objective-c 整数常量不会“减少为整数”
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integer constant does 'not reduce to an integer'
提问by Dan Morgan
I use this code to set my constants
我使用此代码来设置我的常量
// Constants.h
extern NSInteger const KNameIndex;
// Constants.m
NSInteger const KNameIndex = 0;
And in a switch statement within a file that imports the Constant.h file I have this:
在导入 Constant.h 文件的文件中的 switch 语句中，我有这个：
switch (self.sectionFromParentTable) {
case KNameIndex:
self.types = self.facilityTypes;
break;
...
I get error at compile that read this: "error:case label does not reduce to an integer constant"
我在编译时收到错误消息：“错误：案例标签不会减少为整数常量”
Any ideas what might be messed up?
任何想法可能会搞砸？
回答by epatel
For C/C++ and Objective-C must the case statement have fixed values - "reduced to an integer (read value)" at compile time
对于 C/C++ 和 Objective-C，case 语句必须具有固定值 -在编译时“减少为整数（读取值）”
Your constants is not a real "constant" because it is a variable and I imagine it can be changed through a pointer - ie &KNameIndex
您的常量不是真正的“常量”，因为它是一个变量，我想它可以通过指针进行更改 - 即 &KNameIndex
Usually one defines constants as enum
通常将常量定义为 enum
enum {
KNameIndex = 0,
kAnotherConstant = 42
};
If you would use C++, or Objective-C++ (with .mm as file extension) you could use a const statement as
如果您将使用 C++ 或 Objective-C++（以 .mm 作为文件扩展名），您可以使用 const 语句作为
const int KNameIndex = 0;
回答by daniel
You can use
您可以使用
#define KNameIndex 0
...
switch (self.sectionFromParentTable) {
case KNameIndex:
self.types = self.facilityTypes;
break;
...
and it should work.
它应该工作。
Just had the same problem and I decided to go with #define rather than enum. Works for me™ ;-)
刚刚遇到了同样的问题，我决定使用 #define 而不是 enum。对我来说有效™ ;-)
回答by Petriborg
This is a stab in the dark because I haven't used Cocoa / ObjC in a long time now, but is the member variable sectionFromParentTable not of int type?
这是黑暗中的刺，因为我已经很长时间没有使用 Cocoa / ObjC，但是成员变量 sectionFromParentTable 不是 int 类型吗？
回答by derobert
I have not worked with Objective C, but I'd try chucking the 'extern'. At least if this were C++, the Constants.mfile would not be part of the compilation unit of Other.m, so the value of KNameIndexwould be unknown to the compiler. Which would explain the error; an unknowable value can't be a constant.
我没有使用过 Objective C，但我会尝试放弃“extern”。至少如果这是 C++，该Constants.m文件将不是 的编译单元的一部分Other.m，因此KNameIndex编译器不知道的值。这将解释错误；一个不可知的值不能是一个常数。
Does putting the definition, not just the declaration, in the Constants.hfile help?
将定义而不只是声明放在Constants.h文件中是否有帮助？
回答by crashmstr
I think you are stuck with using a const intinstead of a const NSIntegeras switch only works with built in integral types. (not sure about your syntax with const flipped around after the type).
我认为您坚持使用 aconst int而不是 aconst NSInteger作为开关仅适用于内置的整数类型。（不确定你的语法在类型后翻转了 const ）。
Take a look at the related question: Objective-C switch using objects?
