苹果关于异常的详细文档
关于自定义异常或者扩展：
Objective-C中处理异常是依赖于NSException实现的，它是异常处理的基类，它是一个实体类，而并非一个抽象类，所以你可以直接使用它或者继承它扩展使用：
1.直接使用，分两种，抛出默认的异常，和自定义自己的新的种类的异常：
OC代码

#import 
intmain (intargc,constchar* argv[])
{
@autoreleasepool {
NSException* ex = [[NSException alloc]initWithName:@"MyException"
reason:@"b==0"
userInfo:nil];
@try
{
intb = 0;
switch(b)
{
case0:
@throw(ex);//b=0，则抛出异常；
break;
default:
break;
}
}
@catch(NSException *exception)//捕获抛出的异常
{
NSLog(@"exception.name= %@",exception.name);
NSLog(@"exception.reason= %@",exception.reason);
NSLog(@"b==0 Exception!");
}
@finally
{
NSLog(@"finally!");
}
[ex release];
}
return0;
}

ps：
Initializes and returns a newly allocated exception object.

- (id)initWithName:([NSString](http://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Classes/NSString_Class/Reference/NSString.html#//apple_ref/doc/c_ref/NSString)*)*name*reason:([NSString](http://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Classes/NSString_Class/Reference/NSString.html#//apple_ref/doc/c_ref/NSString)*)*reason*userInfo:([NSDictionary](http://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Classes/NSDictionary_Class/Reference/Reference.html#//apple_ref/doc/c_ref/NSDictionary)*)*userInfo*
Parameters
*name*
The name of the exception.
*reason*
A human-readable message string summarizing the reason for the exception.
*userInfo*
A dictionary containing user-defined information relating to the exception
Return Value
The createdNSExceptionobject ornilif the object couldn't be created.
Discussion
This is the designated initializer.
Availability
Available in iOS 2.0 and later.

2.扩展使用，这个推荐你需要自定义一些功能的时候使用，比如，当捕获到指定的异常的时候弹出警告框之类的：
OC代码

![](http://upload-images.jianshu.io/upload_images/1823354-533e19d2a925a0c8.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)

@interface MyException : NSException
-（void）popAlert
@end
Java代码
![](http://upload-images.jianshu.io/upload_images/1823354-4160a2c42246f2e6.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)

@implementationMyException
- （void）popAlert
{
//弹出报告异常原因的警告框 reason
UIAlertView* alert = [[UIAlertView alloc] initWithTitle:@"Tips"
message:self.reason
delegate:nil
cancelButtonTitle:nil
otherButtonTitles:nil];
[alert show];
[alert release];
}
@end

使用：
OC代码

- (IBAction)btnClicked_Exception:(id)sender
{
MyException* ex = [[MyException alloc]initWithName:@"MyException"
reason:@"除数为0了！"
userInfo:nil];
@try
{
intb = 0;
switch(b)
{
case0:
@throw(ex);//b=0，则抛出异常；
break;
default:
break;
}
}
@catch(MyException *exception)//捕获抛出的异常
{
[exception popAlert]；
NSLog(@"b==0 Exception!");
}
@finally
{
NSLog(@"finally!");
}
[ex release];
}

这个时候，捕获到异常，它就会弹出警告框了。当然，你还可以在MyException里面加一些指定的异常的通用处理方法。
只要你愿意，你就可以随意的定制它！