This example shows a very basic add-in function.

// Function:    MySquare 
// Returns:     double 
// Description: Returns the square of the input 
 
//{{XLP_SRC(MySquare) 
    // NOTE - the FunctionWizard will add and remove mapping code here. 
    //    DO NOT EDIT what you see in these blocks of generated code!
IMPLEMENT_XLLFN3(MySquare, MySquare_4, MySquare_12, "BB", "BB", L"MySquare", 0, 
    L"Input", 0, L"14", 0, L"Returns the square of the input", 0, L"Number who"
    L"se square is to be returned\0", 0, 0, L"{MySquare,,,Returns the square o"
    L"f the input,14,1,128,B,{{0,{Input,Double,0,,Number whose square is to be"
    L" returned,,,,}}},{},3,,0,0}", 1)
double MySquare_Impl(double);

extern "C" __declspec(dllexport)
double MySquare_12(double Input)
{
    XLL_FIX_STATE;
    return MySquare_Impl(Input);
}
extern "C" __declspec(dllexport)
double MySquare_4(double Input)
{
    XLL_FIX_STATE;
    return MySquare_Impl(Input);
}

double MySquare_Impl(double Input)
{
    // End of generated code 
//}}XLP_SRC 
 
    return Input * Input;
}

Uses

::IMPLEMENT_XLLFN3