Question

Hello guys I’m using C and GTK+ 2 I want to make a simple paint program like MS program but by these two tools only okay I just started and I want your hands to reach the end 🙂
look to my code here

#include <gtk/gtk.h>
int main(int argc, char *argv[])
{
    GtkWidget *window;
    GtkWidget *drawingArea;
    gtk_init(&argc, &argv);
    window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    g_signal_connect(G_OBJECT(window), "delete_event",
             G_CALLBACK(gtk_main_quit), G_OBJECT(window));
    g_signal_connect(G_OBJECT(window), "destroy", G_CALLBACK(gtk_main_quit),
             G_OBJECT(window));
    gtk_container_set_border_width(GTK_CONTAINER(window), 10);

    drawingArea = gtk_drawing_area_new();
    /*The problem is in the next line */
    gtk_drawing_area_size(G_OBJECT(drawingArea), 200, 200);

    gtk_container_add(GTK_CONTAINER(window), drawingArea);
    gtk_widget_show(drawingArea);

    gtk_widget_show(window);
    gtk_main();
    return 0;
}

my problem is in the commented line

gtk_drawing_area_size(G_OBJECT(drawingArea),200,200);

the error when compiling

ibrahim@ibrahim-PC:~/Desktop$ gcc main.cpp -o base `pkg-config --cflags --libs gtk+-2.0`
main.cpp: In function ‘int main(int, char**)’:
main.cpp:14:52: error: cannot convert ‘GObject* {aka _GObject*}’ to ‘GtkDrawingArea* {aka _GtkDrawingArea*}’ for argument ‘1’ to ‘void gtk_drawing_area_size(GtkDrawingArea*, gint, gint)’

So Please help me

Answer 1

That’s a common gotcha in GTK+. You have to cast the widget to the type that exposes the method (i.e. the implementing type). You should write:

gtk_drawing_area_size(GTK_DRAWING_AREA(drawingArea), 200, 200);

Instead of:

gtk_drawing_area_size(G_OBJECT(drawingArea), 200, 200);

Because the GObject type does not support the gtk_drawing_area_size() method, but the GtkDrawingArea type does.

(Actually, it’s more like gtk_drawing_area_size() does not support taking a GObject instance, since GTK+’s object oriented nature is abstracted that way in C.)