c - C语言的求值顺序

Question

1. x+=x*=x 是未定义的行为吗？
2. 谁能在Order of evaluation中解释这条规则？ ？什么是“单一评价”？ “单一评估”的反义词是什么？

   14) With respect to an indeterminately-sequenced function call, the operation of compound assignment operators, and both prefix and postfix forms of increment and decrement operators are single evaluations.

Answer 1

x+=x*=x 具有未定义的行为，因为 x 在序列点之间被分配了两次。

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C11中14)对应的文字，C17说

A compound assignment of the form E1 op= E2 is equivalent to the simple assignment expression E1 = E1 op (E2), except that the lvalue E1 is evaluated only once, and with respect to an indeterminately-sequenced function call, the operation of a compound assignment is a single evaluation.

我认为它的意思是

int x = 0;

int foo(void) {
    x = 5;
    return x;
}

int main(void) {
    int y = foo() + (x += 2); 
}

将有或者

的行为

int main(void) {
    int _tmp = x += 2;
    int y = foo() + _tmp; 
}

或

int main(void) {
    int _tmp = foo();
    int y = _tmp + (x += 2); 
}

不能分割成例如

int main(void) {
    int _tmp = x;
    int _tmp2 = foo();
    x = _tmp + 2;
    int y = _tmp2 + x; 
}

注意这个保证在C11中是新增的，在C89、C99中是不存在的。