Using C++ classes in native DLL for Unity

Questions & Answers
1

I’d like to use classes written in my C++ native plugin in Unity. Here’s my attempt.

The C++ code (compiles fine):

// frontengine.cpp

#include "stdafx.h"
#include "FrontENGINE.h"

extern "C++" {
	FRONTENGINE_API int Add(int a, int b) {
		return a + b + 10;
	}

	FRONTENGINE_API class TestClass {
		int var;
	public:
		void setvar(int a) {
			var = a;
		}

		int getvar() {
			return var;
		}
	};
}

The Unity Code:

 // dosomething.cs
using System.Runtime.InteropServices;

public class DoStuff : MonoBehaviour {
	[DllImport("FRONT-Engine")]
	static extern int Add(int a, int b);

	void Start() {
		Debug.Log("Test");

		TestClass nq = new TestClass();
		nq.setvar(43);
		Debug.Log(nq.getvar());
	}
}

What I get when I try this:

error CS0246: The type or namespace name `TestClass' could not be found. Are you missing an assembly reference?
2

You can’t directly use C++ classes in C#. The memory management works completely different. In Unity you can only use static “std call” methods that are exported in an extern "C" section. extern “C” is required since C++ uses name mangling to encode the signature of the method into it’s name. This mangling is not really standardised and depends on the C++ compiler you’re using.

If you really need to use a C++ class in C# you have to create a wrapper with static methods like i explained over here. So any instance method you want to call from C# need to have a static wrapper method.