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• Controlling expression of if, switch  switch (selection statement)
• Controlling expression of while, do...while (iteration statement) 
• Second operand of for for (iteration statement) 
• First operand of  ?: (selection statement)
• Either operand of || or && (logical operators) 
• Second operand of comma operator when the comma expression is used in any of these contexts
• Second and third operands  of ?:  (selection statement) where the ternary expression is used in any of these contexts

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In this noncompliant example, the expression x = y is used as the controlling expression of the while statement.

 do { /* ... */ } while ( foo(), x = y ) ;

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do { /* ... */ } while ( foo(), (x = y) != 0) ;

Noncompliant Code Example

In this non-compliant noncompliant example, the expression p = q is used as the controlling expression of the while statement.

 do { /* ... */ } while ( x = y, p = q ) ;

Compliant Solution

This is a compliant example because the expression x = y is not used as the controlling expression of the while statement.

do { /* ... */ } while ( x = y, p == q ) ; 

Exceptions

EXP18-EXP1: Assignment can be used where the result of the assignment is itself a parameter to a comparison expression or relational expression.

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In this compliant example, the expression  x = y is a single primary expression.

if ( ( x = y ) ) { /* ... */ } 

EXP18-EXP3: Assignment can be used in the above contexts if it occurs in a function argument or array index.

In this compliant example, the expression x = y is used in a function argument.

if ( foo( x = y ) ) { /* ... */ } 

if ( ( v = w ) && flag ) { /* ... */ }

if ( ( v == w ) && flag ) { /* ... */ }; 

if ( ( (v = w) != 0 ) && flag ) { /* ... */ };

Errors of omission can result in unintended program flow.

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Bibliography

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