I am a C++ developer I am interested in Cocos2d-x framework. I know that you can write C++ code using the framework, compile it for different platforms and that's it, you have your 2D on Windows, Android, iOS. This is amazing but I don't understand how it is being done and, consequently, I worry that some thing that I have done for one platform will not work on other one. To go into details I would like to open my concerns. In order to do that let's clarify what is compiling and what is running a code.
What does it mean to compile C++ code for a platform (platform is OS + CPU architecture)? It means that C++ source code is mapped to instructions which is understandable for a concrete CPU architecture. And the final set of instructions is packaged into an executable file which is understandable for a concrete OS which means a concrete SO or OSes that understand how to handle the executable file can run it. Also we should not forget that in the set of instructions that the executable contains there could be system calls. Which is also specific to OS.
What does it mean to run the executable? It means that OS knows the format of particular executable file. When you give run command OS loades it into the virtual memory and starts to execute that CPU instructions set step by step. (Very raw but in general it is like that I guess.)
Now returning to the COSOS2D-X. How it is possible to compile a C++ code so that it was able to be recognized and loaded by different OSes and by different CPUs. What mechanisms we use in order to get appropriate .apk, .ipa or .exe files. Is there a trap that we can fall while using system calls or processor specific calls? In general how all this problems are solved? Please explain the process, for example, for Android or it would be great for iOS too. :)
Cocos2d-x has 95% of the same code for all target OS and platforms. And it has 5% of code which is written for the concrete platform. For example there are some Java sources for Android. And there are some Obj-C files for iOS. Also there is some code in C++ for different platform. #define is used to separate this code. Examples of such code is working with files which is written in C++ but differs from platform to platform.
Generating of appropriate output file is responsibility of the compiler and SDK used for target platform. For example xCode with clang compiler will generate the iOS build. While Android NDK with gcc inside will build the apk.