AUTOSAR C++14 Rule M6-5-6

A loop-control-variable other than the loop-counter which is modified in statement shall have type bool

Description

Rule Definition

A loop-control-variable other than the loop-counter which is modified in statement shall have type bool.

Rationale

Loops terminate when the loop-counter value satisfies a specified termination condition. You can use additional loop-control variables to end a loop early if needed.

For instance:

• for(ctr = 0 ; ctr <= 10; ctr++) {…} terminates when the value of ctr is greater than 10.

• for(ctr = 0 ; ctr <= 10 && level > 0; ctr++) {…} terminates when the value of ctr is greater than 10 or when the value of level is greater than 0.

In the second example, the condition level > 0 might not convey the reason for early loop termination. By using a Boolean variable as a loop-control variable, you can provide a more descriptive name that reflects the early termination state.

For example:

for(ctr = 0 ; ctr <= 10 && fuelTankNotEmpty; ctr++) 
{
    /...
    fuelTankNotEmpty = (level >= 0);
}

This Boolean variable is often referred to as a flag. Boolean flags make loop control logic easier to understand.

Polyspace Implementation

Polyspace^® raises this defect whenever a non-Boolean loop-control-variable is modified within the loop statement.

Troubleshooting

If you expect a rule violation but Polyspace does not report it, see Diagnose Why Coding Standard Violations Do Not Appear as Expected.

Examples

Avoid Modifying Non-Boolean Loop-Control-Variables Within the Loop Statement

#include <cstdint>

int32_t ctr, level = 1;
bool fuelTankNotEmpty = true;

void example()
{
	for(ctr = 0 ; ctr <= 10 && level >= 0; ctr++)		//Noncompliant
	{
		level--;
	}
	
	for (ctr = 0; ctr <= 10 && fuelTankNotEmpty; ctr++)	//Compliant
	{
		level--;
		fuelTankNotEmpty = (level >= 0);
	}

}

In the first for loop, because level is not a Boolean and is modified within the statement, Polyspace flags it as noncompliant.

The second loop shows how to use a Boolean flag to be compliant with this rule.

Check Information

Group: Statements

Category: Required, Automated

Version History

Introduced in R2019a

R2025a: Correct handling of struct fields as loop control variables

The rule checker no longer shows false positives when a loop control variable is a field of a struct variable passed by reference, for instance, in the following example:

struct e_struct_T {
   int nVar;
   int numEvals;
};
void t(e_struct_T &obj) {
   for (int idx{0}; idx < obj.nVar; idx++) { // No longer flagged
     obj.numEvals++;
   }
}