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

Solved, split @interface in the .m file of composite into .h
Introduce this .h into main.m and the build will be successful.

But the class classification file can be built only with .m without separating .h

Answer 2

error: linker command failed with exit code 1 (use -v to see invocation)
This error message means that there is an error in the link. If you don't post the detailed error message, it will be of little use.