黑马程序员——Object-C基础(六)面向对象的特性：继承

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1. 继承的基本用法

例子：写一个矩形类，在写一个继承它的正方形类：

#import <Foundation/NSObject.h>

@interface MyRectangle: NSObject {
int width;
int height;
}

-(MyRectangle*) initWithWidth: (int) w height: (int) h;
-(void) setWidth: (int) w;
-(void) setHeight: (int) h;
-(void) setWidth: (int) w height: (int) h;
-(int) width;
-(int) height;
-(void) print;
@end

//2. MyRectangle.m

#import "MyRectangle.h"
#import <stdio.h>

@implementation MyRectangle
-(MyRectangle*) initWithWidth: (int) w height: (int) h {
self = [super init];

if ( self ) {
[self setWidth: w height: h];
}

return self;
}

-(void) setWidth: (int) w {
width = w;
}

-(void) setHeight: (int) h {
height = h;
}

-(void) setWidth: (int) w height: (int) h {
width = w;
height = h;
}

-(int) width {
return width;
}

-(int) height {
return  height;
}

-(void) print {
printf( "width = %i, height = %i", width, height );
}
@end

//3. Square.h
#import "MyRectangle.h"

@interface Square: MyRectangle
-(Square*) initWithSize: (int) s;
-(void) setSize: (int) s;
-(int) size;
@end

//4. Square.m
#import "Square.h"

@implementation Square
-(Square*) initWithSize: (int) s {
self = [super init];

if ( self ) {
[self setSize: s];
}

return self;
}

-(void) setSize: (int) s {
width = s;
height = s;
}

-(int) size {
return width;
}

-(void) setWidth: (int) w {
[self setSize: w];
}

-(void) setHeight: (int) h {
[self setSize: h];
}
@end

//5. main.m
#import "Square.h"
#import "MyRectangle.h"
#import <stdio.h>

int main( int argc, const char *argv[] ) {
MyRectangle *rec = [[MyRectangle alloc] initWithWidth: 10 height: 20];
Square *sq = [[Square alloc] initWithSize: 15];

// print em
printf( "MyRectangle: " );
[rec print];
printf( "\n" );

printf( "Square: " );
[sq print];
printf( "\n" );

// update square
[sq setWidth: 20];
printf( "Square after change: " );
[sq print];
printf( "\n" );

//调用继承自父类的width和height
printf( "Square after width: %i\n", [sq width] );
printf( "Square after height: %i", [sq height] );
// free memory
[rec release];
[sq release];

return 0;
}

2. 编译运行：
gcc -fconstant-string-class=NSConstantString -c main.m -I /GNUstep/System/Library/Headers
gcc -c MyRectangle.m -I /GNUstep/System/Library/Headers
gcc -c Square.m -I /GNUstep/System/Library/Headers
gcc main.o Square.o MyRectangle.o -o main -L /GNUstep/System/Library/Libraries/ -lobjc -lgnustep-base

$ ./main.exe
MyRectangle: width = 10, height = 20
Square: width = 15, height = 15
Square after change: width = 20, height = 20
Square after width: 20Square after height: 20

子类方法和属性的访问过程：如果子类没有，就去访问父类的
父类被继承了还是能照常使用的
父类的静态方法
画继承结构图，从子类抽取到父类
NSObject的引出：全部OC类的最终父类，包含了一些常用方法，比如+new

2. 继承的专业术语

父类\超类 superclass
子类 subclass\subclasses

3. 继承的细节

单继承
子类和父类不能有相同的成员变量
方法的重写

4. super关键字

分别调用父类的对象方法和类方法

5. 继承的好处

不改变原来模型的基础上，拓充方法
建立了类与类之间的联系
抽取了公共代码
坏处：耦合性强

6. 继承的使用场合

它的所有属性都是你想要的，一般就继承
它的部分属性是你想要的，可以抽取出另一个父类