step and explicitly create the union.

Thanks again folks.

John

[Michael Mattias]

When you think about it, all pointers are generic.

A pointer is a pointer.. an address. How one interprets how much data at that address is application-dependent.

>I was just hoping I didn't have to take the step and explicitly create the union.

The UNION statement creates nothing at runtime; it only provides syntax for easy access of data.

An option to creating a UNION defined with PTR members is to define a UNION PTR data item, with the members of the underlying UNION structure as "themselves"

UNION MyUnion
     DW   AS DWORD
     sz     AS ASCIIZ * (Way, way more than enough) 
     C      AS CURRENCY
END UNION
....

   LOCAL pU AS MyUnion PTR   ' only costs your program SIZEOF(pU), which is 4 bytes.

   pU  = Function returning a "generic pointer" 

  IF Pointer_Is_STRING_PTR THEN 
        PRINT @pU.sz     
  ELSEIF Pointer_is_DWORD_PTR THEN
        PRINT @PU.DW
  ELSEIF Pointer_IS_CURRENCY THEN 
       PRINT @pu.C 
  .......

MCM

[John Strasser]

Thanks Mike

JS