This diagnostic rule is based on the software development guidelines developed by MISRA (Motor Industry Software Reliability Association).
The 'sizeof()' operator does not execute the expression passed to it but only evaluates the type of the resulting expression and returns its size at compile time. Therefore, no assembler code is generated for any code inside 'sizeof()' (unevaluated context) and no operations inside it will ever be executed.
For that reason, to avoid such loss of operations, the operand passed to 'sizeof()' must not have any other side effects.
Example of non-compliant code:
int x = ....; .... size_t s = n * sizeof(x++);
To achieve the desired behavior, the snippet should be rewritten as follows:
int x = ....; .... ++x; size_t s = n * sizeof(x);