黑马程序员——Objective-C学习笔记(七) :初始化对象

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分配对象

分配(allocation)是一个新对象诞生的过程。向某个类发送 alloc 消息，就能为类分配一块足够大的内存，以存放该类的全部实例对象。同时alloc 方法还顺便将这块内存区域全部初始化为0.
刚分配的对象并不能立即使用，需要先初始化，然后才能使用。只执行分配而未进行初始化的对象可能会出现bug。

初始化对象

与分配对应的操作是初始化。init方法一般都会返回七正在初始化的对象。

调用的正确方式如下：
Car *car = [[Car alloc] init];

编写初始化方法

(id) init
{
    if (self = [super init])
    {
        engine = [Engine new];
        tires[0] = [Tire new];
        tires[1] = [Tire new];
        tires[2] = [Tire new];
        tires[3] = [Tire new];
    }
    return (self);
} // init

重点在第一行：

`if (self = [super init])`

声明中最先运行的代码是[super init]，作用是让超类完成七自身的初始化。self参数是通过固定的距离寻找实例变量的内存位置的。如果从init方法返回一个新对象，则需要更新self，以便其后的实例变量的引用可以被映射到正确的内存位置。这个赋值操作只影响该init方法中self的值，而不影响该方法范围以外的任何内容。init方法最后一行代码是return (self);init方法返回了刚刚已经被初始化的对象。

Tire类的初始化

类声明如下：

#import <Cocoa/Cocoa.h>
@interface Tire: NSObject
{
    float pressure;
    float treadDepth;
}
- (void) setPressure: (float) pressure;
- (float) pressure;
- (void) setTreadDepth: (float) treadDepth;
- (float) treadDepth;
@end // Tire

Tire类的实现:

#import "Tire.h"
@implementation Tire

-(id) init
{ 
    if (self = [super init])
    {
        pressure = 34.0;
        treadDepth = 20.0;
    }
    return (self);
} // init

- (void) setPressure: (float) p
{
    pressure = p;
} // setPressure
- (float) pressure
{
    return (pressure);
} // pressure
-(void) setTreadDepth:(float) td
{
    treadDepth = td;
} // treadDepth
- (NSString) description
{
    NSString *desc;
    desc = [NSString stringWithFormat: @"Tire: Pressure: %.1f TreadDepth: %.1f",pressure, treadDepth];
    return (desc);
} // description
@end // Tire

main()函数：

#import "Engine.h"
#import "Car.h"
#import "Slant6.h"
#import "AllweatherRadial.h"
int main(int argc,const char *argv[])
{
    @autoreleasepool
    {
        Car *car = [[Car alloc] init];
        for (int i=0; i<4; i++)
        {
            Tire *tire;
            tire = [[Tire alloc] init];
            [tire setPressure: 23+i];
            [tire setTreadDepth: 33-i];
            [car setTire: tire atIndex: i];
            [tire release];
        }
        Engine *engine = [[Slant6 alloc] init];
        [car setEngine: engine];
        [car print];
        [car release];
    }
    return (0);
} // main

Car类

#import <Cocoa/Cocoa.h>
@class Tire;
@class Engine;
@interface Car: NSObject
{
    NSMutableArray: *tires;
    Engine *engine;
}
- (void) setEngine: (Engine *) newEngine;
- (Engine *) engine;
- (void) setTire: (Tire *) tire atIndex: (int) index;
- (Tire *) tireAtIndex: (int) index;
- (void) print;
@end // Car

#import "Car.h"
@implementation Car

- (id) init
{
    if (self = [super init])
    {
        tires = [[NSMutableArray alloc] init];
        for (init i=0; i<4; i++)
        {
            [tires addObject: [NSNull null]];
        }
    }
    return (self);
} // init
- (void) setEngine: (Engine *) newEngine
{
    [newEngine retain];
    [engine release];
    engine = newEngine;
} // setEngine

- (Engine *) engine
{
    return (engine);
} // engine

- (void) setTire: (Tire *) tire atIndex: (int) index
{
    [tire replaceObjectAtIndex: index withObject: tire];
} // setTire: atIndex:

- (Tire *) tireAtIndex: (int) index
{
    Tire *tire;
    tire = [tires objectAtIndex: index];
    return (tire);
} // tireAyIndex:

- (void) dealloc
{
    [tires release];
    [engine release];
    [super dealloc];
} // dealloc
// 一定要调用超类的dealloc方法，并且要确保[super dealloc]是dealloc方法的最后一条语句

- (void) print
{
    for (int i=0; i<4; i++)
    {
        NSLog(@"%@", [self tireAtIndex: i]);
    }
    NSLog (@"%@", engine);
} // print
@end // Car

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