bobj@gli.com (Robert Jacobs) (09/07/90)
Is the following syntax allowable in c++. The reason I ask is because a version
of my compiler used to allow it but the UPGRADE forbids it.
int foo(void *p,int width)
{
void *q;
q = p+width;
}
The compiler complains about not knowing the size information.
In order to get this to work I have to recast p to either an int or a char*.
Note I am not trying to dereference a void. I know that is illegal.
If pointer arithmitic is not allowed on a void* then its usefullness is much
reduced.
Robert Jacobs
bobj@gli.comdaniel@terra.ucsc.edu (Daniel Edelson) (09/11/90)
In article <1990Sep7.144048.2049@gli.com> bobj@gli.com (Robert Jacobs) writes: > >Is the following syntax allowable in c++. The reason I ask is because a version >of my compiler used to allow it but the UPGRADE forbids it. > >int foo(void *p,int width) >{ > void *q; > q = p+width; >} > >The compiler complains about not knowing the size information. >In order to get this to work I have to recast p to either an int or a char*. >Note I am not trying to dereference a void. I know that is illegal. >If pointer arithmitic is not allowed on a void* then its usefullness is much >reduced. >Robert Jacobs >bobj@gli.com Pointer arithmetic on void * is illegal. This does not reduce the usefullness of the void * type because arithmetic on void * would mean ``add an unknown quantity to this pointer.'' According to the ARM section 5.7 additive operators can only be applied to a pointer when the pointer points to an array element. This is the only legal pointer arithmetic. According to 8.2.4 an array may be constructed from any built-in type except void. Therefore no pointer arithmetic with void* is legal. This must be the case because there's no way a void* pointer could be correctly incremented. At the raw machine level, what value should the compiler add to the pointer value? If the pointer pointed at an integer, p+1 would add sizeof(int) to the pointer. If it pointed at a char then sizeof(char) would be added. In general, because of how K&R first defined pointer arithmetic the size of the referenced object must be known for a pointer to be arithmetiked Daniel Edelson | Don't just garbage collect, recycle. daniel@cis.ucsc.edu, or, | ...!sun!practic!peren!daniel |