Is “}while(0);” always equal to “break;}while(1);”?

Question

I have compared gcc assembler output of

do{ 

//some code 

}while(0);

with

do{

//some code

 break; 
}while(1);

The output is equal, with or without optimization but..

It's always that way?

No experiment can prove theories, they can only show they are wrong

Answer 1

There is a slight difference:

do {
  // code
  if ( condition )
    continue;
  // code
  break;
} while(1);

Will restart the loop when condition is true, whereas in the } while(0); version, the continue will be equivalent to break.

If no continue is present, then they should produce exactly the same code.

Answer 2

The forms are not equivalent. This is an infinite loop:

do {
    continue;
    break;
} while (1);

This isn't:

do {
    continue;
} while (0);

Answer 3

Edit: Upon reading various comments on the matter, I will admit that this answer is wrong. Sorry.

Instead of:

do{ 

//some code 

}while(0);

or:

do{

//some code

 break; 
}while(1);

I would just use:

//some code

I'm not 100% sure if you can do this in c++, but if you want to limit the scope of variables, and that is why you are doing this, just use curly braces by themselves:

{

 // Some Code

}

Answer 4

Markus' comment pointed me to this answer: the difference is when using continue keyword.

In this case:

int _tmain(int argc, _TCHAR* argv[])
{
    int i = 0;
    do {
        ++i;
        _tprintf(_T("Iteration %d\n"), i);
        if (i < 30) continue;
    } while(0);

    return 0;
}

you get only one iteration, while in this case:

int _tmain(int argc, _TCHAR* argv[])
{
    int i = 0;
    do {
        ++i;
        _tprintf(_T("Iteration %d\n"), i);
        if (i < 30) continue;
        break;
    } while(1);

    return 0;
}

you get 30 iterations. Tested under VS2008.

Answer 5

The do while clauses are logically equivalent. If they are translated to the same byte code depends on the compiler at hand. I guess that most modern compilers will treat them equally.

Answer 6

EDIT based on your comment that you're using a while with breaks in order to be able to break out of the 'loop' when certain conditions have been met.

If this is what you're trying to accomplish:

do
{ 
  // processing step 1
  if( some_condition )
    break;
  // processing step 2
  if( some_condition )
    break;
  // etcetera..
} while(0)

...then just break the code you have in your while loop out to a stand-alone function with multiple returns:

void processing()
{

  // processing step 1
  if( some_condition )
    return;
  // processing step 2
  if( some_condition )
    return;
  // etcetera..
}

int main()
{
  // ...
  processing();
  return 0;
}