'C++ DLL Not Receiving Passed Variables

I am trying to write a C++ DLL that will receive two long values, sum them, and then return the result. I'm doing this testing as a way to attempt to fix a 20+ year old C++ DLL that needs some fine-tuning to be 64-bit compliant and my skills in C++ are rusty at best. So I'm starting simple with basic math and I can't seem to get it to work. I'm calling the DLL from VBA but I don't think it's my VBA that's the issue.

C++ Header File

#pragma once

#define DLL_EXPORT _declspec (dllexport)

class __declspec(dllexport) CalculationApi
{
public:
    long Addition(long* value1, long* value2);
};

C++ CPP File

#include "pch.h"
#include "Calculation.h"
#include "windows.h"
#include "string"
#include<iostream>

using namespace std;

long DLL_EXPORT __stdcall CalculationApi::Addition(long* value1, long* value2)
{
    long return_result;
    string MsgBoxMessage;

    string Integer1_Type = typeid(*value1).name();

    string test_integer_text = std::to_string(*value1);

    MsgBoxMessage = "Value1 is: " + to_string(*value1);
    MessageBox(NULL, MsgBoxMessage.c_str(), "Value of Value1", MB_OK);

    MsgBoxMessage = "Value1 Data Type is: " + Integer1_Type;
    MessageBox(NULL, MsgBoxMessage.c_str(), "Data Type of Value1", MB_OK);

    MsgBoxMessage = "Value2 is: " + to_string(*value2);
    MessageBox(NULL, MsgBoxMessage.c_str(), "Value of Value2", MB_OK);

    return_result = *value1 + *value2;
    
    *value1 = 50;

    MsgBoxMessage = "Value1 is: " + to_string(*value1);
    MessageBox(NULL, MsgBoxMessage.c_str(), "Value of Value1", MB_OK);

    
    MsgBoxMessage = "Result is: " + to_string(return_result);
    MessageBox(NULL, MsgBoxMessage.c_str(), "Math Results", MB_OK); 

    return return_result;

}

And just in case I messed up the VBA here's how I call the DLL

Private Declare PtrSafe Function Addition Lib "C:\Users\micha\source\repos\CalculationDll\x64\Debug\CalculationDll.dll" (value1 As LongPtr, value2 As LongPtr) As LongPtr   

Function DLL_Testing()
    Dim value1 As LongPtr
    Dim value2 As LongPtr
    Dim Results As LongPtr
        
    value1 = 10
    value2 = 20
    
    Results = Addition(integer1, integer2)
    
    test = 0

End Function

The first messagebox in the C++ comes up blank... I'm sure I'm just not doing something right there but meh doesn't matter much. The second messagebox returns either 0 or some really funky number. The VBA will return the result "583141696^" which of course is not 10+20. Also the value1 doesn't get updated to 50, I'm pretty sure I'm not passing the pointer correctly as I've seen value2 update when I changed the value of value1 in the DLL.

This all seems REALLY basic yet it doesn't work. Thoughts?

c++vbadll


Solution 1:[1]

I think Addition should be static :

static long Addition(long* value1, long* value2);

because if Addition is not static, a third (hidden) parameter (this) exist...

and the class CalculationApi is not really usefull here.

Solution 2:[2]

Well the suggestion to convert this to C instead of C++ paid off (after finding a dumb error of my own). Changes I made...

  • Class gone
  • Converted to C

I couldn't get the "static" suggestion to work but the code in C works fine without it. Not sure what's up there but I'm going to go with it.

Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Source: Stack Overflow

Solution Source
Solution 1 jls28
Solution 2 michaeka

Related Questions