c - (a += 3, 5, a) 中的操作数将如何处理或计算以打印 "a"的值？

Question

代码片段是:

int main()
{
     int a = 1, b = 2, c = 3;
     printf("%d", a += (a += 3, 5, a));
}

虽然它在终端中显示 8 作为输出。但我不明白它背后的概念。

Answer 1

表达式 a += (a += 3, 5, a) 将调用未定义的行为。

C 标准说

C11:6.5.16 赋值运算符(p3):

[...] The side effect of updating the stored value of the left operand is sequenced after the value computations of the left and right operands. The evaluations of the operands are unsequenced.

不保证最左边的a会在(a += 3, 5, a)求值之前或之后被求值。这将导致未定义的行为。