double pointer indirection??

[Nobody]

excuse the "windows" code below, but this is a C++ question... or maybe a C.

Anyways, I want a function where I can pass in an array of strings (variable
count, variable size), so I defined this:

void Test(LPCTSTR* ppsz, int nCount)

{

for (int nIndex = 0; nIndex < nCount; nIndex++)

TRACE(".%s.\n", ppsz[nIndex]);

}

and call it like:

TCHAR sz[5][32] =

{

"United States",

"Canada",

"Mexico",

"United Kingdom",

"South America"

};

Test((LPCTSTR*) &sz, 5);



The Test function crashes no matter what I do (various indirection styles,
etc) even on nIndex = 0?!?!?!

How can I do this? Would the Test function need to know the 32 to get the
next pointer...

Seems like what I want to do is pass in an array of pointers?

Any simple way to do that similar to the above? without new and delete and
so forth and not using a string list class?

this is for an API that I am writing, so I want minimal dependencies and
minimum chance for a developer to pass me a wrong format.

Thanks.

[Karl Heinz Buchegger]

Re: double pointer indirection??



Nobody wrote:[color=blue]
>
> excuse the "windows" code below, but this is a C++ question... or maybe a C.
>
> Anyways, I want a function where I can pass in an array of strings (variable
> count, variable size), so I defined this:
>
> void Test(LPCTSTR* ppsz, int nCount)
>
> {
>
> for (int nIndex = 0; nIndex < nCount; nIndex++)
>
> TRACE(".%s.\n", ppsz[nIndex]);
>
> }
>
> and call it like:
>
> TCHAR sz[5][32] =
>
> {
>
> "United States",
>
> "Canada",
>
> "Mexico",
>
> "United Kingdom",
>
> "South America"
>
> };
>
> Test((LPCTSTR*) &sz, 5);
>
> The Test function crashes no matter what I do (various indirection styles,
> etc) even on nIndex = 0?!?!?!
>
> How can I do this? Would the Test function need to know the 32 to get the
> next pointer...[/color]

:-)

You function expects something like this:

ppsz
+------+ +-----+
| o-------->| o---------------> "Text1"
+------+ +-----+
| o---------------> "Text2"
+-----+
| o---------------> "Text3"
+-----+
| |
. .
. .
+-----+

But your sz Array looks like this

sz
+---+---+---+---+---+- ... 26 chars .. -+---+---+---+---+- .... -+---+---+---+---+- ...
| U | n | i | t | e | | | C | a | n | | M | e | x | i |
+---+---+---+---+---+- ............... -+---+---+---+---+- .... -+---+---+---+---+- ...

As you can see, those 2 data structures are incompatible.
[color=blue]
>
> Seems like what I want to do is pass in an array of pointers?[/color]

Ok. But then you need to build up an array of pointers.

char sz[5][32]

is *not* an array of pointers. It is a flat memory field with a size of 5*32 bytes,
where the index arithmetic divides the memory into the fields.
[color=blue]
>
> Any simple way to do that similar to the above?[/color]

char* sz[] = { "United States", "Canada", "Mexico" }

But note: The pointer point to constant strings! You can't change the strings!
[color=blue]
> without new and delete and
> so forth and not using a string list class?
>
> this is for an API that I am writing, so I want minimal dependencies and
> minimum chance for a developer to pass me a wrong format.[/color]

Drop the idea of passing an array of pointers to character or a 2-dimensional
character array. As you have seen it's a constant source of confusion. If you
got confused, so will the user of your API.

--
Karl Heinz Buchegger
kbuchegg@gascad .at

[Peter van Merkerk]

Re: double pointer indirection??

