如何使用预处理器指令用不同的代码两次包含文件？

Question

I am trying to make a C++/CLI wrapper for my native C++ project so that I can use it in C#. So far I've wrapped the classes and functions that I need, but I've been trying to find a way to do the same for Enums without having to copy and paste the code to the wrapper.

I've tried looking up ways to wrap enums, but it seems that there's no way to use the enums that are from the native C++ code without having to cast/rewrite the code in the wrapper layer. So I found a solution where you use preprocessor directives and include the enums twice into the wrapper, but it seems not to be working for some reason.

Native Code (Enums.h)

#pragma once
#if defined MANAGED && !defined ENUMS_MANAGED_H
#define ENUMS_MANAGED_H
#define NAMESPACE managed
#define ENUM public enum class

#elif !defined MANAGED && !defined ENUMS_NATIVE_H
#define ENUMS_NATIVE_H
#define NAMESPACE native
#define ENUM enum class

#endif

namespace NAMESPACE
{
    ENUM numbers
    {
        ONE = 1,
        TWO = 2,
    }
}

Managed Code (Wrapper.h)

#pragma once
#ifndef WRAPPER_H
#define WRAPPER_H

#include "Native.h" //Other native code which also includes Enums.h

#define MANAGED
#include "Enums.h"

namespace managed
{
    managed::numbers num = managed::numbers::ONE; //managed does not contain 'numbers'
}

#endif //!WRAPPER_H

C# Code (Main.cs)

using managed; //managed project added as reference

static void main(String[] args)
{
   Console.WriteLine((int) numbers.ONE); //managed does not contain 'numbers'
}

I expect to be able to use managed::numbers in the managed project alongside native::numbers. I also expect to be able to use the numbers enum in C#. However, visual studio has not been able to do this. I've tried rearranging the includes as well but that doesn't seem to work.

I made a test file in the managed class that includes Enums.h, but not Native.h:

TestFile.h

#ifndef TESTFILE_H
#define TESTFILE_H

#define MANAGED
#include "Enums.h"

namespace managed
{
    managed::numbers num = managed::numbers::ONE; //Works 
}

#endif //!TESTFILE_H

This works, but I also need to be able to include Native.h and use the enums from there as well

Edit: Using @robthebloke answer, What I got to work is in Enums.h to do:

#pragma once

// to disable error in EnumsImpl.h
#define BUILDING_ENUMS
#include "EnumsImpl.h"
#undef BUILDING_ENUMS

And then in Wrapper.h:

#pragma once
#ifndef WRAPPER_H
#define WRAPPER_H

#include "Native.h"

#define BUILDING_ENUMS
#define MANAGED
#include "EnumsImpl.h"
#undef MANAGED
#undef BUILDING_ENUMS

#endif

Is there any way to just use one file instead of having to use EnumsImpl.h?

Answer 1

EnumsImpl.h

// guard against incorrect usage. 
#if !defined(BUILDING_ENUMS)
#error "Do not include this file directly, include Enums.h instead"
#endif

// choose macros for below
#if defined MANAGED
#define NAMESPACE managed
#define ENUM public enum class
#elif !defined MANAGED
#define NAMESPACE native
#define ENUM enum class
#endif

namespace NAMESPACE
{
    ENUM numbers
    {
        ONE = 1,
        TWO = 2,
    }
}

#undef NAMESPACE 
#undef ENUM 

Enums.h

#pragma once

// to disable error in EnumsImpl.h
#define BUILDING_ENUMS

// unmanaged
#include "EnumsImpl.h"

// managed
#define MANAGED
#include "EnumsImpl.h"

// remove temp defines
#undef MANAGED
#undef BUILDING_ENUMS

An Alternative...

#define DefEnum(NAMESPACE, ENUM) \
namespace NAMESPACE \
{ \
    ENUM numbers \
    { \
        ONE = 1, \
        TWO = 2, \
    }; \
}

DefEnum(dave, enum class);