error: jump to case label

Hello,

Does anyone know what the following g++ compiler error message means:

error: jump to case label

I get this error when switching two case labels together with their bodies.
I have no setjmp/longjmp or gotos in my program.

Thanks,

Neil

Neil Zanella

Neil Zanella wrote:

> I get this error when switching two case labels together with their bodies.

Explain that further or better yet post the offending code.

On Fri, 02 Apr 2004 15:09:00 -0800, Neil Zanella wrote:

> Hello,
>
> Does anyone know what the following g++ compiler error message means:
>
> error: jump to case label
>
> I get this error when switching two case labels together with their bodies.
> I have no setjmp/longjmp or gotos in my program.
>

Perhaps the problem is "jump to case label croses initialization"?

The following is not allowed:

switch (a)
{
case 1:
int a = 6;
//stuff
break;

case 2:
//stuff
break;
}

The following is allowed:

switch (a)
{
case 1:
{
int a = 6;
//stuff
}
break;

case 2:
//stuff
break;
}

James

Bill Seurer <> wrote in message news:<c4kt55$1ahs$>...
> Neil Zanella wrote:
>
> > I get this error when switching two case labels together with their bodies.
>
> Explain that further or better yet post the offending code.

Sure I will. Here is the code. Uncommenting the lines for case 1 produces
the compiler error message. Furthermore, out of curiosity, as an unrelated
matter, I am quite interested in knowing how come the program starts looping
when some large number is entered.

Thanks,

Neil

#include <iostream>

int main() {
unsigned int x; do {
std::cout << "Please enter an integer: " << std::flush;
std::cin >> x;
switch (x) {
case 0:
std::cout << "Hello!" << std::endl;
break;
default:
unsigned int y = ++x;
std::cout << "You could have entered " << y;
std::cout << ". Why didn't you?" << std::endl;
break;
//case 1:
// std::cout << "What??? You entered one?" << std::endl;
// break;
}
} while (x != 0);
}

Neil Zanella wrote:
> Sure I will. Here is the code. Uncommenting the lines for case 1 produces
> the compiler error message.

Try moving them before the 'default'

> Furthermore, out of curiosity, as an unrelated
> matter, I am quite interested in knowing how come the program starts looping
> when some large number is entered.

cin.fail() is set and all subsequent ">>" operations are ignored,
with x unmodified every time. Hence, if x happened to be non-zero,
the loop iterates infinitely.

>
> Thanks,
>
> Neil
>
> #include <iostream>
>
> int main() {
> unsigned int x; do {
> std::cout << "Please enter an integer: " << std::flush;
> std::cin >> x;
> switch (x) {
> case 0:
> std::cout << "Hello!" << std::endl;
> break;
> default:
> unsigned int y = ++x;
> std::cout << "You could have entered " << y;
> std::cout << ". Why didn't you?" << std::endl;
> break;
> //case 1:
> // std::cout << "What??? You entered one?" << std::endl;
> // break;
> }
> } while (x != 0);
> }

HTH,
- J.

Jacek Dziedzic <jacek__NOSPAM__@janowo.net> wrote in message:

> Try moving them before the 'default'

Thank you for your reply...
I know that works but that doens't really explain the nature of the problem.

Regards,

Neil

> > #include <iostream>
> >
> > int main() {
> > unsigned int x; do {
> > std::cout << "Please enter an integer: " << std::flush;
> > std::cin >> x;
> > switch (x) {
> > case 0:
> > std::cout << "Hello!" << std::endl;
> > break;
> > default:
> > unsigned int y = ++x;
> > std::cout << "You could have entered " << y;
> > std::cout << ". Why didn't you?" << std::endl;
> > break;
> > //case 1:
> > // std::cout << "What??? You entered one?" << std::endl;
> > // break;
> > }
> > } while (x != 0);
> > }
>
> HTH,
> - J.

Neil Zanella wrote:

> Jacek Dziedzic <jacek__NOSPAM__@janowo.net> wrote in message:
>
>> Try moving them before the 'default'
>
> Thank you for your reply...
> I know that works but that doens't really explain the nature of the problem.

Please don't top-post.

>>>#include <iostream>
>>>
>>>int main() {
>>> unsigned int x; do {
>>> std::cout << "Please enter an integer: " << std::flush;
>>> std::cin >> x;
>>> switch (x) {
>>> case 0:
>>> std::cout << "Hello!" << std::endl;
>>> break;
>>> default:
>>> unsigned int y = ++x;
>>> std::cout << "You could have entered " << y;
>>> std::cout << ". Why didn't you?" << std::endl;
>>> break;
>>> //case 1:
>>> // std::cout << "What??? You entered one?" << std::endl;
>>> // break;
>>> }
>>> } while (x != 0);
>>>}

James Gregory is right. The 'jump' in the error message is the computed
goto effected by the switch statement. When x = 1, the switch acts like
this:

goto case_label_1;
// ...
unsigned int y = ++ x;
// ...
case_label_1:
// here y is uninitialized

The problem is that the initialization of y is skipped when x == 1.
When the "case 1:" label is reached, stack space has been allocated for
y but its value has not been initialized. This is not allowed.

The general way round this situation is to make the scope of y smaller
by adding braces:

switch (x)
{
default:
unsigned z; // this is OK since z is uninitialized
{
unsigned y = x + 1;
// ...
}
case 1:
z = 2;
// ...
// cannot refer to y here so no problem with y's initialization
}

--
Regards,
Buster.