objective-c - objective-c undefined symbol 编译错误

请帮忙!我在 objective-c 中的第一个程序。跟着一个教程一个字接一个字地讲，但是它给了我这个错误，我不太了解如何阅读 objective-c 。

SimpleCar.h:

#import <Cocoa/Cocoa.h>


@interface SimpleCar : NSObject {
    NSString* make;
    NSString* model;
    NSNumber* vin;
}

// set methods
- (void) setVin:   (NSNumber*)newVin;
- (void) setMake:  (NSString*)newMake;
- (void) setModel: (NSString*)newModel;

// convenience method
- (void) setMake: (NSString*)newMake
        andModel: (NSString*)newModel;

// get methods
- (NSString*) make;
- (NSString*) model;
- (NSNumber*) vin;

@end

SimpleCar.m:

#import "SimpleCar.h"


@implementation SimpleCar
// set methods
- (void) setVin: (NSNumber*)newVin {

    [vin release];
    vin = [[NSNumber alloc] init];
    vin = newVin;

}

- (void) setMake: (NSString*)newMake {

    [make release];
    make = [[NSString alloc] initWithString:newMake];

}

- (void) setModel: (NSString*)newModel {

    [model release];
    model = [[NSString alloc] initWithString:newModel];

}

// convenience method
- (void) setMake: (NSString*)newMake
        andModel: (NSString*)newModel {

    // Reuse our methods from earlier
    [self setMake:newMake];
    [self setModel:newModel];

}

- (NSString*) make {
    return make;
}

- (NSString*) model {
    return model;
}

- (NSNumber*) vin {
    return vin;
}

-(void) dealloc
{
    [vin release];
    [make release];
    [model release];
    [super dealloc];
}

@end

CarApp.m:

#import <Foundation/Foundation.h>
#import "SimpleCar.h"

int main (int argc, const char * argv[]) {

  NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    SimpleCar *myCar = [[SimpleCar alloc] init];

    NSNumber *newVin = [NSNumber numberWithInt:123];

    [myCar setVin:newVin];
    [myCar setMake:@"Honda" andModel:@"Civic"];

    NSLog(@"The car is: %@ %@", [myCar make], [myCar model]);
    NSLog(@"The vin is: %@", [myCar vin]);

    [myCar release];

  [pool drain];

  return 0;
}

编译器调用:

bash-3.2$ gcc CarApp.m SimpleCar.m -g -m64

错误:

Undefined symbols for architecture x86_64:
  "___CFConstantStringClassReference", referenced from:
      CFString in ccR0Zlgm.o
      CFString in ccR0Zlgm.o
      CFString in ccR0Zlgm.o
      CFString in ccR0Zlgm.o
  "_objc_msgSend", referenced from:
      _main in ccR0Zlgm.o
      -[SimpleCar setVin:] in ccJfVliU.o
      -[SimpleCar setMake:] in ccJfVliU.o
      -[SimpleCar setModel:] in ccJfVliU.o
      -[SimpleCar setMake:andModel:] in ccJfVliU.o
     (maybe you meant: l_objc_msgSend_fixup_release, l_objc_msgSend_fixup_alloc )
  "_NSLog", referenced from:
      _main in ccR0Zlgm.o
  "_objc_msgSend_fixup", referenced from:
      l_objc_msgSend_fixup_alloc in ccR0Zlgm.o
      l_objc_msgSend_fixup_release in ccR0Zlgm.o
      l_objc_msgSend_fixup_release in ccJfVliU.o
      l_objc_msgSend_fixup_alloc in ccJfVliU.o
     (maybe you meant: l_objc_msgSend_fixup_release, l_objc_msgSend_fixup_alloc )
  "_OBJC_CLASS_$_NSAutoreleasePool", referenced from:
      objc-class-ref in ccR0Zlgm.o
  "_OBJC_CLASS_$_NSNumber", referenced from:
      objc-class-ref in ccR0Zlgm.o
      objc-class-ref in ccJfVliU.o
  "_objc_msgSendSuper2", referenced from:
      -[SimpleCar dealloc] in ccJfVliU.o
  "_OBJC_METACLASS_$_NSObject", referenced from:
      _OBJC_METACLASS_$_SimpleCar in ccJfVliU.o
  "__objc_empty_cache", referenced from:
      _OBJC_METACLASS_$_SimpleCar in ccJfVliU.o
      _OBJC_CLASS_$_SimpleCar in ccJfVliU.o
  "__objc_empty_vtable", referenced from:
      _OBJC_METACLASS_$_SimpleCar in ccJfVliU.o
      _OBJC_CLASS_$_SimpleCar in ccJfVliU.o
  "_OBJC_CLASS_$_NSObject", referenced from:
      _OBJC_CLASS_$_SimpleCar in ccJfVliU.o
  "_OBJC_CLASS_$_NSString", referenced from:
      objc-class-ref in ccJfVliU.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status

最佳答案

看起来您刚刚创建了一个没有Xcode项目的Xcode工作区。这是您可以使用的项目:

http://www.markdouma.com/developer/CarApp.zip

通常，您只需选择"file">“新建项目”即可创建一个新项目。您可能需要该特定项目的基于基金会的命令行程序。

不幸的是，本教程并未提供SimpleCar类的最佳实现。以下是一种可能的重写:

#import "SimpleCar.h"

@implementation SimpleCar

-(void) dealloc
{
    [vin release];
    [make release];
    [model release];
    [super dealloc];
}


// set methods
- (void) setVin: (NSNumber*)newVin {
    [newVin retain];
    [vin release];
    vin = newVin;
//    [vin release];
//    vin = [[NSNumber alloc] init];
//    vin = newVin;
}

上面的注释代码是原始代码。这既有潜在危险，又会泄漏内存。原始代码的第一行在执行其他任何操作之前先释放vin，这有潜在的危险。例如，如果在CarApp.m中这样做:

NSNumber *vinValue = [car vin];
[car setVin:vin];
NSLog(@"car's vin == %@", [car vin]); // unpredictable results/crash

原始代码会释放现有的vin值，而不必费心确保传入的值实际上不是vin本身。通过设置vin = newVin，它是将vin设置为指向自身，但已被释放。随后将消息发送到vin的任何尝试都将导致崩溃或不可预测的结果。

原始代码还会泄漏内存。 NSNumber的实例是不可变的，因此通过alloc/init创建一个数字实际上没有多大意义(因为它始终为零，并且无法更改)。我的替换代码首先保留传入的值，以防碰巧是vin。然后释放vin，然后将vin分配给newVin。

出于相同的原因，setMake:和setModel:方法存在问题。

- (void) setMake: (NSString*)newMake {
    [newMake retain];
    [make release];
    make = newMake;
//    [make release];
//    make = [[NSString alloc] initWithString:newMake];
}

- (void) setModel: (NSString*)newModel {
    [newModel retain];
    [model release];
    model = newModel;
//    [model release];
//    model = [[NSString alloc] initWithString:newModel];
}

// convenience method
- (void) setMake: (NSString*)newMake
        andModel: (NSString*)newModel {
    // Reuse our methods from earlier
    [self setMake:newMake];
    [self setModel:newModel];
}

- (NSString*) make {
    return make;
}
- (NSString*) model {
    return model;
}
- (NSNumber*) vin {
    return vin;
}
@end

关于objective-c - objective-c undefined symbol 编译错误，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/6231368/