Re: [PATCH 1/2] add macro REALLOCARRAY

René Scharfe [off-list ref] writes:

The macro ALLOC_GROW manages several aspects of dynamic memory
allocations for arrays: It performs overprovisioning in order to avoid
reallocations in future calls, updates the allocation size variable,
multiplies the item size and thus allows users to simply specify the
item count, performs the reallocation and updates the array pointer.

Sometimes this is too much.  Add the macro REALLOCARRAY, which only
takes care of the latter three points and allows users to specify the
number of items an array can store directly.  It can increase and
also decrease its size.  Using this macro avoids duplicating the
array pointer name and takes care of item sizes automatically.

Signed-off-by: Rene Scharfe <redacted>
---

Makes sense.  Originally I had two minor gripes against this

 #1 a macro that modifies its arguments feels a bit too magical, and
    we would not want to overuse them.

 #2 REALLOC_ARRAY(array, size) would have been easier to read.

But #1 is shared with ALLOC_GROW(), and as long as we use it
everywhere (and by the looks of 2/2 we fairly widely do), readers
will get used to the pattern.  #2 still stands, though.

Thanks.

On Mon, Sep 15, 2014 at 11:24:04AM -0700, Junio C Hamano wrote:

René Scharfe [off-list ref] writes:

quoted

The macro ALLOC_GROW manages several aspects of dynamic memory
allocations for arrays: It performs overprovisioning in order to avoid
reallocations in future calls, updates the allocation size variable,
multiplies the item size and thus allows users to simply specify the
item count, performs the reallocation and updates the array pointer.

Sometimes this is too much.  Add the macro REALLOCARRAY, which only
takes care of the latter three points and allows users to specify the
number of items an array can store directly.  It can increase and
also decrease its size.  Using this macro avoids duplicating the
array pointer name and takes care of item sizes automatically.

Signed-off-by: Rene Scharfe <redacted>
---

Makes sense.  Originally I had two minor gripes against this

 #1 a macro that modifies its arguments feels a bit too magical, and
    we would not want to overuse them.

This is probably getting into the too-magical territory, but I have long
considered a macro like:

  #define ALLOC(x) (x) = xmalloc(sizeof(*x))

to prevent obvious size-mismatch errors.  You could even call the macro
NEW() if you wanted to be really disgusting. :)

I rejected it as probably too cutesy (and non-idiomatic for experienced
C programmers), but I feel like this REALLOC_ARRAY is basically the same
thing. I dunno. It does make the code a bit more readable, once you
understand what the macro is doing.

-Peff