C++/CLI Wrapper Implementation Errors

Question

I've been attempting to utilize the C++/CLI wrapper scheme in the following link.

Note: in all the searching I've done over the last week, the below project is the one most cited as providing a workable solution.
http://pragmateek.com/using-c-from-native-c-with-the-help-of-ccli-v2/

The below project is my attempt to link to a 3rd party app that accepts a Win32 Dll. Hy goal is to link the Win32 Dll to a managed C# DLL where the functions should be easier to write & maintain.

However, I keep running into variations of the below errors.

Error   C2664   'std::basic_string<char,std::char_traits<char>,std::allocator<char>>::basic_string(std::initializer_list<_Elem>,const std::allocator<char> &)' : cannot convert argument 1 from 'System::String ^' to 'const std::basic_string<char,std::char_traits<char>,std::allocator<char>> &' Main_interface  

Error (active)  E0415   no suitable constructor exists to convert from "System::String ^" to "std::basic_string<char, std::char_traits<char>, std::allocator<char>>"    Main_interface  

My C# Library: MainUtilities.dll

//Main_Utility.cs

public class MainUtilities
{
    public string GetRevision(string CurrentRev)
    {
        return CurrentRev + "_rev";
    }
}

Main_Interface.h

#pragma once

#define DLL_EXP extern "C" __declspec(dllexport)
DLL_EXP void GetRevision(char* data_in, char *data_out);

class QFUtilitiesWrapperPrivate;

#ifdef BUILDINGQFUTILITIES
#define DLL_EXP_WRAP __declspec(dllexport)
#else
#define DLL_EXP_WRAP __declspec(dllimport)
#endif

#include <string>

class DLL_EXP_WRAP QF_UtilitiesWrapper
{
private: static QFUtilitiesWrapperPrivate* _private;

public: QF_UtilitiesWrapper();

public: ~QF_UtilitiesWrapper();

public: static std::string GetRevisionFunc(const char* rev);

};

Main_Interface.cpp

#include "stdafx.h"
#include "Main_Interface.h"
#include <msclr\auto_gcroot.h>
#using "MainUtilities.dll"
using namespace System::Runtime::InteropServices; // Marshal


class QFUtilitiesWrapperPrivate
{
public: msclr::auto_gcroot<MainUtilities^> main_Utilities;
};

QF_UtilitiesWrapper::QF_UtilitiesWrapper()
{
    _private = new QFUtilitiesWrapperPrivate();
    _private->main_Utilities = gcnew MainUtilities();
}

DLL_EXP void GetRevision(char* data_in, char *data_out)
{
    std::string s_b = QF_UtilitiesWrapper::GetRevisionFunc(data_in);
    strcpy_s(data_out, 100, s_b.c_str());
}

std::string QF_UtilitiesWrapper::GetRevisionFunc(const char* data_in)
{
    return _private->main_Utilities->GetRevision(gcnew System::String(data_in));
} //** ERROR ^^^^^^  ** 

QF_UtilitiesWrapper::~QF_UtilitiesWrapper()
{
    delete _private;
}

Please note the the errors occur at: 'GetRevisionFunc'

---

EDIT: Ok - I've made the following changes:

//Main_Interface.h

private:  QFUtilitiesWrapperPrivate* _private;  // no longer static
public: const char* GetRevisionFunc(const char* rev); //function now: const char*

//Main_Interface.cpp

DLL_EXP void GetRevision(char* data_in, char *data_out)
{
    data_out = QF_UtilitiesWrapper::GetRevisionFunc(data_in);
  // ** ERROR ^^^^^^^^^^ **
}

const char* QF_UtilitiesWrapper::GetRevisionFunc( char* rev) // now: const char*
{
    System::String^ managedCapi = _private->main_Utilities->GetRevision(gcnew System::String(rev));
    return (const char*)Marshal::StringToHGlobalAnsi(managedCapi).ToPointer();
}

Now I'm tying to resolve the following errors:

Error C2352 'QF_UtilitiesWrapper::GetRevisionFunc' : illegal call of non-static member function

Error (active) E0245 a nonstatic member reference must be relative to a specific object

Answer 1

Well you have to marshal the types, this isn't done for you in C++/CLI. You have a System::String^ but you want to return it as an std::string, so you have to marshal it:

std::string QF_UtilitiesWrapper::GetRevisionFunc(const char* data_in)
{
    auto str = _private->main_Utilities->GetRevision(gcnew System::String(data_in));
    return msclr::interop::marshal_as<std::string>(str);
}

For more details, checkout the docs.

Also your QF_UtilitiesWrapper holds _private as a static member and you set and delete this in your constructor / destructor. This will leak memory and probably cause unexpected behavior.