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Do not use the assignment operator in the following contexts because it contexts listed in the following table because doing so typically indicates programmer error and can result in unexpected behavior:.

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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 0, 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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When the assignment is intended, the following is an alternative compliant solutionthis compliant solution explicitly uses inequality as the outermost expression while performing the assignment in the inner expression:

if ((a = b) != 0) {
  /* ... */
}

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Noncompliant Code Example

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

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

The same result can be obtained using the for statement, which is specifically designed to evaluate an expression on each iteration of the loop, just before performing the test in its controlling expression:

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

Compliant Solution (Unintentional Assignment)

When the assignment of y to x is not intended, this conditional block is now executed when x is equal to y, as in this compliant solution:

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

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When the assignment is intended, the following is an alternative this compliant solution can be used:

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

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

Compliant Solution

This is a compliant example because In this compliant solution, the expression x = y is not used as the controlling expression of the while statement:

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EXP45-EX1: Assignment can be used where the result of the assignment is itself a parameter an operand to a comparison expression or relational expression. In this compliant example, the expression x = y  is itself a parameter an operand to a comparison operation:

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EXP45-EX2: Assignment can be used where the expression consists of a single primary expression. In this compliant example, The following code is compliant because the expression  x = y is a single primary expression:

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

EXP45-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:

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

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The following controlling expression is noncompliant because && is not a comparison or relational operator and the entire expression is not primary:

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

When the assignment of v

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to w

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is not intended,

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the following controlling expression can be used to execute the conditional block

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when v

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is equal to w:

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

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When the assignment is intended, the following

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controlling expression can be used:

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

EXP45-EX3: Assignment can be used in a function argument or array index. In this compliant solution, the expression x = y is used in a function argument:

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

Errors of omission can result in unintended program flow.

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Bibliography

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