#include <wx/wx.h>

class BruttoNettoApp : public wxApp {

	public:
		bool OnInit();

};

class BruttoNettoFrame : public wxFrame {

	public:
		BruttoNettoFrame();
	
	private:
		wxPanel *mainPanel;
		wxBoxSizer *mainBoxSizer;
		wxTextCtrl *priceTextCtrl;
		wxStaticText *multiplicatorStaticText;
		wxComboBox *taxComboBox;
		wxButton *calculateButton;
		wxTextCtrl *resultTextCtrl;

};

IMPLEMENT_APP(BruttoNettoApp)

bool BruttoNettoApp::OnInit() {
	BruttoNettoFrame *bruttoNettoFrame = new BruttoNettoFrame;
	bruttoNettoFrame->Show();
	
	SetTopWindow(bruttoNettoFrame);

	return true;
}

BruttoNettoFrame::BruttoNettoFrame() : wxFrame(nullptr, wxID_ANY, "Brutto-Netto-Rechner") {
	mainPanel = new wxPanel(this, wxID_ANY);
	mainBoxSizer = new wxBoxSizer(wxHORIZONTAL);

	priceTextCtrl = new wxTextCtrl(mainPanel, wxID_ANY);
	mainBoxSizer->Add(priceTextCtrl, 1);

	multiplicatorStaticText = new wxStaticText(mainPanel, wxID_ANY, "x");
	mainBoxSizer->Add(multiplicatorStaticText);

	taxComboBox = new wxComboBox(mainPanel, wxID_ANY, "", wxDefaultPosition, wxDefaultSize, 0, NULL, wxCB_READONLY);
	taxComboBox->Append("7%");
	taxComboBox->Append("19%");
	taxComboBox->SetSelection(0);
	mainBoxSizer->Add(taxComboBox);

	calculateButton = new wxButton(mainPanel, wxID_ANY, "=");
	calculateButton->Bind(wxEVT_BUTTON, [this](wxCommandEvent &event) {
		double price = 0.f;
		if(priceTextCtrl->GetValue().ToDouble(&price)) {
			price *= taxComboBox->GetSelection() == 0 ? 1.07 : 1.19;
			resultTextCtrl->SetValue(wxString::Format(_("%lf"), price));
		} else {
			wxMessageBox("Wrong price", "Error");
		}
	});
	mainBoxSizer->Add(calculateButton);
	
	resultTextCtrl = new wxTextCtrl(mainPanel, wxID_ANY, "", wxDefaultPosition, wxDefaultSize, wxTE_READONLY);
	mainBoxSizer->Add(resultTextCtrl, 1);

	mainPanel->SetSizer(mainBoxSizer);
	mainBoxSizer->SetSizeHints(this);

	SetSize(500, -1);

	priceTextCtrl->SetFocus();
	calculateButton->SetDefault();
}

c++ NettoBruttoRechner.cpp -o NettoBruttoRechner -std=c++11 `wx-config --libs --cppflags`