? 在重構(gòu)程序修改了許多地方代碼后運(yùn)行，xcode 報(bào)了1k+的重復(fù)引用錯(cuò)誤容达，經(jīng)過(guò)排查發(fā)現(xiàn)是常量定義出現(xiàn)的，在此引用stackoverflow上的一段回答解釋花盐。
原回答link http://stackoverflow.com/questions/16144666/apple-mach-o-linker-error-with-duplicate-symbols-even-after-linking-the-m-class
? 事實(shí)上是在一個(gè)全局參數(shù)頭文件.h中使用const來(lái)定義常量導(dǎo)致出錯(cuò) ?改用define后就可以。
? 但是只是用define無(wú)法對(duì)指針進(jìn)行比較操作柒昏，更好的方法是使用const來(lái)定義熙揍。
? extern c關(guān)鍵字，表示這個(gè)變量已經(jīng)聲明，只是引用是尖。const關(guān)鍵字表示變量是常量泥耀，不可修改。
#import
extern NSString * const kInitURL;
@interface Constants : NSObject {
}
@end
? .m文件賦值
#import "Constants.h"
NSString * const kInitURL = @"http://marshal.easymorse.com";
@implementation Constants
@end
? 之后引入Constans.h就可以使用常量了痰催。
參考:
? 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.
