example.cpp

#include <epoxy/gl.h>
#include <gtkmm/window.h>
#include <gtkmm/box.h>
#include <gtkmm/glarea.h>
#include <iostream>

class Example : public Gtk::Window
{
public:
    Example();
    ~Example() override;

    bool do_render(const Glib::RefPtr<Gdk::GLContext>&);
    void do_realize();
    void do_unrealize();
private:
    Gtk::GLArea* m_glarea;
    GLuint m_vao;
};

Example::Example()
    : m_glarea()
{
    set_default_size(600, 400);
    set_border_width(10);

    m_glarea = Gtk::make_managed<Gtk::GLArea>();
    m_glarea->set_required_version(4, 1);
    m_glarea->signal_realize().connect(sigc::mem_fun(*this, &Example::do_realize));
    m_glarea->signal_unrealize().connect(sigc::mem_fun(*this, &Example::do_unrealize), false);
    m_glarea->signal_render().connect(sigc::mem_fun(*this, &Example::do_render), false);

    add(*m_glarea);
    show_all();
}

Example::~Example()
{
    printf("Destructor context: %p\n", reinterpret_cast<void*>(m_glarea->get_context().get()));
//    m_glarea.make_current();
}

bool
Example::do_render(const Glib::RefPtr<Gdk::GLContext>&)
{
    m_glarea->make_current();
    try {
        m_glarea->throw_if_error();
        glClearColor(0.5, 0.5, 0.5, 1.0);
        glClear(GL_COLOR_BUFFER_BIT);
        glClearColor(0.5, 0.5, 0.5, 1.0);

        glFlush();
        return true;
    } catch (const Gdk::GLError& gle) {
        std::cerr << "An error occurred in the render callback of the GLArea" << std::endl;
        std::cerr << gle.domain() << "-" << gle.code() << "-" << gle.what() << std::endl;
        return false;
    }
}

void
Example::do_realize()
{
    printf("Realize context: %p\n", reinterpret_cast<void*>(m_glarea->get_context().get()));
    m_glarea->make_current();
    try {
        m_glarea->throw_if_error();
        glGenVertexArrays(1, &m_vao);
        glBindVertexArray(m_vao);
    } catch(const Gdk::GLError& gle) {
        std::cerr << "An error occured making the context current during realize:" << std::endl;
        std::cerr << gle.domain() << "-" << gle.code() << "-" << gle.what() << std::endl;
    }
}

void
Example::do_unrealize()
{
    printf("Unrealize context: %p\n", reinterpret_cast<void*>(m_glarea->get_context().get()));
    m_glarea->make_current();
    try {
        m_glarea->throw_if_error();
        glDeleteBuffers(1, &m_vao);
    } catch (const Gdk::GLError& gle) {
        std::cerr << "An error occured making the context current during unrealize" << std::endl;
        std::cerr << gle.domain() << "-" << gle.code() << "-" << gle.what() << std::endl;
    }
}

int main(int argc, char *argv[])
{
    auto app = Gtk::Application::create("org.gtkmm.Example");
    auto example = std::make_unique<Example>();
    return app->run(*example, argc, argv);
}

Compile with: g++ $(pkg-config --cflags --libs gtkmm-3.0 epoxy) example.cpp

Hi I saw your post on the mailing list. I found that the Error message goes away for me when you assign the window on the heap. ( Not the first time with gtkmm widgets when assigning on the stack I got issues, for example Gtk::Switch can even segfault I found, but as long as assign them dynamically it works.

So instead of

Example example;

Do
Example *example = new Example(); or use a smart pointer, but you get my drift.
....
delete example

then when I run it I no longer get the error message.

Cheers :)