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Testing for
loop termination using an equality operator (==
and !=
) is dangerous, because it could set up an infinite loop. Using a broader relational operator instead casts a wider net, and makes it harder (but not impossible) to accidentally write an infinite loop.
Noncompliant Code Example
for (int i = 1; i != 10; i += 2) // Noncompliant. Infinite; i goes from 9 straight to 11.
{
//...
}
Compliant Solution
for (int i = 1; i <= 10; i += 2) // Compliant
{
//...
}
Exceptions
Equality operators are ignored if the loop counter is not modified within the body of the loop and either:
- starts below the ending value and is incremented by 1 on each iteration.
- starts above the ending value and is decremented by 1 on each iteration.
Equality operators are also ignored when the test is against null
.
for (int i = 0; arr[i] != null; i++) {
// ...
}
for (int i = 0; (item = arr[i]) != null; i++) {
// ...
}
See
- MISRA C++:2008, 6-5-2
- MITRE, CWE-835 - Loop with Unreachable Exit Condition ('Infinite Loop')
- CERT, MSC21-C - Use robust loop termination conditions
- CERT, MSC21-CPP - Use inequality to terminate a loop whose counter changes by more than one
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