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Do not use the assignment operator in the following cases because it typically indicates programmer error and can result in unexpected behavior:

• Controlling expression of if, switch (selection statement)
• Controlling expression of while, do...while (iteration statement) 
• Second operand of 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 of ?:  (selection statement) where the ternary expression is used in any of these contexts

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Although the intent of the code may be to assign b to a and test the value of the result for equality to zero0, it is very frequently a case of the programmer mistakenly using the assignment operator = instead of the equals operator ==. Consequently, many compilers will warn about this condition, making this coding error detectable by adhering to MSC00-C. Compile cleanly at high warning levels.

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

Noncompliant Code Example

In this 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-EX1: Assignment can be used where the result of the assignment is itself a parameter to a comparison expression or relational expression. In this compliant example, the expression x = y  is itself a parameter to a comparison operation:

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EXP18-EX2: Assignment can be used where the expression consists of a single primary expression. In this compliant example, the expression  x = y is a single primary expression:

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EXP18-EX3: 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:

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

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

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

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

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