This question gets asked a fair amount. Be warned that every time someone asks this question a great many people post incorrect answers. Lots of people have incorrect ideas about how these operators work, including people who write programming books and thereby teach other people falsehoods. Read the other answers here very carefully. For a … Read more
Does the definition int a = 0, b = a++, c = a++; have defined behavior in C? Yes, because C 2018 6.8 3 says these initializations (not all, see bottom) are evaluated in the order they appear: … The initializers of objects that have automatic storage duration, and the variable length array declarators of … Read more
This is still undefined behavior (UB) as per expr.compound: The postfix-expression is sequenced before each expression in the expression-list and any default argument. The initialization of a parameter, including every associated value computation and side effect, is indeterminately sequenced with respect to that of any other parameter. (emphasis mine) This means that the postfix expression … Read more
C++98 and C++03 This answer is for the older versions of the C++ standard. The C++11 and C++14 versions of the standard do not formally contain ‘sequence points’; operations are ‘sequenced before’ or ‘unsequenced’ or ‘indeterminately sequenced’ instead. The net effect is essentially the same, but the terminology is different. Disclaimer : Okay. This answer … Read more
In terms of evaluation sequence, it is easier to think of x*f(x) as if it was: operator*(x, f(x)); so that there are no mathematical preconceptions on how multiplication is supposed to work. As @dan04 helpfully pointed out, the standard says: Section 1.9.15: “Except where noted, evaluations of operands of individual operators and of subexpressions of … Read more
C++17 introduced rules on sequences. What was UB before is now well defined. This applies to arguments to function calls as well as a select assortment of operators: sequenced before is an asymmetric, transitive, pair-wise relationship between evaluations within the same thread. If A is sequenced before B (or, equivalently, B is sequenced after A), … Read more
When a sequence point occurs, it basically means that you are guaranteed that all previous operations are complete. Changing a variable twice without an intervening sequence point is one example of undefined behaviour. For example, i = i++; is undefined because there’s no sequence point between the two changes to i. Note that it’s not … Read more
The expression f() + g() contains a minimum of 4 sequence points; one before the call to f() (after all zero of its arguments are evaluated); one before the call to g() (after all zero of its arguments are evaluated); one as the call to f() returns; and one as the call to g() returns. … Read more
No. This is not acceptable. This code invokes Undefined behavior. This is because of the operation on b is not defined. In the expression a=(a+b)-(b=a); it is not certain whether b gets modified first or its value gets used in the expression (a+b) because of the lack of the sequence point. See what standard syas: … Read more
It looks like the code i.operator+=(i.operator ++()); Works perfectly fine with regards to sequence points. Section 1.9.17 of the C++ ISO standard says this about sequence points and function evaluation: When calling a function (whether or not the function is inline), there is a sequence point after the evaluation of all function arguments (if any) … Read more