Objective-C composite object 合成对象问题

Question

最近看《Objective-C 程序设计》 ，11章有个合成对象

自己写了一个例子 Xcode没报错，编译时报错了

//"Rectangle.h"
@interface Rectangle : NSObject

@property int width,height;

-(void) setWidth: (int) w andHeight:(int) h;

- (int) area;

@end

#import "Rectangle.h"

@interface Square_c : NSObject
{
    Rectangle *rect;
}

- (instancetype) init;
- (instancetype) initWithSide:(int) s;
- (void) setSide:(int) s;
- (int) side;
-(int) area;

@end

@implementation Square_c

- (instancetype) init           {
    self = [super init];
    if (self) {
        rect = [[Rectangle alloc] init];
    }
    return [self initWithSide:0];
}

- (instancetype) initWithSide:(int) s
{
    self = [super init];
    if (self)
        [rect setWidth:s andHeight:s];
    return self;
}

- (void) setSide:(int) s
{
    [rect setWidth:s andHeight:s];

}
- (int) side
{
    return rect.width;
}
-(int) area
{
    return [rect area];
}

@end

int main (int argc,char * argv[])
{
    @autoreleasepool {
        Square_c *square_c = [[Square_c alloc] initWithSide:20];

    }
}

Xcode编译后报错

Ld /Users/ma/Library/Developer/Xcode/DerivedData/prog1-eexmdvqxkooeorhbkjzcupkepwqy/Build/Products/Debug/prog1 normal x86_64
    cd /Project/prog1
    export MACOSX_DEPLOYMENT_TARGET=10.10
    /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -arch x86_64 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk -L/Users/ma/Library/Developer/Xcode/DerivedData/prog1-eexmdvqxkooeorhbkjzcupkepwqy/Build/Products/Debug -F/Users/ma/Library/Developer/Xcode/DerivedData/prog1-eexmdvqxkooeorhbkjzcupkepwqy/Build/Products/Debug -filelist /Users/ma/Library/Developer/Xcode/DerivedData/prog1-eexmdvqxkooeorhbkjzcupkepwqy/Build/Intermediates/prog1.build/Debug/prog1.build/Objects-normal/x86_64/prog1.LinkFileList -mmacosx-version-min=10.10 -fobjc-arc -fobjc-link-runtime -Xlinker -dependency_info -Xlinker /Users/ma/Library/Developer/Xcode/DerivedData/prog1-eexmdvqxkooeorhbkjzcupkepwqy/Build/Intermediates/prog1.build/Debug/prog1.build/Objects-normal/x86_64/prog1_dependency_info.dat -o /Users/ma/Library/Developer/Xcode/DerivedData/prog1-eexmdvqxkooeorhbkjzcupkepwqy/Build/Products/Debug/prog1

duplicate symbol _OBJC_IVAR_$_Square2.rect in:
    /Users/ma/Library/Developer/Xcode/DerivedData/prog1-eexmdvqxkooeorhbkjzcupkepwqy/Build/Intermediates/prog1.build/Debug/prog1.build/Objects-normal/x86_64/main.o
    /Users/ma/Library/Developer/Xcode/DerivedData/prog1-eexmdvqxkooeorhbkjzcupkepwqy/Build/Intermediates/prog1.build/Debug/prog1.build/Objects-normal/x86_64/Square_composite.o
duplicate symbol _OBJC_CLASS_$_Square2 in:
    /Users/ma/Library/Developer/Xcode/DerivedData/prog1-eexmdvqxkooeorhbkjzcupkepwqy/Build/Intermediates/prog1.build/Debug/prog1.build/Objects-normal/x86_64/main.o
    /Users/ma/Library/Developer/Xcode/DerivedData/prog1-eexmdvqxkooeorhbkjzcupkepwqy/Build/Intermediates/prog1.build/Debug/prog1.build/Objects-normal/x86_64/Square_composite.o
duplicate symbol _OBJC_METACLASS_$_Square2 in:
    /Users/ma/Library/Developer/Xcode/DerivedData/prog1-eexmdvqxkooeorhbkjzcupkepwqy/Build/Intermediates/prog1.build/Debug/prog1.build/Objects-normal/x86_64/main.o
    /Users/ma/Library/Developer/Xcode/DerivedData/prog1-eexmdvqxkooeorhbkjzcupkepwqy/Build/Intermediates/prog1.build/Debug/prog1.build/Objects-normal/x86_64/Square_composite.o
ld: 3 duplicate symbols for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

求熟手帮忙看下哪里出问题了

Answer 1

해결했습니다. 합성 .m 파일의 @interface를 .h로 분할했습니다
이 .h를 main.m에 추가하면 빌드가 성공합니다.

그러나 클래스 분류 파일은 별도의 .h 없이 .m으로만 구성할 수 있습니다

Answer 2

error: linker command failed with exit code 1 (use -v to see invocation)
이 오류 메시지는 링크에 오류가 있다는 의미입니다. 자세한 오류 메시지를 게시하지 않으면 거의 쓸모가 없습니다.