[03]类和对象

以Car为例：

1、类的声明和实现

//类的声明@interface Car {}@end//类的实现@implementation Car@endint main(){    return 0;}

2、对象的创建

#import<Foundation/Foundation.h>//类的声明@interface Car : NSObject//（继承语法）:NSObject使得类具有创建对象的能力{}@end//类的实现@implementation Car@endint main(){    Car *p = [Car new];//利用类来创建对象：[行为执行者 行为名称]    return 0;}

3、对象的行为

区分函数和对象行为：
都作为功能：
①函数：C语言中 void run();
②行为：OC中 - (void)run;
都有三要素：名称、参数、返回值

#import<Foundation/Foundation.h>//类的声明@interface Car : NSObject//（继承语法）:NSObject使得类具有创建对象的能力,所以需要包含头文件{}- (void)run;@end//类的实现（定义）@implementation Car- (void)run{    NSLog(@"Car is running!");}@endint main(){    Car *p = [Car new];//利用类来创建对象：[行为执行者 行为名称]，在使用类创建对象之前，会将类加载进内存    [p run];//消息机制：给指针p所指的对象发送一条run消息    return 0;}

理解：消息机制

4、成员变量及其赋值

#import<Foundation/Foundation.h>//类的声明@interface Car : NSObject{    int speed;//属性（成员变量、实例变量:默认会初始化为0）    int wheels;}- (void)run;@end//类的实现（定义）@implementation Car- (void)run{    NSLog(@"Car is running!");}@endint main(){    Car *p = [Car new];    [p run];    return 0;}

5、方法与成员变量

#import<Foundation/Foundation.h>//类的声明@interface Car : NSObject{    @public//①如果没有这句代码，会怎么样呢？    int speed;//属性（成员变量、实例变量:默认会初始化为0）    int wheels;}- (void)run;@end//类的实现（定义）@implementation Car- (void)run{    NSLog(@"Car is running!");    NSLog(@"speed is %dkm/s, the car has %d wheels",     speed, wheels);}@endint main(){    Car *p = [Car new];    [p run];    p->speed = 120;    p->wheels = 3;    return 0;}

对于①，如果没有，则：instance variable ‘speed’ is protected
输出结果是：speed is 0km/s, the car has 0 wheels，证明了它会默认为0

利用上面的知识点，看看下面这个程序：

#import<Foundation/Foundation.h>@interface Person : NSObject{    @public    double weight;    int age;}- (void)sing;@end@implementation Person- (void)sing{    NSLog(@"\aThere is a(an) %d-year-old girl singing under the tree.She's %lf kg", age, weight);}@endint main(){    Person *p1 = [Person new];    p1->weight = 45.2;    [p1 sing];    Person *p2 = [Person new];    p2->age = 22;    [p2 sing];    return 0;}

There is a(an) 0-year-old girl singing under the tree.She’s 45.200000 kg
There is a(an) 22-year-old girl singing under the tree.She’s 0.000000 kg

6、对象的简单内存分析

（1）类中有方法列表
（2）每个对象内部默认都有一个isa指针（作用：指向对象所指向的类），这样看来：每个对象都在共用方法，但是访问成员变量是访问对象内部的成员变量（每个对象的都不相同）

方法列表在类中，成员变量在各对象内部

7、对象与函数参数

#import<Foundation/Foundation.h>@interface Person : NSObject{    @public    double weight;    int age;}- (void)sing;@end@implementation Person- (void)sing{    NSLog(@"\aThere is a(an) %d-year-old girl singing under the tree.She's %lf kg", age, weight);}int changeNum(double w, int a){    w = 55;    a = 99;    return 0;}@endint main(){    Person *p = [Person new];    p->weight = 45.2;    p->age = 22;    changeNum(p->weight, p->age);    [p sing];    return 0;}

值传递
There is a(an) 22-year-old girl singing under the tree.She’s 45.200000 kg

#import<Foundation/Foundation.h>@interface Person : NSObject{    @public    double weight;    int age;}- (void)sing;@end@implementation Person- (void)sing{    NSLog(@"\aThere is a(an) %d-year-old girl singing under the tree.She's %lf kg", age, weight);}int changeNum(Person *changeP){    changeP->weight = 55;    changeP->age = 99;    return 0;}@endint main(){    Person *p = [Person new];    p->weight = 45.2;    p->age = 22;    changeNum(p);    [p sing];    return 0;}

更改了实例变量
There is a(an) 99-year-old girl singing under the tree.She’s 55.000000 kg

#import<Foundation/Foundation.h>@interface Person : NSObject{    @public    double weight;    int age;}- (void)sing;@end@implementation Person- (void)sing{    NSLog(@"\aThere is a(an) %d-year-old girl singing under the tree.She's %lf kg", age, weight);}int changeNum(Person *changeP){    Person *p1 = [Person new];//①    p1->weight = 999.99;    p1->age = 999;    changeP = p1;    changeP->age = 100000000;    return 0;}@endint main(){    Person *p = [Person new];//②    p->weight = 45.2;    p->age = 22;    changeNum(p);    [p sing];    return 0;}

There is a(an) 22-year-old girl singing under the tree.She’s 45.200000 kg
结果没改变，为什么？
从①和②看出，p和p1是共用方法不互干扰的两个独立对象，即：p1的属性不影响p
虽然在调用函数时将p地址传给changeP,由于没有进行操作，p属性没有改变，但后来changeP指向p1，改变的也只是p1中的实例变量，下面通过实例，看看是不是这样的

#import<Foundation/Foundation.h>@interface Person : NSObject{    @public    double weight;    int age;}- (void)sing;@end@implementation Person- (void)sing{    NSLog(@"\aThere is a(an) %d-year-old girl singing under the tree.She's %lf kg", age, weight);}int changeNum(Person *changeP){    Person *p1 = [Person new];    p1->weight = 999.99;    p1->age = 999;    changeP = p1;    changeP->age = 111;    NSLog(@"p1->weight = %lf, p1->age = %d", p1->weight, p1->age);//重点    return 0;}@endint main(){    Person *p = [Person new];    p->weight = 45.2;    p->age = 22;    changeNum(p);    [p sing];    return 0;}

2015-11-13 00:04:22.215 a.out[876:89224] p1->weight = 999.990000, p1->age = 111
2015-11-13 00:04:22.216 a.out[876:89224] There is a(an) 22-year-old girl singing under the tree.She’s 45.200000 kg