Using continue in a switch statement

问题

I want to jump from the middle of a switch statement, to the loop statement in the following code:

while (something = get_something())
{
    switch (something)
    {
    case A:
    case B:
        break;
    default:
        // get another something and try again
        continue;
    }
    // do something for a handled something
    do_something();
}

Is this a valid way to use continue? Are continue statements ignored by switch statements? Do C and C++ differ on their behaviour here?

回答1:

It's fine, the continue statement relates to the enclosing loop, and your code should be equivalent to (avoiding such jump statements):

while (something = get_something()) {
    if (something == A || something == B)
        do_something();
}

But if you expect break to exit the loop, as your comment suggest (it always tries again with another something, until it evaluates to false), you'll need a different structure.

For example:

do {
    something = get_something();
} while (!(something == A || something == B));
do_something();

回答2:

Yes, it's OK - it's just like using it in an if statement. Of course, you can't use a break to break out of a loop from inside a switch.

回答3:

Yes, continue will be ignored by the switch statement and will go to the condition of the loop to be tested. I'd like to share this extract from The C Programming Language reference by Ritchie:

The continue statement is related to break, but less often used; it causes the next iteration of the enclosing for, while, or do loop to begin. In the while and do, this means that the test part is executed immediately; in the for, control passes to the increment step.

The continue statement applies only to loops, not to a switch statement. A continue inside a switch inside a loop causes the next loop iteration.

I'm not sure about that for C++.

回答4:

It's syntactically correct and stylistically okay.

Good style requires every case: statement should end with one of the following:

 break;
 continue;
 return (x);
 exit (x);
 throw (x);
 //fallthrough

Additionally, following case (x): immediately with

 case (y):
 default:

is permissible - bundling several cases that have exactly the same effect.

Anything else is suspected to be a mistake, just like if(a=4){...} Of course you need enclosing loop (while, for, do...while) for continue to work. It won't loop back to case() alone. But a construct like:

while(record = getNewRecord())
{
    switch(record.type)
    {
        case RECORD_TYPE_...;
            ...
        break;
        default: //unknown type
            continue; //skip processing this record altogether.
    }
    //...more processing...
}

...is okay.

回答5:

While technically valid, all these jumps obscure control flow -- especially the continue statement.

I would use such a trick as a last resort, not first one.

How about

while (something = get_something())
{
    switch (something)
    {
    case A:
    case B:
        do_something();
    }        
}

It's shorter and perform its stuff in a more clear way.

回答6:

This might be a megabit to late but you can use continue 2.

Some php builds / configs will output this warning:

PHP Warning: "continue" targeting switch is equivalent to "break". Did you mean to use "continue 2"?

For example:

$i = 1;

while ($i <= 10) {
    $mod = $i % 4;
    echo "\r\n out $i";
    $i++;
    switch($mod)
    {
        case 0:
            break;
        case 2:
            continue;
            break;
        default:
            continue 2;
            break;
    }
    echo " is even";
}

This will output:

out 1
out 2 is even
out 3
out 4 is even
out 5
out 6 is even
out 7
out 8 is even
out 9
out 10 is even

Tested with PHP 5.5 and higher.

回答7:

Switch is not considered as loop so you cannot use Continue inside a case statement in switch...

来源：https://stackoverflow.com/questions/2146763/using-continue-in-a-switch-statement

标签

c++

switch-statement

break

continue