On Thu, Oct 19, 2023 at 07:39:43AM -0400, Eric Sokolowsky via Gcc wrote:
> I am using gcc 13.2 on Fedora 38. Consider the following program.
>
> #include <stdio.h>
> int main(int argc, char **argv)
> {
> printf("Enter a number: ");
> int num = 0;
> scanf("%d", &num);
>
> switch (num)
> {
> case 1:
> int a = num + 3;
> printf("The new number is %d.\n", a);
> break;
> case 2:
> int b = num - 4;
> printf("The new number is %d.\n", b);
> break;
> default:
> int c = num * 3;
> printf("The new number is %d.\n", c);
> break;
> }
> }
>
> I would expect that gcc would complain about the declaration of
> variables (a, b, and c) within the case statements. When I run "gcc
> -Wall t.c" I get no warnings. When I run "g++ -Wall t.c" I get
> warnings and errors as expected. I do get warnings when using MinGW on
> Windows (gcc version 6.3 specifically). Did something change in 13.2?
C isn't C++.
In particular, the above is valid C23, which is why it is accepted as an
extension in older C language versions starting with GCC 11.
It is warned about with -pedantic/-Wpedantic and errored on with
-pedantic-errors/-Werror=pedantic unless -std=c2x or -std=gnu2x is used.
The C++ case is completely different. There labels are allowed before
declarations already in C++98, but it is invalid to cross initialization
of some variable using the jump to case 2 or default labels above.
If you rewrite it as:
case 1:
int a;
a = num + 3;
printf("The new number is %d.\n", a);
break;
case 2:
int b;
b = num - 4;
printf("The new number is %d.\n", b);
break;
default:
int c;
c = num * 3;
printf("The new number is %d.\n", c);
break;
it is valid C++ and it won't be diagnosed.
Note, this should have been posted to gcc-help instead.
Jakub
