iOS关键字static extern const使用示例详解

#import "SEObject.h"
//定义在.h文件中时该类被其他文件引入时报重复定义的错误（1 duplicate symbol for architecture x86_64）
NSString * SEString = @"SEString";
@implementation SEObject
@end

#import "SEView.h"
//#import "SEObject.h"
//NSString * SEString;
@implementation SEView
@end

#import "SEView.h"
//#import "SEObject.h"
//NSString * SEString;
@implementation SEView
- (void)add {
    extern NSString * SEString;
    NSLog(@"%@",SEString);
    SEString = @"SEString2";
    NSLog(@"%@",SEString);
}
@end

#import <Foundation/Foundation.h>
NS_ASSUME_NONNULL_BEGIN
extern NSString * SEString;
@interface SEObject : NSObject
@end
NS_ASSUME_NONNULL_END

#import "SEView.h"
#import "SEObject.h"
//NSString * SEString;
@implementation SEView
- (void)add {
    //extern NSString * SEString;
    NSLog(@"%@",SEString);
    SEString = @"SEString2";
    NSLog(@"%@",SEString);
}
@end
SEString
SEString2

#import "SEObject.h"
//定义在.h文件中时该类被其他文件引入时报重复定义的错误（1 duplicate symbol for architecture x86_64）
static NSString * SEString = @"SEString";
@implementation SEObject
+ (void)add {
    NSLog(@"%@",SEString);
    SEString = @"SEString2";
    NSLog(@"%@",SEString);
}
SEString
SEString2
@end

static NSString * const SEString = @"SEString";

static NSString const * SEString = @"SEString";
static const NSString * SEString = @"SEString";

@implementation SEObject

- (void)add {
    NSInteger a = 1;
    NSInteger b = 2;
    NSInteger c = a+b;
    NSLog(@"c = %ld",c);
}

@end

- (void)add {
    NSInteger a = 1;
    NSInteger b = 2;
    static NSInteger c2;
    c2 += a+b;
    NSLog(@"c2 = %ld",c2);
}
调用两次结果：
c2 = 3
c2 = 6