Debugger works not correct with Null pointer strings

[IceSoft]

Code: Select all

Define mystring.s
Debug @""
Debug @mystring  
Debug mystring    ; <<<<<<<< An error message about a null pointer variable is expected but this is handles as an empty string variable ("")

[Trond]

Huh? Null pointer strings are valid strings in PureBasic. There is not supposed to be an error message.

[freak]

Huh? Null pointer strings are valid strings in PureBasic. There is not supposed to be an error message.

correct.

[IceSoft]

You can find the whole discussion here (Sorry its german):
http://www.purebasic.fr/german/viewtopi ... 20&t=24299

[freak]

This is the intended behavior. Strings are only initialized when something is assigned to them. A nullpointer is always interpreted as the empty string internally. This is how it was designed from the start.

[skywalk]

This is the intended behavior. Strings are only initialized when something is assigned to them. A nullpointer is always interpreted as the empty string internally. This is how it was designed from the start.

I noticed this behavior when I try to fill an uninitialized string variable passed to a procedure ByRef.
I remember getting an IMA. It was annoying for sure.
Meaning, I had to define the string and then assign it to a "" before I could modify it in the procedure.

Is this correct?

Code: Select all

Procedure EditStr(*p)
  PokeS(*p,"Hello")
  ProcedureReturn Len(PeekS(*p))
EndProcedure
Define.s r2;=""    ;<--- remove comment and it works
Debug EditStr(@r2)

[ts-soft]

Is this correct?

Code: Select all

Procedure EditStr(*p)
  PokeS(*p,"Hello")
  ProcedureReturn Len(PeekS(*p))
EndProcedure
Define.s r2;=""    ;<--- remove comment and it works
Debug EditStr(@r2)

It works sometimes with removed comment but it is not correct. You poke in a not reserved memory. Enable purifier
to see that it not works!

Greetings - Thomas

[skywalk]

It works sometimes with removed comment but it is not correct. You poke in a not reserved memory. Enable purifier to see that it not works!

True, I left out the other stuff in the procedure for simplicity.
When you say sometimes, I am curious.
What amount of memory is reserved when r2 = ""?
Is there a minimum?

Code: Select all

Procedure EditStr(*p)
  *p = AllocateMemory(RLen)
  ; do stuff...
  PokeS(*p,"Hello")
  FreeMemory(*p)
EndProcedure
Define.s r2;=""    ;<--- remove comment and it works
Debug EditStr(@r2)       

[ts-soft]

What amount of memory is reserved when r2 = ""?

There is only place for the nullbyte, so you can only poke a empty string

[skywalk]

Ok, that's what I thought, so I rearranged the Procedure to return the string directly instead of modifying a ByRef parameter. VB6 handled ByRef strings behind the scenes for me.

[PMV]

PB can only handle the string for you, if you are using normal PB-Strings.
PB ignores memory-functions like PokeS().

Code: Select all

Procedure EditStr(*p)
  Protected *s.STRING = AllocateMemory(SizeOf(STRING))
  PokeI(*s, *p)
  *s\s = "Hello"
  FreeMemory(*s)
  ProcedureReturn Len(PeekS(*p))
EndProcedure
Define.s r2=""
Debug EditStr(@r2)
Debug r2

[helpy]

There is nothing comparable in PureBasic like "ByRef VariableName As String" in Visual Basic (or maybe other programming languages)!

Yes! You can pass a pointer to a PureBasic string to a procedure!
Yes! You can read the string in such a procedure!
Yes! You can change the string in such a procedure, if the string is not getting longer as the original one!
No! You can not handle the string like normal string variables in such a procedure.
No! You cannot use PureBasic string functions in such a Procedure.

The only way you can pass strings ByRef is, if they are inside Structures. See example:

Code: Select all

Procedure AppendString( *First.String, *Second.String )
	*First\s + *Second\s
EndProcedure
Procedure EmptyString( *aString.String )
	*aString\s = ""
EndProcedure

First.String
Second.String

First\s = "Hello"
Debug First\s

Second\s = ", this was appended to the orignial string without any problems :-)"
AppendString( First, Second )
Debug First\s

Second\s = #CRLF$ + #CRLF$ + "Und noch mehr" + RSet("",1024,"-")
AppendString( First, Second )
Debug First\s

EmptyString( First )
Debug #DQUOTE$ + First\s +#DQUOTE$

Hope this helps!
cu, guido