How to break out of a loop from inside a switch?

Question

Hi,

I'm writing some code that looks like this:

while(true) {
    switch(msg->state) {
    case MSGTYPE: // ... 
        break;
    // ... more stuff ...
    case DONE:
        break; // **HERE, I want to break out of the loop itself**
    }
}

Is there any direct way to do that?

I know I can use a flag, and break from the loop by putting a conditional break just after the switch. I just want to know if C++ has some construct for this already.

Thanks,
jrh

Answer 1

You can use goto.

while ( ... ) {
   switch( ... ) {
     case ...:
         goto exit_loop;

   }
}
exit_loop: ;

Answer 2

You could potentially use goto.

But I would prefer to set a flag that stops the loop.
Then break out of the switch.

Answer 3

AFAIK there is no "double break" or similar construct in C++. The closest would be a goto - which, while it has a bad connotation to its name, exists in the language for a reason - as long as it's used carefully and sparingly, it's quite a viable option.

Answer 4

A neatish way to do this would be to put this into a function:

int yourfunc() {

    while(true) {

        switch(msg->state) {
        case MSGTYPE: // ... 
            break;
        // ... more stuff ...
        case DONE:
            return; 
        }

    }
}

Optionally (but 'bad practices'): as already suggested you could use a goto, or throw an exception inside the switch.

Answer 5

There's no C++ construct for breaking out of the loop in this case.

Either use a flag to interrupt the loop or (if appropriate) extract your code into a function and use return.

Answer 6

while( true )

The following code should be considered bad form, regardless of language or desired functionality:

while( true ) {
}

Reasons

The while( true ) loop is poor form because it:

• Breaks the implied contract of a while loop.
  • The while loop declaration should explicitly state the only exit condition.
• Implies that it loops forever.
  • Code within the loop must be read to understand the terminating clause.
  • Loops that repeat forever prevent the user from terminating the program from within the program.
• Is inefficient.
  • There are multiple loop termination conditions, including checking for "true".
• Is prone to bugs.
  • Cannot easily determine where to put code that will always execute for each iteration.
• Is more difficult to prove as correct.

Alternative to Goto

The following code is better form:

while( isValidState() ) {
  execute();
}

bool isValidState() {
  return msg->state != DONE;
}

Advantages

No flag. No goto. No exception. Easy to change. Easy to read. Easy to fix. Additionally the code:

1. Isolates the knowledge of the loop's workload from the loop itself.
2. Allows someone maintaining the code to easily extend the functionality.
3. Allows multiple terminating conditions to be assigned in one place.
4. Separates the terminating clause from the code to execute.
5. Is safer for Nuclear Power plants. ;-)

The second point is important. Without knowing how the code works, if someone asked me to make the main loop let other threads (or processes) have some CPU time, two solutions come to mind:

Option #1

Readily insert the pause:

while( isValidState() ) {
  execute();
  sleep();
}

Option #2

Override execute:

void execute() {
  super->execute();
  sleep();
}

This code is simpler (thus easier to read) than a loop with an embedded switch. The isValidState method should only determine if the loop should continue. The workhorse of the method should be abstracted into the execute method, which allows subclasses to override the default behaviour (a difficult task using an embedded switch and goto).

Answer 7

Even if you don't like goto, do not use an exception to exit a loop. The following sample shows how ugly it could be:

try {
  while ( ... ) {
    switch( ... ) {
      case ...:
        throw 777; // I'm afraid of goto
     }
  }
}
catch ( int )
{
}

I would use goto as in this answer. In this case goto will make code more clear then any other option. I hope that this question will be helpful.

But I think that using goto is the only option here because of the string while(true). You should consider refactoring of your loop. I'd suppose the following solution:

bool end_loop = false;
while ( !end_loop ) {
    switch( msg->state ) {
    case MSGTYPE: // ... 
        break;
    // ... more stuff ...
    case DONE:
        end_loop = true; break;
    }
}

Or even the following:

while ( msg->state != DONE ) {
    switch( msg->state ) {
    case MSGTYPE: // ... 
        break;
    // ... more stuff ...
}

Answer 8

You could put your switch into a separate function like this:

bool myswitchfunction()
{
    switch(msg->state) {
    case MSGTYPE: // ... 
        break;
    // ... more stuff ...
    case DONE:
        return false; // **HERE, I want to break out of the loop itself**
    }
    return true;
}

while(myswitchfunction())
    ;

Answer 9

An alternate solution is to use the keyword continue in combination with break, i.e.:

for (;;) {
    switch(msg->state) {
    case MSGTYPE
        // code
        continue; // continue with loop
    case DONE:
        break;
    }
    break;
}

Use the continue statement to finish each case label where you want the loop to continue and use the break statement to finish case labels that should terminate the loop.

Of course this solution only works if there is no additional code to execute after the switch statement.

Answer 10

If I remember C++ syntax well, you can add a label to break statements, just like for goto. So what you want would be easily written:

while(true) {
    switch(msg->state) {
    case MSGTYPE: // ...
        break;
    // ... more stuff ...
    case DONE:
        break outofloop; // **HERE, I want to break out of the loop itself**
    }
}

outofloop:
// rest of your code here

Answer 11

The simplest way to do it is to put a simple IF before you do the SWITCH , and that IF test your condition for exiting the loop .......... as simple as it can be