Objective-C++ compilation errors due to Xcode not respecting file type - xcode

Question

I'm following a tutorial in a book (iPhone 3D Programming), which uses:
Objective-C header and source files (file extensions .h, .m - respectively),
Objective-C++ header and source files (file extensions .h, .mm - respectively)
C++ header and source files (file extensions .hpp, .mpp - respectively)
A sample Xcode project is included which compiles successfully.
Before I found the sample project, I had manually typed out the code from the book but I was getting the following compilation errors for the files detailed below:
Unknown type name 'virtual'
Expected member name or ';' after declaration specifiers
IRenderingEngine.hpp (Xcode File Inspector - File Type = "Default - C++ Header")
...
struct IRenderingEngine {
virtual void Initialize(int width, int height) = 0; //2 errors as marked above
virtual void Render() const = 0; //2 errors as marked above
virtual void UpdateAnimation(float timeStep) = 0; //2 errors as marked above
virtual void OnRotate(DeviceOrientation newOrientation) = 0; //2 errors as marked above
virtual ~IRenderingEngine() {} //2 errors as marked above
};
...
Must use 'struct' tag to refer to type 'IRenderingEngine'
GLView.h (Xcode File Inspector - File Type = "Default - C Header")
#import "IRenderingEngine.hpp"
#import <QuartzCore/QuartzCore.h>
#interface GLView : UIView {
#private
EAGLContext* m_context;
IRenderingEngine* m_renderingEngine; //1 error marked above
float m_timestamp;
}
- (void) drawView:(CADisplayLink*)displayLink;
- (void) didRotate:(NSNotification*)notification;
#end
The file types for all the other files also defaulted to their expected file types in the Xcode File Inspector and as such should have worked correctly with the Build Setting - Apple LLVM compiler 4.2 - Language - "Compile Sources As = According to File Type" - which is identical to the Build Setting in the sample project that compiles successfully.
For some odd reason changing the Build Setting to "Compile Sources As = Objective-C++" in my manually created project removed the compilation errors and the application ran as expected.
Can anyone offer a reason as to why this setting is not consistent between seemingly identical (source-code-wise) projects?

Answer 1

Header files are not compiled. Header files are used by the preprocessor — anywhere you have a #include or a #import the actual text of the original is treated as though you'd copied and pasted it into the original.
Hence it doesn't matter if your file is called .hpp, .h or anything else. If a .m file imports a .h file that includes a .hpp file then the .hpp code will be compiled as part of the .m file, i.e. as Objective-C.
I am therefore going to guess that you've got GLView.m. If that's going to import a .hpp file, whether directly or indirectly, it needs to be compiled as Objective-C++. One way to do it is to rename it .mm, the other is to tell the project not to try to guess language types by file extension.

Answer 2

Tommy and HotLicks gave you the right answer to your immediate problem -- you need to make sure that all source files that include GLView.h are Objective-C files, by naming them .mm.
However, I want to add another side to this. Blindly making all files Objective-C++ is a bad solution. It should lead you to ask yourself: why do all these files need to be Objective-C++, if they are not using C++ features? The answer is that they import GLView.h, and the GLView class contains an instance variable whose type is a pointer to a struct that contains C++ features. Why do these other files care about that? They shouldn't.
There are various things you can do about it.
IRenderingEngine can be forward-declared in GLView.h. It is unnecessary to import IRenderingEngine.hpp, since the header doesn't care about the internal structure of IRenderingEngine; it only needs to know that it's some type in order to have a pointer to it. A forward declaration suffices for this. (However it would be necessary to write struct IRenderingEngine* m_renderingEngine; to be compatible with C.)
Furthermore, the instance variables for the GLView class do not need to be declared in the header in the first place. Instance variables can instead be declared in the implementation (.m) file either in the "class extension" (i.e. #interface GLView () { ... }), or, in newer versions of Xcode, directly in the class implementation (#implementation GLView { ... })