incomprehensible behavior of this

Question

I have few problems dealing with the behavior of this in vala class.

here is my code: (build_and_send1 and build_and_send2 are signals raised by pressing a button)

using GLib;
using Gtk;

public class Main : Object 
{

    /* 
     * Uncomment this line when you are done testing and building a tarball
     * or installing
     */
    //const string UI_FILE = Config.PACKAGE_DATA_DIR + "/" + "gtk_httpclient.ui";
    const string UI_FILE = "src/gtk_httpclient.ui";

    /* ANJUTA: Widgets declaration for gtk_httpclient.ui - DO NOT REMOVE */


    private Builder builder;

    public Main ()
    {

        try 
        {
            this.builder = new Builder ();
            this.builder.add_from_file (UI_FILE);
            this.builder.connect_signals (this);

            var window = this.builder.get_object ("window") as Window;
            /* ANJUTA: Widgets initialization for gtk_httpclient.ui - DO NOT REMOVE */
            window.show_all ();

            stderr.printf ("constructor:\n");
            stderr.printf ("this: %p\n", this);
            stderr.printf ("builder: %p\n", this.builder);
        } 
        catch (Error e) {
            stderr.printf ("Could not load UI: %s\n", e.message);
        } 

    }

    [CCode (instance_pos = -1)]
    public void on_destroy (Widget window) 
    {
        Gtk.main_quit();
    }

    public void build_and_send1 (Widget button)
    {
        stderr.printf ("\nbuild_and_send1:\n");
        stderr.printf ("this: %p\n", this);
        stderr.printf ("builder: %p\n", this.builder);
    }

    [CCode (instance_pos = -1)]
    public void build_and_send2 (Widget button)
    {
        stderr.printf ("\nbuild_and_send2:\n");
        stderr.printf ("this: %p\n", this);
        stderr.printf ("builder: %p\n", this.builder);
    }

    static int main (string[] args) 
    {
        Gtk.init (ref args);
        new Main ();

        Gtk.main ();

        return 0;
    }
}

I've got this output :

constructor:
this: 0x1a524a0
builder: 0x1a6a230

build_and_send1:
this: 0x1aa2030
builder: 0x5a4fe823

build_and_send2:
this: 0x1a524a0
Program has been terminated receiving signal 11 (Segmentation fault)

I assume that if I want the same instance of Main in my signals, they have to be preceded by [CCode (instance_pos = -1)]. But then what is th goal of builder.connect_signals (this); ?

And why this.builder raises a segmentation fault in build_and_send2 and not in the constructor ?

Answer 1

Most likely, the result of new Main() is going out of scope after construction and being garbage collected. Assign it to a variable.