This example uses CXlOper::IsVector(), CXlOper::GetVectorCount() and CXlOper::VectorCell() to iterate the cells in a CXlOper argument containing a vector.

//{{XLP_SRC(VectorSum) 
    // NOTE - the FunctionWizard will add and remove mapping code here. 
    //    DO NOT EDIT what you see in these blocks of generated code!
IMPLEMENT_XLLFN3(VectorSum, VectorSum_4, VectorSum_12, "RP", "UQ", L"VectorSum",
    0, L"Input", 0, L"Demo functions", 0, L"Returns the sum of a vector", 0, 
    L"Input vector\0", 0, 0, L"{VectorSum,,,Returns the sum of a vector,Demo f"
    L"unctions,1,128,U,{{0,{Input,Value,0,,Input vector,,,,}}},{},3,,0,0}", 1)
CXlOper* VectorSum_Impl(CXlOper&, const CXlOper*);

extern "C" __declspec(dllexport)
LPXLOPER12 VectorSum_12(LPXLOPER12 Input)
{
    XLL_FIX_STATE;
    CXlOper xloResult, Input__port(Input);
    try {
        CXlStructuredExceptionHandler _seh_;
        xloResult.HandleResult(VectorSum_Impl(xloResult, &Input__port));
    }
    catch(const CXlRuntimeException& ex) {
        CXllApp::Instance()->DisplayException(xloResult, ex);
    }
    return xloResult.Ret12();
}
extern "C" __declspec(dllexport)
LPXLOPER4 VectorSum_4(LPXLOPER4 Input)
{
    XLL_FIX_STATE;
    CXlOper xloResult, Input__port(Input);
    try {
        CXlStructuredExceptionHandler _seh_;
        xloResult.HandleResult(VectorSum_Impl(xloResult, &Input__port));
    }
    catch(const CXlRuntimeException& ex) {
        CXllApp::Instance()->DisplayException(xloResult, ex);
    }
    return xloResult.Ret4();
}

CXlOper* VectorSum_Impl(CXlOper& xloResult, const CXlOper* Input)
{
    // End of generated code 
//}}XLP_SRC

    BOOL bOk = TRUE;

    // This function doesn't work on 2-dimensional arrays 
    // so return an error 
    if ( !Input->IsVector() )
    {
        xloResult = xlerrValue;
        return xloResult.Ret();
    }

    // Iterate vector's cells, adding up  
    // any numeric values
    USHORT i, cCells;
    double dSum = 0.0;
    cCells = (USHORT)Input->GetVectorCount();
    for ( i = 0; i < cCells; i++ )
    {
        if ( Input->VectorCell(i).IsDouble() )
            dSum += (double)Input->VectorCell(i);
    }

    // Return the total
    xloResult = dSum;
    return xloResult.Ret();
}

Uses

CXlOper::VectorCell | CXlOper::IsVector | CXlOper::GetVectorCount