How to set CSS class for specific button in gtkmm3 in c++ on Linux

Question

I'm new in Linux programming. Recently I've started to learn gtkmm3 to make GUI for my study project, but I stuck on this problem. I have few buttons, which are styled with CSS file and I want to set other style to selected button, but nothing works. This is how I make buttons and read CSS file:

Glib::RefPtr<Gtk::Application> app = Gtk::Application::create(argc, argv);
Gtk::Window window;
window.set_default_size(800, 480);

Glib::RefPtr<Gtk::CssProvider> cssProvider = Gtk::CssProvider::create();
cssProvider->load_from_path("style.css");   
Glib::RefPtr<Gtk::StyleContext> styleContext = Gtk::StyleContext::create();
Glib::RefPtr<Gdk::Screen> screen = Gdk::Screen::get_default();//get default screen
styleContext->add_provider_for_screen(screen, cssProvider, GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);//add provider for screen in all application

Gtk::Box *vbox = Gtk::manage(new Gtk::Box(Gtk::ORIENTATION_VERTICAL, 0));
window.add(*vbox); 

Gtk::Button *b1 = new Gtk::Button("b1");
b1->signal_clicked().connect(sigc::ptr_fun(&on_b1_click));

Gtk::Button *b2 = new Gtk::Button("b2");
b2->signal_clicked().connect(sigc::ptr_fun(&on_b1_click));

vbox->add(*b1);
vbox->add(*b2);        

and CSS file:

.button {
	color: red;
	background: red;
	border-color: red;
}
.button:hover { 
	color: yellow;
    background-color: yellow;
	border-color: yellow;
}
.myButton{
	color: red;
	background: blue;
	border-color: red;
}

I tried to set other style for button like this:

GtkStyleContext *context;
context = gtk_widget_get_style_context (GTK_WIDGET (b1));
gtk_style_context_add_class (context, "myButton");

from here, but get error while running app:

GLib-GObject-WARNING **: invalid cast from '(null)' to 'GtkWidget'
Gtk-CRITICAL **: gtk_widget_get_style_context: assertion 'GTK_IS_WIDGET (widget)' failed
Gtk-CRITICAL **: gtk_style_context_add_class: assertion 'GTK_IS_STYLE_CONTEXT (context)' failed

and

gtk_widget_set_name (b1, "myButton");

from here but get compile error:

main.cpp: In function ‘int main(int, char**)’: main.cpp:34:40: error: cannot convert ‘Gtk::Button*’ to ‘GtkWidget* {aka _GtkWidget*}’ for argument ‘1’ to ‘void gtk_widget_set_name(GtkWidget*, const gchar*)’
 gtk_widget_set_name (b1, "myButton");

Any help appreciated :)

Answer 1

You are mixing GTK+ with gtkmm (C++ interface for GTK+). b1 is not a GtkButton but Gtk::Button.

Use Gtk::StyleContext:

Glib::RefPtr<StyleContext> context = b1->get_style_context();
context->add_class("myButton");