goto statement usage in C++ - c++

Question

I have a funcA which I call every msec. by another funcB. I want to use goto statement. But when I look at the flow (when m_tempdata is not NULL), after printing "stage 2", it is also printing "cleanup starts". Normally, I expect to return after printing "stage 2" for the next turn. Am I wrong?
void ClassA::funcA()
{
m_tempdata = m_freedata;
printf("stage 1 \n");
if (NULL == m_tempdata)
{
printf("going cleaning \n" );
goto cleanup;
}
m_freedata = m_tempdata->next;
printf("stage 2 \n");
cleanup: printf("cleanup starts \n");
// ... some additional work todo
}

Answer 1

Normally, I expect to return after printing "stage 2" for the next
turn. Am I wrong?
Yes, you're wrong.
In the case where the label wasn't jumped to, the code will simply fall through to it and continue. A label isn't an instruction. It doesn't cause the program to magically return or jump somewhere else.
Here is a much better way to write this:
void ClassA::funcA()
{
m_tempdata = m_freedata;
printf("stage 1 \n");
if (NULL == m_tempdata)
{
printf("going cleaning \n" );
Cleanup();
return;
}
m_freedata = m_tempdata->next;
}
void ClassA::Cleanup()
{
printf("cleanup starts \n");
// ... some additional work todo
}
Please don't use goto. It make the code much more difficult to understand, debug and maintain. Plus you end up making silly mistakes like you did here because you made poor design decisions earlier. Use modern flow-control constructs instead.

Answer 2

You don't seem to have a good/compelling reason to use goto in this code, so don't. Use more readable control flow structures.
printf("stage 1\n");
if (NULL != m_tempdata)
{
printf("stage 2\n");
// ...
}
else
{
printf("cleanup starts\n");
// ...
}
If you need to do the cleanup regardless of whether "stage 2" runs or not, remove the else block and have the cleanup code at the end.

Answer 3

Why should it stop? There's nothing to interrupt control flow between the statement printf("stage 2 \n"); and the statement printf("cleanup starts \n"); (which happens to be labelled cleanup:).
If you really want it to interrupt, you'll have to insert a return statement before the cleanup: label.
But please, seriously reconsider using goto. It makes programs very difficult to reason about - always prefer structured flow (ifs, loops, function calls).

Answer 4

There's nothing to tell the compiler that the function should exit after "stage 2". If you want it to return you have to actually tell the compiler that by adding a return statement.

Answer 5

I suspect you are trying to use a vintage C programming pattern like this one:
void do_hot_stuff (void)
{
void * data1 = malloc (123);
if (data1 == NULL) goto fail1;
void * data2 = malloc (123);
if (data2 == NULL) goto fail2;
void * data3 = malloc (123);
if (data3 == NULL) goto fail3;
// do your hot stuff here
free (data3);
fail3:
free(data2);
fail2:
free(data1);
fail1:
}
Frankly I did not use this since I last wrote code for my Amiga back in the late 80's.
Just don't.