Day 10 - Pointer(指标)

Objective-C中的变数分为储存基本资料型态的数值(value)以及指标(pointer)

Pointer 其实就是一个变数而其值代表数值的位置，有点类似地址的概念。要想取得指标可以使用符号(&)，它表示内存中的地址访问。

ex.

int a = 10;
NSLog(@"a的值：%i",a);
NSLog(@"a的pointer: %p",&a);

结果：

2021-10-01 17:13:41.847800+0800 TestOC[33699:577420] a的值：10
2021-10-01 17:13:41.847858+0800 TestOC[33699:577420] a的pointer: 0x16d847fcc

Pointer本身是一个characters字串的记忆体位置，ex. 0x16b1bffcc，也就是不管 Pointer 指向的 object 多大，Pointer 的大小都是固定的，大大节省记忆体空间。

声明形式

用来声明指针的是星号(*)

type *var-name;

ex.

int a = 20;
int *ip;
    
ip = &a;
    
NSLog(@"a的pointer: %p",&a);
    
NSLog(@"储存ip值的地址: %p",ip);
    
NSLog(@"*ip的value值: %d",*ip);

结果：

2021-10-01 17:24:11.709216+0800 TestOC[33825:583275] a的pointer: 0x16d717fcc
2021-10-01 17:24:11.709264+0800 TestOC[33825:583275] 储存ip值的地址: 0x16d717fcc
2021-10-01 17:24:11.709299+0800 TestOC[33825:583275] *ip2的value值: 20

我们尝试把一个值传入方法，然後修改其值，返回後再传回原本的值，来比较看看他们的差异。

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    
    int i = 5;
    NSLog(@"i address is %p",&i);
    i = [self addOne:i];
    NSLog(@"i address is %p",&i);
}


- (int) addOne: (int) x {
    x += 1;
    NSLog(@"x is %i",x);
    NSLog(@"x address is %p",&x);
    return x;
}

2021-10-01 17:27:12.611427+0800 TestOC[33861:585111] i address is 0x16d6f7fcc
2021-10-01 17:27:12.611473+0800 TestOC[33861:585111] x is 6
2021-10-01 17:27:12.611500+0800 TestOC[33861:585111] x address is 0x16d6f7f8c
2021-10-01 17:27:12.611532+0800 TestOC[33861:585111] i address is 0x16d6f7fcc

可以发现到 x 与 i 的位置不同，这代表 x又多开了一个记忆体位置

如果不传入"值"，改成 pointer 的话

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    
    int i = 5;
    NSLog(@"i address is %p",&i);
    i = [self addOne:&i];
    NSLog(@"i address is %p",&i);
}


- (int) addOne: (int *) x {
    *x += 1;
    NSLog(@"x is %i",*x);
    NSLog(@"x address is %p",x);
    return *x;
}

结果：

21-10-01 17:35:31.573388+0800 TestOC[34045:590438] i address is 0x16b27ffcc
2021-10-01 17:35:31.573437+0800 TestOC[34045:590438] x is 6
2021-10-01 17:35:31.573474+0800 TestOC[34045:590438] x address is 0x16b27ffcc
2021-10-01 17:35:31.573506+0800 TestOC[34045:590438] i address is 0x16b27ffcc

可以发现记忆体位置都相同，指标都指向同一个位置