What will be the result of this condition? Is it defined or compiler-dependant?
c++, conditional-statements, operator-precedence
Solution
The order of evaluation of the operand of `==` is not defined (I am assuming that `==` is not overloaded). It is not guaranteed whether `oldval` will evaluate first or `oldval = new_value()` will. Behavior is undefined in this case.
Avoid writing such expressions that access the value of a variable and also modify that variable elsewhere in the expression.
NOTE: Only the operators, `,` , `&&`, `||` and `?:` guarantee that the operand evaluation takes place from left to right. So, there exist sequence point:
- Between evaluation of the left and right operands of the `&&` (logical AND), `||` (logical OR) (as part of short-circuit evaluation), and comma operators.
- Between the evaluation of the first operand of the ternary "question-mark" operator and the second or third operand.
Problem
If I want to assign a new value to a variable and check if the new value is the same as the old value, I would tend to avoid using temporary variables. Could I do something like this: ``` if (oldval == (oldval = new_value())) { ... do something } ``` ? Would this behaviour be well-defined or is the evaluation priority language or compiler dependant? I could try this and see the result, but it would not guarantee that it will work on other systems as well. I am doing it in C++, so this is the language which interests me the most, but if the behaviour is consistent (or inconsistent) amongst other languages, I would like to know. Thanks!