CodeQL documentation

Sizeof with side effects

ID: cpp/sizeof-side-effect
Kind: problem
Severity: recommendation
Precision: high
   - reliability
   - correctness
   - external/jsf
Query suites:
   - cpp-security-and-quality.qls

Click to see the query in the CodeQL repository

This rule finds sizeof expressions that have a parameter with side-effects. sizeof only uses the type of the parameter, so the parameter will not be evaluated. In the C99 standard, using sizeof on expressions with dynamic arrays may or may not evaluate the side-effect, so it is better to avoid it completely.


Simplify the sizeof parameter to use only the subexpression that is of the type you need.


int f(void){
	int i = 0;
	char arr[20];
	int size = sizeof(arr[i++]); //wrong: sizeof expression has side effect
	cout << i; //would output 0 instead of 1


  • AV Rule 166, Joint Strike Fighter Air Vehicle C++ Coding Standards. Lockheed Martin Corporation, 2005.
  • Tutorialspoint - The C++ Programming Language: C++ sizeof Operator