Disallow assignments in conditions (#2907332) · Issues · project / coding_standards

Disallow assignments in conditions

>>> [!note] Migrated issue   Reported by: [JvE](https://www.drupal.org/user/892998) >>> <h3 id="summary-problem-motivation">Problem/Motivation</h3> I thought it might be interesting to propose this as a new standard. From the SEI CERT Oracle Coding Standard for Java (<a href="https://www.securecoding.cert.org/confluence/display/java/EXP51-J.+Do+not+perform+assignments+in+conditional+expressions">https://www.securecoding.cert.org/confluence/display/java/EXP51-J.+Do+not+perform+assignments+in+conditional+expressions</a>): Using the assignment operator in conditional expressions frequently indicates programmer error and can result in unexpected behavior. The assignment operator should not be used in the following contexts: <ul> <li><code>if</code> (controlling expression)</li> <li><code>while</code> (controlling expression)</li> <li><code>do ... while</code> (controlling expression)</li> <li><code>for</code> (second operand)</li> <li><code>switch</code> (controlling expression)</li> <li><code>?:</code> (first operand)</li> <li><code>&&</code> (either operand)</li> <li><code>||</code> (either operand)</li> <li><code>?:</code> (second or third operands) where the ternary expression is used in any of these contexts</li> </ul> <h3>Noncompliant Code Example</h3> In this noncompliant code example, the controlling expression in the <code>if</code> statement is an assignment expression: <div class="codeblock"> <pre><?php public function f($a, $b) {   if ($a = $b) {     /* ... */   } } ?></pre></div> Although the programmer's intent could have been to assign <code>b</code> to <code>a</code> and test the value of the result, this usage frequently occurs when the programmer mistakenly used the assignment operator <code>=</code> rather than the equality operator <code>===</code>. <h2>Compliant Solution</h2> The conditional block shown in this compliant solution executes only when <code>a</code> is equal to <code>b</code>: <div class="codeblock"> <pre><?php public function f($a, $b) {   if ($a === $b) {     /* ... */   } } ?></pre></div> Unintended assignment of <code>b</code> to <code>a</code> cannot occur. <h2>Compliant Solution</h2> When the assignment is intended, this compliant solution clarifies the programmer's intent: <div class="codeblock"> <pre><?php public function f($a, $b) {   if (($a = $b) === TRUE) {     /* ... */   } } ?></pre></div> <h2>Compliant Solution</h2> It may be clearer to express the logic as an explicit assignment followed by the <code>if</code> condition: <div class="codeblock"> <pre><?php public function f($a, $b) {   $a = $b;   if ($a) {     /* ... */   } } ?></pre></div> <h3>Noncompliant Code Example</h3> In this noncompliant code example, an assignment expression appears as an operand of the <code>&&</code> operator: <div class="codeblock"> <pre><?php public function f($a, $b, $flag) {   while(($a = $b) && $flag) {     /* ... */   } } ?></pre></div> Because <code>&&</code> is not a comparison operator, assignment is an illegal operand. Again, this is frequently a case of the programmer mistakenly using the assignment operator <code>=</code> instead of the equals operator <code>===</code>. <h2>Compliant Solution</h2> When the assignment of <code>b</code> to <code>a</code> is unintended, this conditional block is now executed only when <code>a</code> is equal to <code>b</code> and <code>flag</code> is <code>true</code>: <div class="codeblock"> <pre><?php public function f($a, $b, $flag) {   while(($a === $b) && $flag) {     /* ... */   } } ?></pre></div> <h2>Applicability</h2> The use of the assignment operator in controlling conditional expressions frequently indicates programmer error and can result in unexpected behavior. As an exception to this guideline, it is permitted to use the assignment operator in conditional expressions when the assignment is not the controlling expression (that is, the assignment is a subexpression), as shown in the following compliant solution: <div class="codeblock"> <pre><?php public function readLines($reader) {   $line = '';   while(($line = $reader->readLine()) !== NULL) {     // ... Work with $line.   } } ?></pre></div>

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