extern导致链接器错误

Question

似乎几乎是this question的副本，但答案并没有解决问题。

first.h

extern PlanningDocumentService *folderService; 

first.m

#import first.h 

- (void)serviceDefinition 
{  
    PlanningDocumentService *folderService = [_myDocumentViews objectAtIndex:[_gridView indexOfSelectedItem]]; 
    detail.service = folderService; 
} 

second.h

#import first.h 

second.m

#import second.h 

+(NSArray*)getDocumentData 
{ 
    NSString *passedCaption = folderService.caption; 
} 

给出了错误：

Undefined symbols for architecture i386: 
    "_folderService", referenced from: 
     +[GenericService getDocumentData] in GenericService.o 
ld: symbol(s) not found for architecture i386 
clang: error: linker command failed with exit code 1 (use -v to see invocation) 

我没有声明或定义除上述文件以外的任何地方folderService。 欢迎任何想法。

Answer 1

您需要定义folderService有全局范围：

first.m：

#import "first.h" 

PlanningDocumentService *folderService = nil; 

- (void)serviceDefinition 
{  
    folderService = [_myDocumentViews objectAtIndex:[_gridView indexOfSelectedItem]]; 
    detail.service = folderService; 
} 

Answer 2

PlanningDocumentService *folderService内- (void)serviceDefinition使局部变量的函数，而不是全球性的。但extern PlanningDocumentService *folderService声明了全局变量的存在，这是从未定义的。