Callback function does't work Multi-threaded Debug (/ MTd)

Question

I need to create a dll that works with callback function. When I set Runtime Libary = Multi-threaded Debug (/ MTd) in project properties, it generates this error message:

But when I set Runtime Libary = Multi-threaded Debug DLL (/ MDd), the application works perfectly

Look at my DLL:

callbackproc.h

#include <string>

#ifdef CALLBACKPROC_EXPORTS
#define CALLBACKPROC_API __declspec(dllexport)
#else
#define CALLBACKPROC_API __declspec(dllimport)
#endif

// our sample callback will only take 1 string parameter
typedef void (CALLBACK * fnCallBackFunc)(std::string value);

// marked as extern "C" to avoid name mangling issue
extern "C"
{
    //this is the export function for subscriber to register the callback function
    CALLBACKPROC_API void Register_Callback(fnCallBackFunc func);
}

callbackpro.cpp

#include "stdafx.h"
#include "callbackproc.h"
#include <sstream>

void Register_Callback(fnCallBackFunc func)
{
    int count = 0;

    // let's send 10 messages to the subscriber
    while(count < 10)
    {
        // format the message
        std::stringstream msg;
        msg << "Message #" << count;

        // call the callback function
        func(msg.str());

        count++;

        // Sleep for 2 seconds
        Sleep(2000);
    }
}

stdafx.h

#pragma once

#include "targetver.h"    
#define WIN32_LEAN_AND_MEAN           
// Windows Header Files:
#include <windows.h>

My application that consumes a dll

#include <windows.h>
#include <string>

#include "callbackproc.h"

// Callback function to print message receive from DLL
void CALLBACK MyCallbackFunc(std::string value)
{
    printf("callback: %s\n", value.c_str());
}

int _tmain(int argc, _TCHAR* argv[])
{
    // Register the callback to the DLL
    Register_Callback(MyCallbackFunc);

    return 0;
}

Where am I going wrong? Tanks!

Answer 1

Seems to me as a classic problem of passing std types (in this case std::string) across DLL boundaries.

As a best practice, pass only "native" data types across DLL boundaries, I'm sure in 99% that if you change your prototype from

typedef void (CALLBACK * fnCallBackFunc)(std::string value);

to

typedef void (CALLBACK * fnCallBackFunc)(const char* value);

Your code will work regardless of your underlying runtime