Objective-C composite object 合成物件問題

Question

最近看《Objective-C 程式設計》 ，11章有個合成物件

自己寫了一個例子 Xcode沒報錯，編譯時報錯了

//"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編譯後報錯

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)

求熟手幫忙看下哪裡出問題了

Answer 1

解決了，把composite的那個.m檔裡的@interface拆分出.h
main.m 裡再引入這個.h 就可以成功build了。

但是類別的分類 檔案只用.m 不分出.h 是可以build的

Answer 2

error: linker command failed with exit code 1 (use -v to see invocation)
這個錯誤訊息的意思是link出錯了，如果你不詳細的出錯訊息貼出來的話，那麼它並沒有什麼卵用。