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

问题

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.

回答1:

Premise

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

while( true ) {
}

Supporting Arguments

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.
Leads to unnecessarily complex code.
Automatic source code analysis.
- To find bugs, program complexity analysis, security checks, or automatically derive any other source code behaviour without code execution, specifying the initial breaking condition(s) allows algorithms to determine useful invariants, thereby improving automatic source code analysis metrics.
Infinite loops.
- If everyone always uses while(true) for loops that are not infinite, we lose the ability to concisely communicate when loops actually have no terminating condition. (Arguably, this has already happened, so the point is moot.)

Alternative to "Go To"

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:

Isolates the knowledge of the loop's workload from the loop itself.
Allows someone maintaining the code to easily extend the functionality.
Allows multiple terminating conditions to be assigned in one place.
Separates the terminating clause from the code to execute.
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).

Python Example

Contrast the following answer (to a Python question) that was posted on StackOverflow:

Loop forever.
Ask the user to input their choice.
If the user's input is 'restart', continue looping forever.
Otherwise, stop looping forever.
End.

Code

while True: 
    choice = raw_input('What do you want? ')

    if choice == 'restart':
        continue
    else:
        break

print 'Break!'

Versus:

Initialize the user's choice.
Loop while the user's choice is the word 'restart'.
Ask the user to input their choice.
End.

Code

choice = 'restart';

while choice == 'restart': 
    choice = raw_input('What do you want? ')

print 'Break!'

Here, while True results in misleading and overly complex code.

回答2:

You can use goto.

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

   }
}
exit_loop: ;

回答3:

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.

回答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.

回答5:

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 a viable option.

回答6:

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())
    ;

回答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 ...
}

回答8:

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.

回答9:

You could potentially use goto, but I would prefer to set a flag that stops the loop. Then break out of the switch.

回答10:

Why not just fix the condition in your while loop, causing the problem to disappear?

while(msg->state != DONE)
{
    switch(msg->state) {
    case MSGTYPE: // ... 
        break;
    // ... more stuff ...
    case DONE:
        // We can't get here, but for completeness we list it.
        break; // **HERE, I want to break out of the loop itself**
    }
}

回答11:

No, C++ does not have a construct for this, given that the keyword "break" is already reserved for exiting the switch block. Alternatively a do..while() with an exit flag could suffice.

do { 
    switch(option){
        case 1: ..; break;
        ...
        case n: .. ;break;
        default: flag = false; break;
    }
} while(flag);

回答12:

I think;

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

回答13:

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

回答14:

The break keyword in C++ only terminates the most-nested enclosing iteration or switch statement. Thus, you couldn't break out of the while (true) loop directly within the switch statement; however you could use the following code, which I think is an excellent pattern for this type of problem:

for (; msg->state != DONE; msg = next_message()) {
    switch (msg->state) {
    case MSGTYPE:
        //...
        break;

    //...
    }
}

If you needed to do something when msg->state equals DONE (such as run a cleanup routine), then place that code immediately after the for loop; i.e. if you currently have:

while (true) {
    switch (msg->state) {
    case MSGTYPE:
        //... 
        break;

    //...

    case DONE:
        do_cleanup();
        break;
    }

    if (msg->state == DONE)
        break;

    msg = next_message();
}

Then use instead:

for (; msg->state != DONE; msg = next_message()) {
    switch (msg->state) {
    case MSGTYPE:
        //...
        break;

    //...
    }
}

assert(msg->state == DONE);
do_cleanup();

回答15:

It amazes me how simple this is considering the depth of explanations... Here's all you need...

bool imLoopin = true;

while(imLoopin) {

    switch(msg->state) {

        case MSGTYPE: // ... 
            break;

        // ... more stuff ...

        case DONE:
            imLoopin = false;
            break;

    }

}

LOL!! Really! That's all you need! One extra variable!

回答16:

while(MyCondition) {
switch(msg->state) {
case MSGTYPE: // ... 
    break;
// ... more stuff ...
case DONE:
   MyCondition=false; // just add this code and you will be out of loop.
    break; // **HERE, you want to break out of the loop itself**
}
}

回答17:

I got same problem and solved using a flag.

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

回答18:

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

回答19:

  while(true)
  {
    switch(x)
    {
     case 1:
     {
      break;
     }
    break;
   case 2:
    //some code here
   break;
  default:
  //some code here
  }
}

来源：https://stackoverflow.com/questions/1420029/how-to-break-out-of-a-loop-from-inside-a-switch

标签

c++

loops

syntax

switch-statement

break