> excuse the "windows" code below, but this is a C++ question... or
maybe a C.[color=blue]
>
> Anyways, I want a function where I can pass in an array of strings[/color]
(variable[color=blue]
> count, variable size), so I defined this:
>
> void Test(LPCTSTR* ppsz, int nCount)
> {
> for (int nIndex = 0; nIndex < nCount; nIndex++)
> TRACE(".%s.\n", ppsz[nIndex]);
> }
>
> and call it like:
>
> TCHAR sz[5][32] =
> {
> "United States",
> "Canada",
> "Mexico",
> "United Kingdom",
> "South America"
> };
>
> Test((LPCTSTR*) &sz, 5);
>
> The Test function crashes no matter what I do (various indirection[/color]
styles,[color=blue]
> etc) even on nIndex = 0?!?!?![/color]

That is not surprising considering that you pass an array of char array
to a function that expect a pointer to char pointer. Since the character
array is interpreted as a pointer all kinds of nasty things may happen.
[color=blue]
> How can I do this? Would the Test function need to know the 32 to get[/color]
the[color=blue]
> next pointer...[/color]

One way to fix the problem is to make sz[] an array of const char
pointers:

const char* sz[] =
{
"United States",
"Canada",
"Mexico",
"United Kingdom",
"South America"
};

--
Peter van Merkerk
peter.van.merke rk(at)dse.nl

[Daniel Schüle]

Re: double pointer indirection??

> TCHAR sz[5][32] =[color=blue]
>
> {
>
> "United States",
>
> "Canada",
>
> "Mexico",
>
> "United Kingdom",
>
> "South America"
>
> };
>
> Test((LPCTSTR*) &sz, 5);[/color]

why not

void print1(const char (* const x)[32])
{
for(int i=0; i<5; i++)
printf("%s\n", x[i]);
}

//or ;)

void print2(const char (* x)[32])
{
for(int i=0; i<5; i++)
printf("%s\n", *x++);
}

int main()
{
char text[5][32] = {"a1", "b2", "c3", "d4", "e5"};
print1(&text[0]);
print2(&text[0]);
return 0;
}

[EventHelix.com]

Re: double pointer indirection??

I think the problem is due to the sz declaration. This is not declaring
an array of pointers. It is a two dimensional array of characters.

Try declaring an array of LPCTSTR with the
same initialization list.
[color=blue]
> TCHAR sz[5][32] =[/color]

Sandeep
--

http://www.EventHelix.com/EventStudio

EventStudio 2.0 - Generate Sequence Diagrams and Use Case Diagrams in PDF

[Nobody]

Re: double pointer indirection??

that did the trick. thanks.

"Peter van Merkerk" <merkerk@deadsp am.com> wrote in message
news:boq7ri$1fi tqd$1@ID-133164.news.uni-berlin.de...[color=blue][color=green]
> > excuse the "windows" code below, but this is a C++ question... or[/color]
> maybe a C.[color=green]
> >
> > Anyways, I want a function where I can pass in an array of strings[/color]
> (variable[color=green]
> > count, variable size), so I defined this:
> >
> > void Test(LPCTSTR* ppsz, int nCount)
> > {
> > for (int nIndex = 0; nIndex < nCount; nIndex++)
> > TRACE(".%s.\n", ppsz[nIndex]);
> > }
> >
> > and call it like:
> >
> > TCHAR sz[5][32] =
> > {
> > "United States",
> > "Canada",
> > "Mexico",
> > "United Kingdom",
> > "South America"
> > };
> >
> > Test((LPCTSTR*) &sz, 5);
> >
> > The Test function crashes no matter what I do (various indirection[/color]
> styles,[color=green]
> > etc) even on nIndex = 0?!?!?![/color]
>
> That is not surprising considering that you pass an array of char array
> to a function that expect a pointer to char pointer. Since the character
> array is interpreted as a pointer all kinds of nasty things may happen.
>[color=green]
> > How can I do this? Would the Test function need to know the 32 to get[/color]
> the[color=green]
> > next pointer...[/color]
>
> One way to fix the problem is to make sz[] an array of const char
> pointers:
>
> const char* sz[] =
> {
> "United States",
> "Canada",
> "Mexico",
> "United Kingdom",
> "South America"
> };
>
> --
> Peter van Merkerk
> peter.van.merke rk(at)dse.nl
>
>
>[/color]