Undefined symbols for architecture x86_64 on Xcode 6.1

All of a sudden Xcode threw me this error at compilation time:

Undefined symbols for architecture x86_64:
"_OBJC_CLASS_$_Format", referenced from: objc-class-ref in WOExerciseListViewController.old: 
symbol(s) not found for architecture x86_64clang: 
error: linker command failed with exit code 1 (use -v to see invocation)

解決方案：
Common Causes
The common causes for "Undefined symbols for architecture armv7" are:

1.You import a header and do not link against the correct library. This is common, especially for headers for libraries like QuartzCore since it is not included in projects by default. To resolve:

• Add the correct libraries in the ***Link Binary With Libraries
  *** section of the **Build Phases
  .

• If you want to add a library outside of the default search path you can include the path in the ***Library Search Paths
  *** value in the Build Settings and add -l{library_name_without_lib_and_suffix} (eg. for libz.a use -lz) to the ***Other Linker Flags
  *** section of ***Build Settings
  ***.

2.You copy files into your project but forgot to check the target to add the files to. To resolve:

• Open the ***Build Phases
  *** for the correct target, expand ***Compile Sources
  *** and add the missing .m files. If this is your issue please upvote Cortex's answer below as well.

3.You include a static library that is built for another architecture like i386, the simulator on your host machine. To resolve:

• If you have multiple library files from your libraries vendor to include in the project you need to include the one for the simulator (i386) and the one for the device (armv7 for example).

• Optionally, you could create a fat static library that contains both architectures.

轉載地址：
https://stackoverflow.com/questions/6429494/undefined-symbols-for-architecture-armv7