Passing strings from VBA to C++ DLL

Question

I\'m really confused about passing strings from VBA to C++. Here\'s the VBA code:

Private Declare Sub passBSTRVal Lib \"         


        

Answer 1

Ok, so I know I set bounty to draw out a fuller response on the IDL idea but I have had a go at this myself. So I opened a ATL project, changed the idl to the following

// IDLForModules.idl : IDL source for IDLForModules
//

// This file will be processed by the MIDL tool to
// produce the type library (IDLForModules.tlb) and marshalling code.

import "oaidl.idl";
import "ocidl.idl";

[
    helpstring("Idl For Modules"),
    uuid(EA8C8803-2E90-45B1-8B87-2674A9E41DF1),
    version(1.0),
]
library IDLForModulesLib
{
    importlib("stdole2.tlb");

    [
        /* dllname attribute https://msdn.microsoft.com/en-us/library/windows/desktop/aa367099(v=vs.85).aspx */
        dllname("IdlForModules.dll"),
        uuid(4C1884B3-9C24-4B4E-BDF8-C6B2E0D8B695)
    ]
    module Math{
        /* entry attribute https://msdn.microsoft.com/en-us/library/windows/desktop/aa366815(v=vs.85).aspx */
        [entry(656)] /* map function by entry point ordinal */
        Long _stdcall Abs([in] Long Number);
    }
    module Strings{
        [entry("pUpper")] /* map function by entry point name */
        BSTR _stdcall Upper([in] BSTR Number);
    }
};

Then in the main cpp file I added

#include 
#include 

INT32 __stdcall _MyAbs(INT32 Number) {
    return abs(Number);
}

BSTR __stdcall pUpper(BSTR sBstr)
{
    // Get the BSTR into the wonderful world of std::wstrings immediately
    std::wstring sStd(sBstr);

    // Do some "Mordern C++" iterator style op on the string
    std::transform(sStd.begin(), sStd.end(), sStd.begin(), ::toupper);

    // Dig out the char* and pass to create a return BSTR
    return SysAllocString(sStd.c_str());
}

And in the DEF file I edited it to be

; MidlForModules.def : Declares the module parameters.

LIBRARY

EXPORTS
    DllCanUnloadNow     PRIVATE
    DllGetClassObject   PRIVATE
    DllRegisterServer   PRIVATE
    DllUnregisterServer PRIVATE
    DllInstall      PRIVATE
    _MyAbs @656
    pUpper

In a macro-enmabled workbook called TestClient.xlsm placed in same directory as the Debug output Dll I write the following in the ThisWorkbook module

Option Explicit

Private Declare Function LoadLibrary Lib "kernel32" Alias "LoadLibraryA" (ByVal lpLibFileName As String) As Long

Private Sub Workbook_Open()
    '* next line establishes relative position of Dll
    Debug.Assert Dir(ThisWorkbook.Path & "\IDLForModules.dll") = "IDLForModules.dll"

    '* next line loads the Dll so we can avoid very long Lib "c:\foo\bar\baz\barry.dll"
    LoadLibrary ThisWorkbook.Path & "\IDLForModules.dll"

    '* next go to  Tools References are check "Idl For Modules"
    '* "Idl For Modules" Iis set in the IDL with helpstring("Idl For Modules")

End Sub

I then add a Tools Reference to the newly created Type library and now I can complete by adding a standard module and adding the following

Option Explicit

Sub TestAbs()
    Debug.Print IDLForModulesLib.Math.Abs(-5)
End Sub

Sub TestUpper()
    Debug.Print IDLForModulesLib.Strings.Upper("foobar")
End Sub

This works for me on Windows 8.1 Professional 64 bit, VS2013, Excel 15. Fuller instructions for C++ newbies can found here Throw away Declare Function with IDL for Modules.