• Status: Solved
  • Priority: Medium
  • Security: Public
  • Views: 601
  • Last Modified:

Untyped var parameters

A function declares a formal parameter as untyped var:
  function Read(var buffer; ...)

If I pass a string variable to it, it doesn't work. I even tried presetting SetLength. What is the correct way to pass a string to the above paramter?

Thanks,
Sanjay
0
skanade
Asked:
skanade
1 Solution
 
ZifNabCommented:
Hi skanade,

I don't understand you correclty : shouldn't it be :  function Read(var buffer: string; ...)

ZiF.
0
 
skanadeAuthor Commented:
Hi ZiF,

That's what an 'untyped' parameter is. There is no type spec. Here is an example of BlockRead from Delphi:

procedure BlockRead(var F: File; var Buf; Count: Integer [; var AmtTransferred: Integer]);

The second parameter is untyped. That's where a string passed doesn't seem to work. I tried all sorts of conversions, PChar, Pointer, etc.

Sanjay
0
 
kjtengCommented:
Did you type cast the untyped parameter in the function?

Here is an example:

procedure test( var src);
begin
  showmessage( string( src));
end;

IMHO you should try to use the Type-Safe Open Arrays
(array of const) as parameter because it is safer.

0
Cloud Class® Course: Certified Penetration Testing

This CPTE Certified Penetration Testing Engineer course covers everything you need to know about becoming a Certified Penetration Testing Engineer. Career Path: Professional roles include Ethical Hackers, Security Consultants, System Administrators, and Chief Security Officers.

 
skanadeAuthor Commented:
>>Did you type cast the untyped parameter in the function?

It's not my own function so I have no control. I am trying to use BlockWrite from the library which uses the untypted parameter. Please see BlockWrite from Delphi help.

I have tried sending PChar typecase and even tried SetLength before sending. But the value of the string never reaches the file. If I send from a zero-based array of characters, it works. What I want is to somehow get strings to it without allocating arrays of huge length.

Sanjay
0
 
skanadeAuthor Commented:
Anyway, I found the answer on my own after some experimentation. I need to pass it as
   PChar(mystr)^

to the BlockWrite function's buffer untyped parameter. For BlockRead, I am reading it into dynamically allocated memory in a PChar pointer by P^.

Regards,
Sanjay
0
 
ZifNabCommented:
Hi all,
 
didn't know about this feature, never needed such a thing and then again, why not using a variant type?

c.U. Zif.
0
 
d003303Commented:
Hi Zif,
a variant is a high-level type that can be any type, but also stores its type information. An untyped parameter is low-level, in fact just a pointer to the memory a variable allocates. Thus, PChar(MyString)^ does the trick.

Slash/d003303
0
 
ZifNabCommented:
Thanks d003303, now I understand it.
0
Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.

Join & Write a Comment

Featured Post

The 14th Annual Expert Award Winners

The results are in! Meet the top members of our 2017 Expert Awards. Congratulations to all who qualified!

Tackle projects and never again get stuck behind a technical roadblock.
Join Now