一个异常的例子

• 異常情況（Exceptions）

  • 注意：異常處理只有 Mac OS X 10.3 以上才支援。
  • 基於 "Programming in Objective-C," Copyright © 2004 by Sams Publishing一書中的範例，並經過允許而刊載。
  • CupWarningException.h

    #import <Foundation/NSException.h>@interface CupWarningException: NSException@end

  • CupWarningException.m

    #import "CupWarningException.h"@implementation CupWarningException@end

  • CupOverflowException.h

    #import <Foundation/NSException.h>@interface CupOverflowException: NSException@end

  • CupOverflowException.m

    #import "CupOverflowException.h"@implementation CupOverflowException@end

  • Cup.h

    #import <Foundation/NSObject.h>@interface Cup: NSObject {    int level;}-(int) level;-(void) setLevel: (int) l;-(void) fill;-(void) empty;-(void) print;@end

  • Cup.m

    #import "Cup.h"#import "CupOverflowException.h"#import "CupWarningException.h"#import <Foundation/NSException.h>#import <Foundation/NSString.h>@implementation Cup-(id) init {    self = [super init];    if ( self ) {        [self setLevel: 0];    }    return self;}-(int) level {    return level;}-(void) setLevel: (int) l {    level = l;    if ( level > 100 ) {        // throw overflow        NSException *e = [CupOverflowException            exceptionWithName: @"CupOverflowException"            reason: @"The level is above 100"            userInfo: nil];        @throw e;    } else if ( level >= 50 ) {        // throw warning        NSException *e = [CupWarningException            exceptionWithName: @"CupWarningException"            reason: @"The level is above or at 50"            userInfo: nil];        @throw e;    } else if ( level < 0 ) {        // throw exception        NSException *e = [NSException            exceptionWithName: @"CupUnderflowException"            reason: @"The level is below 0"            userInfo: nil];        @throw e;    }}-(void) fill {    [self setLevel: level + 10];}-(void) empty {    [self setLevel: level - 10];}-(void) print {    printf( "Cup level is: %i\n", level );}@end

  • main.m

    #import "Cup.h"#import "CupOverflowException.h"#import "CupWarningException.h"#import <Foundation/NSString.h>#import <Foundation/NSException.h>#import <Foundation/NSAutoreleasePool.h>#import <stdio.h>int main( int argc, const char *argv[] ) {    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];    Cup *cup = [[Cup alloc] init];    int i;    // this will work    for ( i = 0; i < 4; i++ ) {        [cup fill];        [cup print];    }    // this will throw exceptions    for ( i = 0; i < 7; i++ ) {        @try {            [cup fill];        } @catch ( CupWarningException *e ) {            printf( "%s: ", [[e name] cString] );        } @catch ( CupOverflowException *e ) {            printf( "%s: ", [[e name] cString] );        } @finally {            [cup print];        }    }    // throw a generic exception    @try {        [cup setLevel: -1];    } @catch ( NSException *e ) {        printf( "%s: %s\n", [[e name] cString], [[e reason] cString] );    }    // free memory     [cup release];    [pool release];}

  • output

    Cup level is: 10Cup level is: 20Cup level is: 30Cup level is: 40CupWarningException: Cup level is: 50CupWarningException: Cup level is: 60CupWarningException: Cup level is: 70CupWarningException: Cup level is: 80CupWarningException: Cup level is: 90CupWarningException: Cup level is: 100CupOverflowException: Cup level is: 110CupUnderflowException: The level is below 0

  • NSAutoreleasePool 是一個記憶體管理類別。現在先別管它是幹嘛的。
  • Exceptions（異常情況）的丟出不需要擴充（extend）NSException 物件，你可簡單的用 id 來代表它： @catch ( id e ) { ... }
  • 還有一個 finally 區塊，它的行為就像 Java 的異常處理方式，finally 區塊的內容保證會被呼叫。
  • Cup.m 裡的 @"CupOverflowException" 是一個 NSString 常數物件。在 Objective-C 中，@ 符號通常用來代表這是語言的衍生部分。C 語言形式的字串（C string）就像 C/C++ 一樣是 "String constant" 的形式，型別為 char *。

e name 中的name函数是NSException中提供的 cString是NSString提供的