pass a pointer-to-pointer, or return a pointer? Post: 302273896

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Top Forums Programming pass a pointer-to-pointer, or return a pointer? Post 302273896 by spirtle on Tuesday 6th of January 2009 05:43:33 AM

01-06-2009

Registered User

I prefer the second form because:

The interface is simpler: one parameter in, the answer returned. You just check whether the pointer is null to verify success. The first form has some kind of other return value to check, which could be boolean, but I cannot tell, so I would have to look up the definitions of OK and NOK. And it is unclear from the interface whether I also need to check the pointer value, or indeed whether the pointer is NULL on failure. Looking into you implementation I can see that OK is returned if and only if the pointer is valid and the pointer is NULL on failure, but then I can write
Code:
```
char *my_pmem;
my_malloc(64, &my_pmem);
if(my_pmem){
  ...
}
```
so the return value is redundant -- it gives me no extra information.
However, if you want the function to indicate more than just a simple fail/succeed (e.g. different failure modes) then the first way is the only way to do it.
It mimics the standard malloc(3) function -- or it would do if the parameter was of type size_t rather than int -- and therefore has the benefit of familiarity, and makes it easier to port code written with malloc to use my_malloc.

spirtle

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LEARN ABOUT DEBIAN

dispatch_api

dispatch_api(3) 					   BSD Library Functions Manual 					   dispatch_api(3)

NAME

     dispatch_api -- Designing API using dispatch

DESCRIPTION

     The following is a brief summary of some of the common design patterns to consider when designing and implementing API in terms of dispatch
     queues and blocks.

     A general recommendation is to allow both a callback block and target dispatch queue to be specified. This gives the application the greatest
     flexibility in handling asynchronous events.

     It's also recommended that interfaces take only a single block as the last parameter. This is both for consistency across projects, as well
     as the visual aesthetics of multiline blocks that are declared inline. The dispatch queue to which the block will be submitted should immedi-
     ately precede the block argument (second-to-last argument). For example:

	   read_async(file, callback_queue, ^{
		   printf("received callback.
");
	   });

     When function pointer alternatives to interfaces that take blocks are provided, the argument order of the function signature should be iden-
     tical to the block variant; with the exception that the block argument is replaced with a context pointer, and a new last parameter is added,
     which is the function to call.

     The function based callback should pass the context pointer as the first argument, and the subsequent arguments should be identical to the
     block based variant (albeit offset by one in order).

     It is also important to use consistent naming. The dispatch API, for example, uses the suffix "_f" for function based variants.

SEE ALSO

     dispatch(3), dispatch_async(3), dispatch_queue_create(3)

Darwin								    May 1, 2009 							    Darwin