EXP18-C. Do not perform assignments in selection statements

Do not use the assignment operator in the outermost expression of a selection statement (if or switch) or an iteration statement (while, do, or for) because it typically indicates programmer error and can result in unexpected behavior.

Noncompliant Code Example

In this noncompliant code example, an assignment expression is the outermost expression in an if statement.

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

Although the intent of the code may be to assign b to a and test the value of the result for equality to zero, 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.

Compliant Solution

When the assignment of b to a is not intended, this conditional block is now executed when a is equal to b.

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

When the assignment is intended, the following is an alternative compliant solution:

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

It is less desirable in general, depending on what was intended, because it mixes the assignment in the condition, but it is clear that the programmer intended the assignment to occur.

Risk Assessment

Errors of omission can result in unintended program flow.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography