在 switch 语句中声明变量 [英] Declaring variables inside a switch statement

问题描述

我看到了一些关于这个问题的答案，我明白了——你不能在 switch 中声明和分配变量.但我想知道以下在抛出错误时是否正确

I saw a few answers to this issue, and I get it — you can't declare and assign variables inside a switch. But I'm wondering if the following is correct at throwing an error saying

错误:'int' 之前的预期表达式

error: expected expression before 'int'

代码:

switch (i) {
    case 0:
        int j = 1;
        break;
}

为什么要在没有错误之前调用 NSLog() ?

Why would putting a call to NSLog() before it result in no errors?

switch (i) {
    case 0:
        NSLog(@"wtf");
        int j = 1;
        break;
}

推荐答案

如果您根据语言的语法进行声明，您实际上可以在开关中声明变量.你会得到一个错误，因为case 0:"是一个标签，在 C 中将 declaration 作为标签后的第一个语句是非法的——注意编译器需要一个表达式，例如方法调用、正常赋值等.(虽然可能很奇怪，但这是规则.)

You actually can declare variables within a switch if you do it according to the syntax of the language. You're getting an error because "case 0:" is a label, and in C it's illegal to have a declaration as the first statement after a label — note that the compiler expects an expression, such as a method call, normal assignment, etc. (Bizarre though it may be, that's the rule.)

当您将 NSLog() 放在首位时，您就避免了这个限制.您可以将 case 的内容括在 { } 大括号中以引入作用域块，或者您可以将变量声明移到 switch 之外.您选择哪个是个人喜好的问题.请注意，在 { } 大括号中声明的变量仅在该范围内有效，因此使用它的任何其他代码也必须出现在这些大括号中.

When you put the NSLog() first, you avoided this limitation. You can enclose the contents of a case in { } braces to introduce a scoping block, or you can move the variable declaration outside the switch. Which you choose is a matter of personal preference. Just be aware that a variable declared in { } braces is only valid within that scope, so any other code that uses it must also appear within those braces.

顺便说一下，这种怪癖并不像您想象的那么罕见.在 C 和 Java 中，在 for、while 或 中使用局部变量声明作为单独的语句(意思是没有用大括号括起来)"也是非法的do 循环，甚至在 if 和 else 子句中.(实际上，这在 "Java Puzzlers"，我强烈推荐.)我认为我们通常不会写这样的错误开始，因为声明一个变量没有意义作为此类上下文中的唯一语句.但是，对于 switch/case 结构，有些人会省略大括号，因为 break 语句是关键语句用于控制流.

By the way, this quirk isn't as uncommon as you might think. In C and Java, it's also illegal to use a local variable declaration as the lone statement (meaning "not surrounded by braces) in a for, while, or do loop, or even in if and else clauses. (In fact, this is covered in puzzler #55 of "Java Puzzlers", which I highly recommend.) I think we generally don't write such errors to begin with because it makes little sense to declare a variable as the only statement in such contexts. With switch / case constructs, though, some people omit the braces since the break statement is the critical statement for control flow.

要查看编译器是否合适，请将这个可怕的、毫无意义的片段复制到您的 (Objective-)C 代码中:

To see the compiler throw fits, copy this horrific, pointless snippet into your (Objective-)C code:

if (1)
    int i;
else
    int i;
for (int answer = 1; answer <= 42; answer ++)
    int i;
while (1)
    int i;
do
    int i;
while (1);

总是使用 { } 大括号来分隔此类结构体的另一个原因.:-)

Yet another reason to always use { } braces to delimit the body of such constructs. :-)

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