Window with image GTK C++ - c++

Question

I'm trying to open a new window from an existing window by clicking on a button. The new window should display an image. When I click the button, the new window is shown, but the image is not displayed. There are no errors.
I made sure that my image is in the current directory and is readable. What is happening?
Code:
void ExampleWindow::on_button_clicked()
{
std::cout << "The Button was clicked." << std::endl;
Gtk::Window *window = new Gtk::Window();
Gtk::VBox mainLayout;
window->add(mainLayout);
Gtk::Image image("Vampire.png");
mainLayout.pack_start(image);
window->show_all();
}

Answer 1

The problem is that after your handler is called (end of scope), all local variables are destroyed, and that includes image.
Here is code that works:
#include <iostream>
#include <memory>
#include <gtkmm.h>
class ExampleWindow : public Gtk::Window
{
public:
ExampleWindow()
: m_image("Vampire.png")
{
add(m_btn);
m_btn.signal_clicked().connect([this](){OnButtonClicked();});
}
private:
void OnButtonClicked()
{
std::cout << __FUNCTION__ << " enter scope" << std::endl;
m_window = std::make_unique<Gtk::Window>();
m_window->add(m_image);
m_window->show_all();
std::cout << __FUNCTION__ << " leaving scope" << std::endl;
// With your code, image is destroyed here. The window still
// lives because it was newed, but you lost your reference to
// it and so the program will leak memory (i.e. you will be
// unable to call delete on it, unless it is a class member).
}
Gtk::Button m_btn("Show image");
Gtk::Image m_image;
std::unique_ptr<Gtk::Window> m_window;
};
int main(int argc, char *argv[])
{
auto app = Gtk::Application::create(argc, argv, "so.question.q65011763");
ExampleWindow window;
window.show_all();
return app->run(window);
// With this code, the image and the window are both destroyed here.
// Since the window is in a unique_ptr, delete will be automatically
// called on it.
}
Note that I have made all variables class members, so that they outlive the end of scope of the handler. I have also stored the window containing the image inside a smart pointer so that I don't have to call delete myself.