函数参数的评估顺序
以下操作的结果将打印什么：
x=5;
printf("%d,%d,%d\n",x,x<<2,x>>2);
答案：5,20,1
我以为顺序未定义，但我在很多网站上发现了上面的面试问题。
What will be printed as the result of the operation below:
x=5;
printf("%d,%d,%d\n",x,x<<2,x>>2);
Answer: 5,20,1
I thought order is undefined yet I found above as interview question on many sites.
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评论（5）
来自 C++ 标准：
但是，如果参数为
x>>=2和x<<=2（这样 x 就会被修改），您的示例才会有未定义的行为。From the C++ standard:
However, your example would only have undefined behavior if the arguments were
x>>=2andx<<=2, such that x were being modified.位移位运算符不会修改变量的值...因此顺序并不重要。
Bit shift operators don't modify the value of the variable... so order doesn't matter.
评估顺序未指定，但这并不重要，因为您根本没有修改
x。所以程序是明确定义的，答案也是给定的。
以下内容将具有未定义的语义：
The order of evaluation is unspecified, but it doesn't matter because you're not modifying
xat all.So the program is well-defined, and the answer is as given.
The following would have undefined semantics:
我在 c++ 标准 中找到了答案。
第 5.2.2.8 段：
换句话说，它仅取决于编译器。
I found the answer in c++ standards.
Paragraph 5.2.2.8:
In other words, It depends on compiler only.
官方 C 规范中未定义求值顺序。
然而，出于实用性的考虑，参数通常是从右到左计算的。
在您的问题中，位移运算符不会更改 X 的值，因此评估顺序并不重要。无论是从左到右、从右到左还是从中间开始计算，您都会得到 5,20,1。
在 C 中，参数按从右到左的顺序推送到堆栈上，因此第一个参数（在本例中为 char* "%d,%d,%d"）位于堆栈的顶部。参数通常（但并非总是）按照它们推送的顺序进行评估。
一个能更好地说明您所讨论内容的问题是：
官方答案是“未定义”。
实际的答案（在我尝试过的几个编译器/平台中）是“3,2,1”。
The order of evaluation is undefined in the Official C Specification.
However, as a matter of practicality, parameters are usually evaluated right-to-left.
In your problem, the bit-shift operator doesn't change the value of X, so the order of evaluation is not important. You'd get 5,20,1, whether evaluated left-to-right, right-to-left, or middle-first.
In C, parameters are pushed on to the stack in a right-to-left order, so that the 1st param (in this case, the char* "%d,%d,%d") is at the top of the stack. Parameters are usually (but not always) evaluated in the same order they are pushed.
A problem that better illustrates what you're talking about is:
The official answer is "undefined".
The practical answer, (in the several compilers/platforms I've tried), is "3, 2, 1".