Why is this an issue?
When the execution is not explicitly terminated at the end of a switch case, it continues to execute the statements of the following case. While
this is sometimes intentional, it often is a mistake which leads to unexpected behavior.
Noncompliant code example
switch (myVariable) {
case 1:
foo();
break;
case 2: // Both 'doSomething()' and 'doSomethingElse()' will be executed. Is it on purpose ?
doSomething();
default:
doSomethingElse();
break;
}
Compliant solution
switch (myVariable) {
case 1:
foo();
break;
case 2:
doSomething();
break;
default:
doSomethingElse();
break;
}
Exceptions
This rule is relaxed in the following cases:
switch (myVariable) {
case 0: // Empty case used to specify the same behavior for a group of cases.
case 1:
doSomething();
break;
case 2: // Use of return statement
return;
case 3: // Use of throw statement
throw 1;
case 4: // Use of an attribute to make explicit the fact that we want to fall through the next case
doSomething();
[[fallthrough]];
case 5: // Use of continue statement, if the switch is inside a loop
continue;
default: // For the last case, use of break statement is optional
doSomethingElse();
}
Resources
- MISRA C:2004, 15.0 - The MISRA C switch syntax shall be used.
- MISRA C:2004, 15.2 - An unconditional break statement shall terminate every non-empty switch clause
- MISRA C++:2008, 6-4-3 - A switch statement shall be a well-formed switch statement.
- MISRA C++:2008, 6-4-5 - An unconditional throw or break statement shall terminate every non-empty switch-clause
- MISRA C:2012, 16.1 - All switch statements shall be well-formed
- MISRA C:2012, 16.3 - An unconditional break statement shall terminate every switch-clause
- MITRE, CWE-484 - Omitted Break Statement in Switch
- CERT, MSC17-C. - Finish every set of statements associated with a case label with a
break statement
