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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: [ On topic, amazingtly ]: labels and declarations
Date: Sun, 17 Mar 2019 16:49:52 -0700
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Lew Pitcher <lew.pitcher@digitalfreehold.ca> writes:
> Keith Thompson wrote:
>> Richard Damon <Richard@Damon-Family.org> writes:
>> [...]
>>> This unfortunately leads to the following being grammatically correct:
>>>
>>> if(flag) int x;
>>>
>>> which would mean that the following code might be valid or not depending
>>> of if flag was true at run time, or
>>>
>>> if(flag) int x;
>>> else     float x;
>>>
>>> where the type of x changes at run time, or even
>> [...]
>> 
>> C++ allows this; a declaration (if it appears inside a function
>> defintion) is a kind of statement.  The scope of x is limited to the
>> declaration-statement, so the above is equivalent to:
>> 
>>     if (flag) { int x; }
>>     else      { float x; }
>> 
>> which is valid in C.
>
> By implication, even current C supports this, afaict. Consider
>
>   if (!flag) goto label1;
>   int x;
>   goto label2;
>   label1: ;
>   float x;
>   label2: ;
>
> Doesn't this accomplish the same end-state as Richard's
>  if(flag) int x;
>  else     float x;
> ?

No.  In your example with gotos, the declaration `float x;` is illegal,
because there's already an `x` defined in the same scope.

In Richard's example, if the scopes of the two `x`s extended beyond the
if-else statement, it would be difficult to define the semantics or,
more likely, the second declaration would be illegal for the same
reason.  If the scopes were defined as they are in C++, they would be
non-overlapping and unambiguous (and not useful, since nothing could
refer to either `x`).

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Will write code for food.
void Void(void) { Void(); } /* The recursive call of the void */