Why don't the extra parentheses compile? (Can't parenthesize a declaration, maybe?)

[K. Frank]

Hello Group!

Why doesn't the line flagged with the error compile?
(I'm compiling as c++ with g++.)

void f() {
if (false);
if ((false));
bool b;
if (b = false);
if ((b = false));
if (bool b = false);
if ((bool b = false)); // error: expected primary-expression before 'bool'
}

All but the last of the variations compile, but the last
triggers the noted error. What's the language-standard
analysis that explains this?


Thanks for any ideas.


K. Frank

[Bo Persson]

It's a grammar thing.

A primary-expression can inclucde a declaration of a variable, but as
soon as you add a '(' it is an '(expression)', which cannot declare
variables.



Bo Persson

[K. Frank]

Hi Bo!

On Saturday, February 18, 2017 at 6:27:08 PM UTC-5, Bo Persson wrote:
> On 2017-02-18 23:29, K. Frank wrote:
> > Hello Group!
> >
> > Why doesn't the line flagged with the error compile?

> > ...

> > if ((bool b = false)); // error: expected primary-expression before 'bool'

> ...

> It's a grammar thing.
>
> A primary-expression can inclucde a declaration of a variable, but as
> soon as you add a '(' it is an '(expression)', which cannot declare
> variables.

Thanks. That makes sense (I guess ...)

By the way, I'm speaking from memory here, but is this the
same reason you can use an extra set of parentheses to "fix"
the "most vexing parse" in that the extra set of parentheses
prevents the expression from being parsed as the unwanted
declaration? (I never understood that either.)

> Bo Persson


Best.


K. Frank