TITLE: return and void

(Newsgroups: comp.lang.c++,comp.std.c++, 8 Oct 98)

NARAN: sbnaran@KILL.uiuc.edu wrote:
>
>
> Is something like this possible?  I've seen in done in one of the
> examples in Stroustrup III.  It's very convenient.
>
> void f() { /*stuff*/ }
> void g() { /*stuff*/  return f();  /*stuff*/ }

HYSLOP: jim.hyslop@leitch.com

First off, if Stroustrup uses it, that's a pretty strong indicatation that
it's allowable syntax.	However, even Stroustrup is human, and can make
mistakes (did I actually say that :-O )

Anyway, MSVC5 gives an error, complaining "g: 'void' function returning a
value".  But we all know that one compiler's opinion does not necessarily
indicate the truth, so let's see what the Standard says...

[pause while I dig through the Standard] Section 3.9.1 (Fundamental Types)
states "The void type has an empty set of values.... Any expression can be
explicitly converted to type cv void (5.2).... An expression of type void
shall be used only as ... the expression in a return statement (6.6.3) for a
function with the return type void."

OK, that suggests to me that "return void;" would be valid syntax.  Let's see
what MSVC says... nope, "error: type 'void' unexpected."  How about "return
(void) f();"... nope, back to the original error.

Let's look at 6.6.3 to see if it gives any clues.  Ah, here we go - clause 3:
"A return statement with an expression of type 'cv void' can be used only in
functions with a return type of cv void; the expression is evaluated just
before the function returns to its caller".  Well, as I understand it, "f()"
is an expression of type 'cv void', so "return f();" should be valid syntax.

OK, now what do you real experts say to this?  Is my analysis correct?


CLAMAGE: stephen.clamage@sun.com (Steve Clamage)

It's a recent addition to the standard. It was formerly
forbidden. The reason for the change was not to write
code like the above, but to make it easier to write templates.
Without the relaxed rule, you would have to write a special
version of templates for instantiation on type void.