obj-c编程05:类的多态与id动态绑定

    说实在的，写这第5篇的时候十分纠结，代码老是不能动态绑定，在编译时就会出错，最后发现这是开了ARC的原因。开了ARC obj-c就不能动态绑定了吗？这个问题还不清楚哦。把ARC关闭后，虽然会有警告，但运行是正确的。下面上代码：

#import <Foundation/Foundation.h>

@interface A:NSObject{
	double i;
}
	@property double i;
	-(double)mul:(double)x;
	-(void)show;
@end

@implementation A
	@synthesize i;

	-(double)mul:(double)x{
		return i * x;
	}

	-(void)show{
		NSLog(@"[A obj]i : %f",i);
	}
@end

@interface B:NSObject{
	int i;
}
	@property int i;

	-(int)mul:(int)x;
	-(void)show;
@end

@implementation B
	@synthesize i;

	-(int)mul:(int)x{
		return i * x;
	}

	-(void)show{
		NSLog(@"[B obj]i : %d",i);
	}
@end

int main(int argc,char *argv[])
{
	@autoreleasepool{
		NSLog(@"hello obj-c!");

		//A *a = [[A alloc] init];
		//id obj = a;
		id obj = [[A alloc] init];
		[obj setI:123.123];
		//[obj mul:123];
		[obj show];
		NSLog(@"%f * %f is %f",[obj i],99.99,[obj mul:99.99]);
	}
	return 0;
}

这是开了ARC之后的编译,无法动态绑定鸟:

apple@kissAir: objc_src$clang -fobjc-arc -framework Foundation 2.m -o 2

2.m:52:3: error: multiple methods named 'setI:' found with mismatched result,

      parameter type or attributes

                [obj setI:123.123];

                ^~~~~~~~~~~~~~~~~~

2.m:6:19: note:
one possibility

        @property double i;

                         ^

2.m:26:16: note: also found

        @property int i;

                      ^

2.m:55:26: error: multiple methods named 'i' found with mismatched result,

      parameter type or attributes

                NSLog(@"%f * %f is %f",[obj i],99.99,[obj mul:99.99]);

                                       ^~~~~~~

2.m:6:19: note:
one possibility

        @property double i;

                         ^

2.m:26:16: note: also found

        @property int i;

                      ^

2.m:55:40: error: multiple methods named 'mul:' found with mismatched result,

      parameter type or attributes

                NSLog(@"%f * %f is %f",[obj i],99.99,[obj mul:99.99]);

                                                     ^~~~~~~~~~~~~~~

2.m:7:2: note:
one possibility

        -(double)mul:(double)x;

        ^~~~~~~~~~~~~~~~~~~~~~~

2.m:28:2: note:
also found

        -(int)mul:(int)x;

        ^~~~~~~~~~~~~~~~~

3 errors generated.

这是关了ARC之后的编译和运行：

apple@kissAir: objc_src$clang -framework Foundation 2.m -o 2

2.m:52:3: warning: multiple methods named 'setI:' found

                [obj setI:123.123];

                ^~~~~~~~~~~~~~~~~~

2.m:6:19: note: using

        @property double i;

                         ^

2.m:26:16: note: also found

        @property int i;

                      ^

2.m:55:26: warning: multiple methods named 'i' found

                NSLog(@"%f * %f is %f",[obj i],99.99,[obj mul:99.99]);

                                       ^~~~~~~

2.m:6:19: note: using

        @property double i;

                         ^

2.m:26:16: note: also found

        @property int i;

                      ^

2.m:55:40: warning: multiple methods named 'mul:' found

                NSLog(@"%f * %f is %f",[obj i],99.99,[obj mul:99.99]);

                                                     ^~~~~~~~~~~~~~~

2.m:7:2: note: using

        -(double)mul:(double)x;

        ^~~~~~~~~~~~~~~~~~~~~~~

2.m:28:2: note:
also found

        -(int)mul:(int)x;

        ^~~~~~~~~~~~~~~~~

3 warnings generated.

apple@kissAir: objc_src$./2

2014-06-30 08:51:28.972 2[828:507] hello obj-c!

2014-06-30 08:51:28.974 2[828:507] [A obj]i : 123.123000

2014-06-30 08:51:28.974 2[828:507] 123.123000 * 99.990000 is 12311.068770

这个问题还没得到解决，希望哪位高人给予解答一下，在这之前我先跳过往后学鸟，毕竟时间不等笨猫啊！

[2014-07-05 第1次新增]:多态的第二个例子

我们来看看一个简单的代码,其正确实现了对象的多态功能,虽然编译时会有警告:

int main(int argc, char *argv[]){
	@autoreleasepool {
		msg(@"Hello World!");
		NSString *str = @"hello apple";
		NSObject *obj = str;
		id item = str;

		msg(@"%lu %lu",[obj length],[item length]);
	}
	return 0;
}

编译运行如下:

wisy@wisy-ThinkPad-X61:~/src/objc_src$ clang -objc-arc -O3 -g0 $OBJC_OPTS -lobjc -lgnustep-base -o 1 1.m
1.m:12:23: warning: 'NSObject' may not respond to 'length'
                msg(@"%lu %lu",[obj length],[item length]);
                                ~~~ ^
1.m:3:24: note: expanded from macro 'msg'
#define msg(...) NSLog(__VA_ARGS__)
                       ^
1 warning generated.
wisy@wisy-ThinkPad-X61:~/src/objc_src$ ./1
2014-07-05 09:45:58.092 1[3309] Hello World!
2014-07-05 09:45:58.095 1[3309] 11 11

前面一个问题答案貌似也清楚了,类B没有从A继承啊!你从NSObject继承,当运行时动态绑定的对象自然不知道到底是调用A还是B对象的方法鸟.