Xcode 6.2 OSX Undefined symbols for functions in other files - xcode

Question

I'm sure this is a total nube question but if someone could explain to me what's going wrong I'd be very grateful.
I create a new Cocoa App in XCode. Call it LinkerTest. This basic app will build and run, putting a simple blank window up.
Add a new .cpp file using the C++ File template. Call it Test.cpp. This creates Test.h too.
In Test.cpp add a simple function:
int TestMe(void)
{
return 1;
}
Declare my function in Test.h
int TestMe(void);
In my AppDelegate.m (which was created automatically when I create the app) add
#include "Test.h"
In the applicationDidFinishLaunching method add:
printf("Test = %d\n", TestMe());
Now try to build. Everything compiles ok, but it fails to link. This is the linker command:
Ld Build/Products/Debug/LinkerTest.app/Contents/MacOS/LinkerTest normal x86_64
cd /Users/chip/LinkerTest
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/chip/LinkerTest/Build/Products/Debug -F/Users/chip/LinkerTest/Build/Products/Debug -filelist /Users/chip/LinkerTest/Build/Intermediates/LinkerTest.build/Debug/LinkerTest.build/Objects-normal/x86_64/LinkerTest.LinkFileList -Xlinker -rpath -Xlinker #executable_path/../Frameworks -mmacosx-version-min=10.10 -stdlib=libc++ -fobjc-arc -fobjc-link-runtime -Xlinker -dependency_info -Xlinker /Users/chip/LinkerTest/Build/Intermediates/LinkerTest.build/Debug/LinkerTest.build/Objects-normal/x86_64/LinkerTest_dependency_info.dat -o /Users/chip/LinkerTest/Build/Products/Debug/LinkerTest.app/Contents/MacOS/LinkerTest
And this is the error I get:
Undefined symbols for architecture x86_64:
"_TestMe", referenced from:
-[AppDelegate applicationDidFinishLaunching:] 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)
So any idea what I've done wrong? Seems to me that adding the file it should link too, but it doesn't. I know the file Test.cpp gets compiled, if I add garbage to the file, then the compiler immediately yacks on that garbage.
EDIT: A couple more things.
1) Looking at this stackoverflow(Getting XCode to include, compile and link existing (C++) codebase in XCode 4.3(.1)) question it seems similar but not my issue. I can confirm that my test.cpp is listed in my Compile Sources under Build Phases.
2) Looking in the LinkerTest/Build/Intermediates/LinkerTest.build/Debug/LinkerTest.build/Objects-normal/x86_64 folder I find Test.o, Test.d, and Test.dia which suggests to me that Test.cpp is in fact compiling. In that folder I also find LinkerTest.LinkFileList which when opened with a text editor shows that Test.o should be linked.

Answer 1

The answer is that when you create a default Cocoa app the AppDelegate file is a .m file. Changing that file to be AppDelegate.mm fixes the problem. My uninformed guess is that a .m file will only handle .c files and to correctly handle .cpp files you must use .mm files. Why xcode's default files for a default Cocoa App wouldn't be .mm files is beyond my pay grade, but there it is.