我可以在 Objective-C switch 语句中声明变量吗？
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Can I declare variables inside an Objective-C switch statement?
提问by JoBu1324
I think I'm going blind, because I can't figure out where the syntax error is in this code:
我想我要瞎了，因为我无法弄清楚这段代码中的语法错误在哪里：
if( cell == nil ) {
titledCell = [ [ [ TitledCell alloc ] initWithFrame:CGRectZero
reuseIdentifier:CellIdentifier ] autorelease
];
switch( cellNumber ) {
case 1:
NSString *viewDataKey = @"Name";
etc...
When I try to compile it, I'm getting an Error: syntax error before '*' tokenon the last line.
当我尝试编译它时，我在最后一行的'*' 标记之前收到一个错误：语法错误。
Sorry for such a basic question, but what am I missing?
抱歉问这么基本的问题，但我错过了什么？
回答by ephemient
I don't have a suitable Objective-C compiler on hand, but as long as the C constructs are identical:
我手头没有合适的 Objective-C 编译器，但只要 C 结构相同：
switch { … }gives you oneblock-level scope, not one for each case. Declaring a variable anywhere other than the beginning of the scope is illegal, and inside a switchis especiallydangerous because its initialization may be jumped over.
switch { … }为您提供一个块级作用域，而不是每个case. 声明比任何地方范围的开始以外的变量是非法的，里面switch是特别危险的，因为它的初始化，可以跳过。
Do either of the following resolve the issue?
以下任一操作是否可以解决问题？
NSString *viewDataKey;
switch (cellNumber) {
case 1:
viewDataKey = @"Name";
…
}
switch (cellNumber) {
case 1: {
NSString *viewDataKey = @"Name";
…
}
…
}
回答by Chuck
You can't declare a variable at the beginning of a case statement. Make a test case that just consists of that and you'll get the same error.
不能在 case 语句的开头声明变量。制作一个只包含它的测试用例，你会得到同样的错误。
It doesn't have to do with variables being declared in the middle of a block — even adopting a standard that allows that won't make GCC accept a declaration at the beginning of a case statement. It appears that GCC views the case label as part of the line and thus won't allow a declaration there.
它与在块中间声明的变量无关——即使采用允许不会让 GCC 在 case 语句开头接受声明的标准。似乎 GCC 将 case 标签视为行的一部分，因此不允许在那里声明。
A simple workaround is just to put a semicolon at the beginning of the case so the declaration is not at the start.
一个简单的解决方法是在案例的开头放一个分号，这样声明就不会在开头。
回答by Aragorn
In C you only can declare variables at the begining of a block before any non-declare statements.
在 C 中，您只能在块的开头在任何非声明语句之前声明变量。
{
/* you can declare variables here */
/* block statements */
/* You can't declare variables here */
}
In C++ you can declare variables any where you need them.
在 C++ 中，您可以在任何需要的地方声明变量。
回答by Phil Miller
Might it not be valid to declare a variable within a switch block?
在 switch 块中声明变量可能无效吗？
回答by egarlock
You can create a variable within a switch statement but you will have to create it within a block so that the scope of that variable is defined.
您可以在 switch 语句中创建变量，但必须在块中创建它，以便定义该变量的范围。
Example:
例子：
switch(number){
case 1:
{
// Create object here
// object is defined only for the scope of this block
}
break;
case 2:
{
// etc.
}
break;
default:
break;
}
回答by lin zheng
How to solve the warning:
如何解决警告：
1.Insert one ;in the first line of your case block
1.;在案例块的第一行插入一个
2.Put codes inside braces
2.将代码放在大括号内
