How do I create a shared library in C++ for both Android and iOS?

Question

I am using LibGDX to write apps for both Android and iOS and I want to be able to add C++ code to my apps to optimize certain parts and to port some functions etc.

I have been searching the internet and tried to follow some tutorials, but did not find what I need.

How can I write a very basic C++ library which I can load in LibGDX? Which tools do I need to use? Visual Studio? I develop in Android Studio.

I think that I need an .so file for Android and an .a file for iOS, is that correct?

Answer 1

On both platforms, it's possible to include a precompiled library as well as C++ source code directly.

On Android, you'll want to look into using the Android NDK. This allows you to include native C/C++ code that can bridge over to Java. The connection between Java and C/C++ is managed with the JNI. It's a fairly tedious, awkward system for communicating between C++ and Java. You'll want to look into setting up an Android.mk makefile that specifies how to include your library (or source code) into your build.

On iOS, it's a little more tightly linked. You can have Objective-C++ files that can run both C++ and Objective-C code. If you're using Swift, it's a little different (bridging between Objective-C++ and Swift).

In some cases, when the platform (Android/iOS) provides functionality that is superior to what is possible or realistic with C++, you might find yourself architecting the code such that your C++ can reach out to the platform as needed. This means that you might have headers with separate implementation files per platform.

• thing.h
• thing_android.cpp
• thing_ios.mm

The android app's Android.mk file will include thing_android.cpp (but not thing_ios.mm). This file could cross the JNI bridge to talk to Java as needed, whenever you need something from Android SDK.

The iOS app will include thing_ios.mm (but not thing_android.cpp). The .mm extension signifies Objective-C++, so that file could directly call powerful Cocoa libraries as needed.

Finally, on all platforms, you'll want to be sure to either scale back your usage of C++ to the lowest common denominator platform. In other words, if iOS supports a particular feature of C++, and Android doesn't, then you cannot use that particular feature.