To compile a C++ plug-inA set of code created outside of Unity that creates functionality in Unity. There are two kinds of plug-ins you can use in Unity: Managed plug-ins (managed .NET assemblies created with tools like Visual Studio) and Native plug-ins (platform-specific native code libraries). More info
See in Glossary for Android, use the Android NDK and familiarize yourself with the steps required to build a shared library or a static library.
If you use C++ to implement the plug-in, you must declare with C linkage to avoid name mangling issues. By default, only C source files that have a .c file extension in the plug-ins have C linkage (not C++).
extern "C" {
float Foopluginmethod ();
}