Unifying the cross-platform code
Right now, we have two different versions of a simple program (Win_Min2 and App3). Let us see how to unify the common parts of the code.
Getting ready
In Android, the application initialization phase is different, and since we use a mixed Java plus C++ approach, the entry points will be different. In C++, we are tied to, int main() or DWORD WinMain() functions; whereas in Android it is up to us to choose which JNI function we may call from our Java starter code. Event handling and rendering the initialization code are also quite different, too. To do so, we mark sections of the code with pre-processor definitions and put the different OS code into different files—Wrappers_Android.h and Wrappers_Windows.h.
How to do it...
We use the standard macros to detect the OS for which the program is being compiled: Windows-targeted compilers provide the _WIN32 symbol definition, and the __linux__ macro is defined on any Linux-based OS, including Android. However, the __linux__ defination is not enough, since some of the APIs are missing in Android. The macro ANDROID is a non-standard macro and we pass the -DANDROID switch to our compiler to identify the Android target in our C++ code. To make this for every source file, we modify the CFLAGS variable in the Android.mk file.
Finally, when we write the low-level code, the detection looks like the following code:
#if defined(_WIN32) // windows-specific code #elif defined(ANDROID) // android-specific code #endif
For example, to make an entry point look the same for both the Android and Windows versions, we write the following code:
#if defined(_WIN32) # define APP_ENTRY_POINT() int main() #elif defined(ANDROID) # define APP_ENTRY_POINT() int App_Init() #endif
Later we will replace the int main() definition with the APP_ENTRY_POINT() macro.
There's more...
To detect more operating systems, compilers, and CPU architectures, it is useful to check out a list of predefined macros at http://predef.sourceforge.net.