Xcode 中大量报重复引用错误的一种可能

  在重构程序修改了许多地方代码后运行，xcode 报了1k+的重复引用错误，经过排查发现是常量定义出现的，在此引用stackoverflow上的一段回答解释。

原回答link http://stackoverflow.com/questions/16144666/apple-mach-o-linker-error-with-duplicate-symbols-even-after-linking-the-m-class

  事实上是在一个全局参数头文件.h中使用const来定义常量导致出错  改用define后就可以。

  但是只是用define无法对指针进行比较操作，更好的方法是使用const来定义。

  extern c关键字，表示这个变量已经声明，只是引用。const关键字表示变量是常量，不可修改。

#import

extern NSString * const kInitURL;

@interface Constants : NSObject {

}

@end

  .m文件赋值

#import "Constants.h"

NSString * const kInitURL = @"http://marshal.easymorse.com";

@implementation Constants

@end

  之后引入Constans.h就可以使用常量了。

参考:

  This is really really basic beginner stuff that suggests to me you don't understand the difference between C-family language's "header files" and "implementation files".Did you search your project and not notice that you are declaring variables in header files but without extern? You even had the name of the global variable in your linker error. Yes, global variables are emitted as symbols into object files (.o) and if you include a header into three files and all three files then declare the same global variable, you get this error. It was for just this issue that the mighty and wonderful extern keyword was invented.In the context of header files you should not declare actual variables, only extern references to them. This is something you should learn on your first day of C, C++ or Objective C programming. You can however put code into header files, but then, every file you include that header into will generate a duplicate set of symbols, leading to link errors, and further accelerating the pace of global warming, and causing the melting of icebergs. (Just kidding about that last part.)Here is your mistake, this is a header file but you are treating it like it was an implementation file and declaring not external-variable-declarations but actual-emit-the-global-variable-into-the-object-file declarations:

#importUIImage *SavedImage;

UIImage *SavedImage1;

NSString * vvvalue;

//UILabel * l2;

NSString * lll;

@interface NSObject (ShareClass)

@end

  You tell me now. Where should the keyword extern appear in the code sample above? Where does the real "emit this variable into object file" declaration of a variable go? In a header file? NO. In the implementation file (.m or .c or .cpp as the case may be).

  Secondly, after you add the extern keyword, you now have a second problem. You probably need to add the real declaration of that global variable in your corresponding .m file. Yes, that's right you have to "declare" every global twice in a C-family language program. once as extern in header files if you really need all the modules in your system to be able to see that global variable, and once without the extern keyword, in only one of your .m files.

  Of course, you do know that such "global variable mess" is to be avoided, and is terrible programming practice right? You should be thinking about keeping global variables out of your code, and have variables that limit their scope to a single implementation module wherever possible.

  In .h file:

extern UIImage * myImage;

  In .m file: 

UIImage *myImage;

  Go read your book on what the extern keyword means please. Also, I suspect that you might have meant to do this:

@interface NSObject (ShareClass)

UIImage * InstanceVariable;

@end

  The above is an instance variable (belongs to each instance of the class). It is common if you need to group some stuff and put it inside an object not to make it a global variable.