逗号运算符的局限性

发布于 2024-10-09 18:39:37 字数 366 浏览 8 评论 0原文

我已阅读这个问题，我想补充一点，可以做什么不能使用逗号运算符来完成。这让我很困惑，因为我可以这样做：

int arr[3];
arr[0]=1,arr[1]=2,arr[2]=3;

但是当我这样做时：

int arr[3],arr[0]=1,arr[1]=2,arr[2]=3;

它给了我一个编译器错误。

我想问一下，逗号运算符在实际应用中有何局限性？

原文

I have read this question and I want to add to it that what are the things that can not be done using the comma operator. This has confused me a lot, as I can do this:

int arr[3];
arr[0]=1,arr[1]=2,arr[2]=3;

But when I do:

int arr[3],arr[0]=1,arr[1]=2,arr[2]=3;

It gives me a compiler error.

I want to ask that what are the limitations of the comma operator in real practice?

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往事随风而去 2024-10-16 18:39:37

需要认识到的一件事是，并非 C 中逗号的所有使用都是逗号运算符的实例。将第二个示例更改为语法声明：

int a0=1,a1=2,a2=3;

逗号不是运算符，它们只是分隔列表中声明符实例所需的语法。

此外，参数/参数列表中使用的逗号不是逗号运算符。

在我看来，使用逗号运算符几乎总是一个坏主意 - 它只会导致不必要的混乱。在大多数情况下，使用逗号运算符所做的事情可以使用单独的语句更清楚地完成。

很容易想到的两个例外是 for 语句的控制子句内部，以及绝对需要将多个“事物”塞入单个表达式中的宏，即使这样也应该这样做当没有其他合理选择时）。

One thing to realize is that not all uses of a comma in C are instances of the comma operator. Changing your second example to be a syntactically declaration:

int a0=1,a1=2,a2=3;

the commas are not operators, they're just syntax required to separate instances of declarators in a list.

Also, the comma used in parameter/argument lists is not the comma operator.

In my opinion the use of the comma operator is almost always a bad idea - it just causes needless confusion. In most cases, what's done using a comma operator can be more clearly done using separate statements.

Two exceptions that come to mind easily are inside the control clauses of a for statement, and in macros that absolutely need to cram more than one 'thing' into a single expression, and even this should only be done when there's no other reasonable option).

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无可置疑 2024-10-16 18:39:37

您可以在表达式可以出现的大多数地方使用逗号运算符。但也有一些例外；值得注意的是，不能在常量表达式中使用逗号运算符。

使用逗号运算符时还必须小心，其中逗号也用作分隔符，例如，在调用函数时，必须使用括号对逗号表达式进行分组：

void f(int, bool);

f(42, 32, true);   // wrong
f((42, 32), true); // right (if such a thing can be considered "right")

您的示例是声明：

int arr[3],arr[0]=1,arr[1]=2,arr[2]=3;

在声明中，您可以声明通过用逗号分隔多个内容，因此这里也使用逗号作为分隔符。另外，您不能像这样将表达式添加到声明的末尾。（请注意，您可以使用 int arr[3] = { 1, 2, 3 }; 获得所需的结果）。

You can use the comma operator most anywhere that an expression can appear. There are a few exceptions; notably, you cannot use the comma operator in a constant expression.

You also have to be careful when using the comma operator where the comma is also used as a separator, for example, when calling functions you must use parentheses to group the comma expression:

void f(int, bool);

f(42, 32, true);   // wrong
f((42, 32), true); // right (if such a thing can be considered "right")

Your example is a declaration:

int arr[3],arr[0]=1,arr[1]=2,arr[2]=3;

In a declaration, you can declare multiple things by separating them with the comma, so here too the comma is used as a separator. Also, you can't just tack on an expression to the end of a declaration like this. (Note that you can get the desired result by using int arr[3] = { 1, 2, 3 };).

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