object_getIvar

object_getIvar 获得类的变量（包括“所谓私有变量”）

/** 
 * Reads the value of an instance variable in an object.
 * 
 * @param obj The object containing the instance variable whose value you want to read.
 * @param ivar The Ivar describing the instance variable whose value you want to read.
 * 
 * @return The value of the instance variable specified by \e ivar, or \c nil if \e object is \c nil.
 * 
 * @note \c object_getIvar is faster than \c object_getInstanceVariable if the Ivar
 *  for the instance variable is already known.
 */
OBJC_EXPORT id object_getIvar(id obj, Ivar ivar) 
     __OSX_AVAILABLE_STARTING(__MAC_10_5, __IPHONE_2_0);

#import <Foundation/Foundation.h>

@interface Person : NSObject

@property(nonatomic, assign) NSInteger age;
@property(nonatomic, strong) NSString * name;

@end

#import "Person.h"

@interface Person ()

@property(nonatomic, strong) NSString * sex;

@end

@implementation Person

- (instancetype)init
{
    self = [super init];
    if (self) {
        self.sex = @"-----------";
    }
    return self;    
}
@end

#import "ViewController.h"
#import <objc/runtime.h>
#import "Person.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    
    
    Person * p1 = [[Person alloc] init];
    
    Ivar ivar = class_getInstanceVariable([p1 class], "_sex");
    
    id sex = object_getIvar(p1, ivar);
    
    NSLog(@"%@", sex);
    
}

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