继承 子类 可以继承父类的方法，从而实现子类可以利用父类的方法， 重写 是 子类可以重写父类的方法，这样子类的这个方法将覆盖掉父类的方法。 虚方法 使用时 可以通过父类的指针指向子类对象，从而子类对象到父类的引用。
父类的实现
#import<Foundation/Foundation.h>

@interfacefather :NSObject
{
@private
inta;

@protected
intb;

@public
intc;
int_age;
}

- (id) init;
- (void) setA:(int)newa;
- (int) getA;
- (void) setage:(int)newage;
- (void) printage;
- (void) jump;  //定义一个虚方法，在子类中进行实现。

@end

#import"father.h"

@implementation father

- (id) init
{
   if (self = [super init])
   {
b = 4;
c = 5;
   }
   return self;
}

- (void) setA:(int)newa  //这两个就可以作为私有变量的桥梁了
{
a = newa;
}

- (int) getA  
{
   return a;
}

- (void) setage:(int)newage
{
_age = newage;
}

- (void) printage
{
NSLog(@"the age is %d", _age);
}

- (void) jump
{
   return;
}


- (NSString *) description  //后续在子类的方法中会有该方法的重写
{

NSString *descrip = [[NSStringalloc] initWithFormat:@"the valuable is a = %d, b = %d, c = %d", a, b, c];

   return ([descrip autorelease]);
}

@end

子类继承的实现
#import<Foundation/Foundation.h>
#import"father.h"

@interfaceson :father
{
intd;
inte;
}

- (id) init;

@end

#import"son.h"

@implementation son
- (id) init
{
   if (self = [super init])
   {
d = 49;
e = 59;
   }
   return self;
}

- (void) jump
{
NSLog(@"the son is jump tall");
   return;
}

- (NSString *) description   //子类的重写，这样就会将父类的方法进行覆盖
{
// [self description];
   int newa =  [super getA];   //这里应该用父类的一个调用，而不应该仅仅是getA
NSString * str = [[NSStringalloc] initWithFormat:@"a = %d, b = %d, c = %d, d = %d, e = %d", newa, b, c, d, e];

   return ([str autorelease]);
}

@end



#import"father.h"

@interface doughter : father

@end

#import"doughter.h"

@implementation doughter
- (void) jump
{
NSLog(@"the doughter is jump down");
   return;
}

@end

主程序的实现：这里jump即为虚方法：
#import<Foundation/Foundation.h>
#import"father.h"
#import"son.h"
#import"doughter.h"

intmain(intargc,constchar* argv[])
{

@autoreleasepool{

// insert code here...
NSLog(@"Hello, World!");

father*fat = [[fatheralloc]init];

NSLog(@"%@", fat);

       [fatsetA:6];
NSLog(@"%@", fat);


son*so = [[sonalloc]init];
NSLog(@"the son is %@", so);
       [sosetA:10];   //子类继承了父类的setA方法，直接进行使用

NSLog(@"the father is %@", fat);
NSLog(@"the newson is %@", so);  //这里用到了description方法，对father父类方法进行了重写覆盖
       [sosetage:13];
       [soprintage];  //子类继承了父类的setA方法，直接进行使用

//虚方法的使用
//调用方法时不看指针看对象，
//由于父类的指针可以指向子类的对象，也即子类对象指向父类引用
father*fatson = [[sonalloc]init];
       [fatsonjump];

father*fatdoughter = [[doughteralloc]init];
       [fatdoughterjump];



       [fatrelease];
       [sorelease];
       [fatsonrelease];
       [fatdoughterrelease];
   }
return0;
}


运行结果：
2014-03-20 22:12:20.480集成[1270:403] Hello, World!
2014-03-20 22:12:20.484集成[1270:403] the valuable is a = 0, b = 4, c = 5
2014-03-20 22:12:20.486集成[1270:403] the valuable is a = 6, b = 4, c = 5
2014-03-20 22:12:20.486集成[1270:403] the son is a = 0, b = 4, c = 5, d = 49, e = 59
2014-03-20 22:12:20.487集成[1270:403] the father is the valuable is a = 6, b = 4, c = 5
2014-03-20 22:12:20.488集成[1270:403] the newson is a = 10, b = 4, c = 5, d = 49, e = 59
2014-03-20 22:12:20.489集成[1270:403] the age is 13
2014-03-20 22:12:20.489集成[1270:403] the son is jump tall
2014-03-20 22:12:20.490集成[1270:403] the doughter is jump down