Callback functions in VBA: VBA calls DLL calls VBA.

callback.bas

Option Explicit

Declare Sub squareItAndThen Lib "callback.dll" _
  (ByVal x As Double, ByVal f As Long)

Public Sub EchoSquares()
    Dim x As Double
    
    ChDir ActiveWorkbook.Path
    
    For x = 1 To 10 Step 0.5
        squareItAndThen x, AddressOf Echo
    Next x
End Sub

Public Sub Echo(ByVal d As Double)
    Debug.Print d
End Sub

callback.cpp

extern "C" {

using callback [[gnu::stdcall]] = void (*)(double);

[[gnu::dllexport]]
void squareItAndThen [[gnu::stdcall]] (double x, callback f)
{
    f(x * x);
}

}

Makefile

CXX=g++
CXXOPTS=-std=c++11 -pedantic -Wall -Wextra -Werror -static-libgcc -static-libstdc++
OPTOPTS=-O3 -fno-exceptions -fno-rtti -ffast-math -mfpmath=387 -msse3 -mtune=core2 -funroll-loops -s
DLLOPTS=-shared
LINKOPTS=-Wl,--add-stdcall-alias
LOG=
#LOG=-DLOGGING="log.txt"

callback.dll: callback.o
	$(CXX) $(CXXOPTS) $(LOG) $(OPTOPTS) $(DLLOPTS) -o callback.dll callback.o $(LINKOPTS)

callback.o: callback.cpp
	$(CXX) $(CXXOPTS) $(LOG) $(OPTOPTS) -c callback.cpp

clean:
	-rm callback.dll
	-rm callback.o