用逻辑运算符对子表达式求值的C

用逻辑运算符对子表达式求值的C

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问题描述

关于优先级评估顺序的概念有很多疑问,但我找不到与我的特殊情况有关的问题.

There are lots of questions on concepts of precedence and order of evaluation but I failed to find one that refers to my special case.

考虑以下语句:

if(f(0) && g(0)) {};

保证会首先评估f(0)吗?请注意,运算符是&&.

Is it guaranteed that f(0) will be evaluated first? Notice that the operator is &&.

我的困惑源于我在"The C ++ Programming Language,(Stroustrup,4ed,2013)"中阅读的内容.

My confusion stems from what I've read in "The C++ Programming Language, (Stroustrup, 4ed, 2013)".

这本书的10.3.2节说:

In section 10.3.2 of the book, it says:

int x = f(2)+ g(3); //不确定是先调用f()还是g()

int x = f(2)+g(3); // undefined whether f() or g() is called first

这似乎适用于所有运营商,包括和&&运算符,但在以下段落中表示:

This seems to apply to all operators including && operator, but in a following paragraph it says:

在11.1.1节中还另外提到了这一点:

There is also another mention of this in section 11.1.1:

while(p&!whitespace(p))++ p;

while (p && !whitespace(p)) ++p;

在这里,如果p是nullptr,则不会取消引用.

Here, p is not dereferenced if it is the nullptr.

最后一个引号表示&&和||首先评估他们的第一个参数,因此似乎加强了我的假设,即第二个引号中提到的运算符是第一个引号的例外,但我也不能从最后一个例子中得出明确的结论,因为该表达式仅包含一个子表达式,而不是我的示例,其中包含两个.

This last quote implies that && and || evaluate their 1st argument first, so it seems to reinforce my assumption that operators mentioned in 2nd quote are exceptions to 1st quote, but I cannot draw a definitive conclusion from this last example either, as the expression contains only one subexpression as opposed to my example, which contains two.

推荐答案

&&||,的特殊排序行为在C和C ++中已得到很好的确立.您引用的第一句话应为通常未指定表达式中子表达式的求值顺序"或除少数特定例外情况之外,未指定表达式中子表达式的求值顺序".

The special sequencing behavior of &&, ||, and , is well-established in C and C++. The first sentence you quoted should say "The order of evaluation of subexpressions within an expression is generally unspecified" or "With a few specific exceptions, the order of evaluation of subexpressions within an expression is unspecified".

您问过有关C ++的问题,但此问题 http://c-faq.com/expr/seqpointops.html"rel =" nofollow noreferrer> C常见问题解答列表是相关的.

You asked about C++, but this question in the C FAQ list is pertinent.

附录:我刚刚意识到,在这些规则中,未指定"比未定义"更好.编写类似f() + g()的东西不会给您未定义的行为.您只是无法知道先调用f还是g.

Addendum: I just realized that "unspecified" is a better word in these rules than "undefined". Writing something like f() + g() doesn't give you undefined behavior. You just have no way of knowing whether f or g might be called first.

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08-06 18:09