Func Param Confusion - in/out/inout/ref/const/immutable

[bearophile]

Ali Çehreli:

> > why someone would want to use 'out' instead of 'ref'
>
> There is an idiom in C where a function first sets its "out 
> parameter" to null (or another initial value) and then does 
> some more work to fill that to actual result of the function:
>
> int make_something(Something ** p)
> {
>     int err = 0;
>
>     assert(p);
>     *p = NULL;    // first, initialize
>
>     // fill in p later on
>
>     return err;
> }
>
> To me, 'out' of D does that initialization automatically.

Another advantage of "out" is that it clearly documents that 
argument as another result of the function.

In practice in not-performance critical functions I sometimes 
prefer to return multiple values in a tuple, despite D doesn't 
yet have a tuple unpack syntax.

One disadvantage of "out" arguments in D is that D (unlike Ada, I 
think) doesn't forbid you to set a out argument to some value 
before the function call. Such assignment is always useless, and 
sometimes it risks being a programmer mistake. A tuple result 
doesn't have such bug risk.

Bye,
bearophile