Dll access by excel from c++ code - c++

Question

I am trying to create a DLL from C++ code using visual studio 2015.
I have a DLL_Tutorial.h file :
#ifndef _DLL_TUTORIAL_H_
#define _DLL_TUTORIAL_H_
#include <iostream>
extern "C"
{
DECLDIR int Add( int a, int b );
}
#endif
\\
\ Then I created a DLL_Tutorial.cpp
#include <iostream>
#include "DLL_Tutorial.h"
#define DLL_EXPORT
extern "C"
{
__declspec(dllexport) int Add(int a, int b)
{
return(a + b);
}
}
I got a Dll file
I would like to call my function in VBA and apply that to an excel sheet
so in VBA I did :
Public Declare Function Add _
Lib "C:\Users\hasna\Desktop\Projet VBA-C++\projet5\Debug\projet5.dll" (byval a As integer,byval b As integer) As integer
Then in an excel sheet, I enter 2 values (for example 6 and 4 ) I call add function but it gave me : #VALEUR!
Where is the problem ?
Could you please help me resolve that
Thanks

Answer 1

In your DLL header file, it does not appear that you've prefixed the exported API with __declspec(dllexport). Which is usually defined so that the API can use __declspec(dllexport) when building the DLL, and __declspec(dllimport), when the header is being used in an external project. The best thing would be to just create the DLL using the VS 2015 project template, it includes the correct headers, and export definitions for you, so you just need to focus on writing the APIs. Here's a working example from a VS 2015 project:
Example *.h:
#pragma once
// The following ifdef block is the standard way of creating macros which make exporting
// from a DLL simpler. All files within this DLL are compiled with the DLLAPI_EXPORTS
// symbol defined on the command line. This symbol should not be defined on any project
// that uses this DLL. This way any other project whose source files include this file see
// DLLAPI_API functions as being imported from a DLL, whereas this DLL sees symbols
// defined with this macro as being exported.
#ifdef DLLAPI_EXPORTS
#define DLLAPI_API __declspec(dllexport)
#else
#define DLLAPI_API __declspec(dllimport)
#endif
// This is an example of an exported function.
DLLAPI_API int fnDLLAPI(void);
Example *.cpp:
// DLLAPI.cpp : Defines the exported functions for the DLL application.
//
#include "DLLAPI.h"
// This is an example of an exported function.
DLLAPI_API int fnDLLAPI(void)
{
return 42;
}
The Windows Via C/C++ book is also a very good resource regarding writing DLLs. Now regarding the VB import, I am not sure as I am not familiar with importing APIs via VB.