轉自：
http://www.xuebuyuan.com/2160769.html
xcode6.0升級到xcode6.1後，程序編譯報錯，錯誤如下：這是個很常見的錯誤，很多項目在更換xcode版本後，都會出現類似的問題。
Undefined symbols for architecture x86_64:
"_OBJC_CLASS_$_QQApiInterface", referenced from:
objc-class-ref in AppDelegate.o
"_OBJC_CLASS_$_ShareSDK", referenced from:
objc-class-ref in AppDelegate.o
objc-class-ref in RecipeDetailViewController.o
objc-class-ref in showViewController.o
objc-class-ref in video_show.o
"_OBJC_CLASS_$_TencentOAuth", referenced from:
objc-class-ref in AppDelegate.o
"_OBJC_CLASS_$_WXApi", referenced from:
objc-class-ref in AppDelegate.o
"_OBJC_CLASS_$_WeiboApi", referenced from:
objc-class-ref in AppDelegate.o
"_OBJC_CLASS_$_YXApi", referenced from:
objc-class-ref in AppDelegate.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
解決方案：
Xcode升級到6.1.1 默認讓所有App都通過64位編譯器編譯。原來在Xcode6.0的時候默認的Architectures只有（arm7,armv7s），到6.1.1默認就帶上arm64的參數了。
1.把1.選中Targets—>Build Settings—>Architectures。
把build active architectures only 改爲 NO。
2. 把最下面的Valid Architectures中的arm64參數刪掉就可以了
或者：
雙擊Architectures，選擇other，刪除$(ARCH_STANDARD)，然後增加armv7和armv7s（寫上：$(ARCHS_STANDARD_32_BIT)）。
3.clean 再build。
結果設置如下圖：
目前設置完成後，問題順利解決
