EXP04-J. Do not perform assignments in conditional statements

Do not use the assignment operator in the outermost expression of an if or switch statement or a looping statement (while, do, or for) because this 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) {
  /* ... */
}

While 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 frequently a case of the programmer mistakenly using the assignment operator = instead of the equals operator ==.

Compliant Solution

When the assignment of b to a is unintended, this conditional block is now executed when a is equal to b.

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

When the assignment is intended, the following compliant solution may be used as the programmer intent is clearer:

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

Although it may be preferable to express this same logic as an assignment followed by a conditional:

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

Risk Assessment

Errors of omission can result in unintended program flow.

Related Guidelines

CERT C++ Secure Coding Standard: EXP19-CPP. Do not perform assignments in conditional expressions
CERT C Secure Coding Standard: EXP18-C. Do not perform assignments in selection statements

ISO/IEC TR 24772 "KOA Likely Incorrect Expressions"

MITRE CWE: CWE-480, "Use of Incorrect Operator"

Bibliography

[[Hatton 1995]] Section 2.7.2, "Errors of omission and addition"

EXP17-C. Do not perform bitwise operations in conditional expressions      03. Expressions (EXP)      EXP19-C. Use braces for the body of an if, for, or while statement