Re: Pointer question about Discrete2d

[Tom Wainwright]

Paul E Johnson wrote:
> 
> There is a pointer usage in swarm, src/space/Discrete2d.m, that I don't
> understand.
> 
> Discrete2d.h declares
> long * offsets;

Here, "offsets" is a "pointer to long", so "*offsets" is a "long".

> 
> It is allocated in Discrete2d.m
> 
> - makeOffsets
> {
>   unsigned i;
>   // precalculate offsets based on the y coordinate. This lets
>   // us avoid arbitrary multiplication in array lookup.
>   offsets = [getZone (self) alloc: ysize * sizeof (*offsets)];
>   for (i = 0; i < ysize; i++)
>     offsets[i] = xsize * i; // cache this multipliction
>   return self;
> }
> 
> I don't understand why the aloc is not
> offsets = [getZone (self) alloc: ysize * sizeof (long)];
> 
> I don't get what sizeof(*offsets) means.

As "*offsets" is declared as a long in the header, sizeof(*offets) will
return the size of a long.  This notation just makes sure that the
allocation will match the declared type of "offsets", otherwise a coder
might change the declaration in the header, but not the allocation size
here.

> 
> Can someone explain for a poor, old country doctor?

Hope this makes sense.

-- 
Tom Wainwright
NOAA/NMFS/NWFSC
2030 S Marine Science Dr
Newport, OR 97365 USA
address@hidden

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