load dylib in Excel through vba for mac - c++

Question

I am trying to expose a C++ DLL to Excel using mac. The DLL was written and compiled with Xcode 4 and I am using Excel 2011.
For simple functions, extern "C" does the work and I am able to use the dylib in Excel. Specifically, if the C++ code is something like
extern "C"
{
double sumC( double a, double b)
{
return a + b;
}
}
and the VBA code is:
Private Declare Function addFunction _
Lib "Macintosh HD:Users:SVM:Documents:Excel:lib:libTestDLLtoVBA.dylib" _
Alias "sumC" (ByVal a As Double, ByVal b As Double) As Double
Function Addition(a As Double, b As Double)
Addition = addFunction(a, b)
End Function
everything works fine. But, I am interested in exposing to Excel more complex code with classes defined in header files - as in the example below - and in that case Excel returns #VALUE!. My C++ code is something like this
header file:
#ifndef TestDLLtoVBA_TestFunction_h
#define TestDLLtoVBA_TestFunction_h
class AdditionVBATest{
public:
AdditionVBATest(){};
AdditionVBATest( double ){ m_AdditionResult = 0.0; }
~AdditionVBATest(){};
void setResult( double nAddition ){ m_AdditionResult = nAddition; }
double getResult(){ return m_AdditionResult; }
void addFunct( double x, double y, double &nResult );
double addFunct( double, double );
private:
double m_AdditionResult;
};
double addFunctionC( double a, double b);
#endif
cpp file:
#include <iostream>
#include "TestFunction.h"
void AdditionVBATest::addFunct(double x, double y, double &nResult)
{
nResult = 0.0;
nResult = x + y;
AdditionVBATest::setResult(nResult);
}
double AdditionVBATest::addFunct( double a, double b )
{
double nResult(0.0);
AdditionVBATest addCompute;
addCompute.AdditionVBATest::addFunct(a, b, nResult);
AdditionVBATest addResult;
return addResult.getResult();
}
And finally this is the file that contains the function I would like to expose to Excel:
#include <iostream>
#include "TestFunction.h"
extern "C"
{
double addFunctionC( double a, double b)
{
AdditionVBATest *resAddition;
double result(0.0);
result = resAddition->AdditionVBATest::addFunct(a, b);
return result;
}
}
I tried to use the same dylib in a C++ app and works fine, so I believe it is something related to exposing the library through VBA.
The VBA code I used is
Private Declare Function addFunction _
Lib "Macintosh HD:Users:SVM:Documents:Excel:lib:libTestDLLtoVBA.dylib" Alias "addFunctionC" _
(ByVal a As Double, ByVal b As Double) As Double
Function Addition(a As Double, b As Double)
Addition = addFunction(a, b)
End Function
Any help would be greatly appreciated.

Answer 1

This is of course not necessary but to simplify I will assume from now on that your header file and both source files are in the same directory, and that all future commands are executed in this directory. Then replace in both source files
#include "TestFunction.h"
with
#include "./TestFunction.h"
Then, compile as follows :
g++ -m32 -Wall -g -c ./TestFunction.cpp -o ./TestFunction.o
g++ -m32 -dynamiclib ./file.cpp ./TestFunction.o -o ./libTestDLLtoVBA.dylib
where
g++ is your gcc. I guess it is clang's one, on my computer it is gcc 5.2.0, but both should work fine
file.cpp is the source file containing the declaration of addFunctionC
the -m32 option asks to produce a 32 bits dylib, as excel/VBA on mac are 32 bits
Now do a nm -gU libTestDLLtoVBA.dylib and you will see a line with _addFunctionC showing that the function addFunctionC is exported indeed.
In the VBA, declare as follows :
Declare Function addFunctionC Lib "/Path/to/your/libTestDLLtoVBA.dylib" (ByVal x As Double, ByVal y As Double) As Double
Function Addition(a As Double, b As Double)
Addition = addFunction(a, b)
End Function
and it should work. This was a great source of inspiration.
Now, if you don't want to used command line and want to use XCode for instance, I would say that the only thing you have to take care of is to ensure that XCode produces a 32 bits dylib, as excel/VBA are 32 bits only on mac OS X. I really think that this 32 bits stuff is your problem.

Answer 2

You likely want to read up on COM automation objects. Sometimes known as "OLE" or "ActiveX". It's essentially a way to expose classes and objects written in C++ to other programming languages.
For scripting environments (VBA and Javascript), the traditional way to do this is by registering a COM object that exposes an IDispatch interface.
Some links to get you started:
https://msdn.microsoft.com/en-us/library/windows/desktop/ms221375(v=vs.85).aspx
https://msdn.microsoft.com/en-us/library/windows/desktop/ms221326(v=vs.85).aspx
Don't get hung up on the details of "controls" and embedded UI windows. Foucs on getting a simple COM object class and interface declared in an IDL, generate a typelib, implement a C++ class that implements the interface and IDispatch. ATL (Active Template Library) makes this stuff easy.