[openbeos] Largely Offtopic C/C++ question about pointers
- From: "Bruno van Dooren" <bruno_van_dooren@xxxxxxxxxxx>
- To: <openbeos@xxxxxxxxxxxxx>
- Date: Wed, 27 Jul 2005 21:09:17 +0200
Hi All,
i have some (3) different weird pointer problems that have me stumped. i
suspect that the compiler behavior is correct because both gcc and msvc show
the same results.
i know that this is largely offtopic, but since there are a lot of
knowledgeable people here, i thought i'd give it a shot. because i can't
find an answer.
i have found a temporary workaround, but the only thing that is worse than
code that doesn't work is code that works and not knowing why.
i anyone could explain to me what i do wrong (or what i don't understand) i
would be very grateful.
kind regards,
Bruno.
----------------------------------------------
//example 1:
typedef int t_Array[10];
int main(int argc, char* argv[])
{
t_Array array;
array[0] = 123;
array[9] = 321;
t_Array *ptrArray[] = {&array};
t_Array *element = ptrArray[0];
printf("array = 0x%08x, ptrArray[0] = 0x%08x, element = 0x%08x, *element =
0x%08x\n",
array, ptrArray[0], element, *element);
printf("array[9] = %d, (*element)[9] = %d, element[9] = %d\n",
array[9], (*element)[9], element[9]);
return 0;
}
now according to the print statements, the different pointers all point to
the same thing, even though 'element' is dereferenced at one place, and not
dereferenced at another place. if i try to print a value from the array, i
get 2 different results.
??
----------------------------------------------
example 2:
typedef int t_Array[10];
void function(t_Array Array)
{
t_Array *ptrArray[] = {&Array}; //error
}
int main(int argc, char* argv[])
{
t_Array array;
t_Array *ptrArray[] = {&array}; //no error
return 0;
}
the 2 declarations of the arrays look the same to me, but 1 gives an error,
and the other one doesn't. i have looked up the documentation of that
error (C2440 with msvc), but that didn't shine much light.
??
----------------------------------------------
example 3:
typedef int t_Array[10];
void function(t_Array Array)
{
t_Array *element = NULL;
void * ptrArray[] = {NULL};
ptrArray[0] = &Array; //first element should be pointer to a t_Array
element = (t_Array *)ptrArray[0];
printf("Weird Failure: (*element)[9] = %d\n", (*element)[9]);
ptrArray[0] = Array; //first element should be Array itself
element = (t_Array *)ptrArray[0];
printf("Weird Success: (*element)[9] = %d\n", (*element)[9]);
}
int main(int argc, char* argv[])
{
t_Array array;
array[9] = 321;
function(array);
return 0;
}
this is what really caused my headaches. i used void pointers to get rid of
the error of example 2, but that caused weird behavior. in the first
situation in
'function', i expect to dereference a pointer to end up with an array, but
that clearly is not what happens.
in the second situation i expect to do something illegal: ie to use a
pointer to an array as the array itself, and somehow that seems to work.
what do i miss here?
i would be very grateful for some light on this murky behavior (or at least
my murky understanding).
kind regards,
Bruno.
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