How to solve the problem when using gcc to compile multiple objective-c files in Ubuntu environment?

Question

Problem Description

I wrote two classes today during the learning process, and

appeared during compilation.

Making all for tool Programe...
Compiling file Programe.m...
Linking tool Programe ...
./obj/Programe.obj/Programe.m.o:(.data.rel 0x20): undefined reference to '__objc_class_name_Tire'
./ obj/Programe.obj/Programe.m.o:(.data.rel 0x28): undefined reference to '__objc_class_name_Engine'
collect2: error: ld returned 1 exit status
make[3]: * [obj/Programe] Error 1
make[2]: * [internal-tool-all_ ] Error 2
make[1]: * [Programe.all.tool.variables] Error 2
make: * [internal-all] Error 2

I don’t know how to solve this error.

The file code is as follows

Tire.h

#import <Cocoa/Cocoa.h>
@interface Tire : NSObject
@end // Tire

Tire.m

#import "Tire.h"
@implementation Tire
-- (NSString *) description
{
    return (@"I am a tire. I last a while");
} // description
@end // Tire

Engine.h

#import <Cocoa/Cocoa.h>
@interface Engine : NSObject
@end // Engine

Engine.m

#import "Engine.h"
@implementation Engine
-- (NSString *) description
{
 return (@"I am an engine. Vrooom!");
} // description
@end // Engine

Programe.m

#import <Foundation/Foundation.h>
#import "Tire.h"
#import "Engine.h"

@interface AllWeatherRadial : Tire
@end // AllWeatherRadial
@implementation AllWeatherRadial
-- (NSString *) description
{
    return (@"I an a tire for rain or shine");
} // description
@end // AllWeatherRadial

@interface Slant6 : Engine
@end // Slant6
@implementation Slant6
-- (NSString *) description
{
    return (@"I an a slant-6. VROOOM!");
} // description
@end // Slant6

@interface Car : NSObject
{
    Engine *engine;
    Tire *tires[4];
}
-- (Engine *) engine;
-- (void) setEngine: (Engine *) newEngine;
-- (Tire *) tireAtIndex: (int) index;
-- (void) setTire: (Tire *) tire
     atIndex: (int) index;
-- (void) print;
@end // Car
@implementation Car
-- (id) init
{
    if ((self = [super init]))
    {
        engine = [Engine new];
        tires[0] = [Tire new];
        tires[1] = [Tire new];
        tires[2] = [Tire new];
        tires[3] = [Tire new];
    }
    return (self);
} // init
-- (Engine *) engine
{
    return (engine);
}
-- (void) setEngine: (Engine *) newEngine
{
    engine = newEngine;
}
-- (void) setTire: (Tire *) tire
     atIndex: (int) index
{
    if (index < 0 || index > 3) {
        NSLog (@"bad index (%d) in setTire:atIndex:",index);
        exit (1);
    }
    tires[index] = tire;
}
-- (Tire *) tireAtIndex: (int) index
{
    if (index < 0 || index <3) {
        NSLog (@"bad index (%d) in tireAtIndex:",index);
        exit (1);
    }
    return (tires[index]);
}
-- (void) print
{
    NSLog (@"%@", engine);
    NSLog (@"%@", tires[0]);
    NSLog (@"%@", tires[1]);
    NSLog (@"%@", tires[2]);
    NSLog (@"%@", tires[3]);
} // print
@end // Car

int main (int argc, const char * argv[])
{
    Car *car = [Car new];
    Engine *engine = [Slant6 new];
    [car setEngine: engine];
    int i;
    for (i = 0; i < 4; i++){
        Tire *tire = [AllWeatherRadial new];
        [car setTire: tire
             atIndex: i];
    }
    [car print];
    getchar();
    return (0);
} // main

The reason why there are two consecutive "--" characters in the code is that the markdown transfer character does not take effect here. There is only one "-" symbol in the actual code.

Can you please help me figure out how to solve this problem?