error: void value not ignored as it ought to be - C/GTK+

Question

I am trying to create a window in GTK+ that has tab looks like this:

enter image description here

Whenever the user clicks in the "New" button, there is a new tab appear.

However, when I was compiling my program I got an error: void value not ignored as it ought to be

phongcao@phongcao:~$ g++ /home/phongcao/C++/GTK+/newtab.cc -o /home/phongcao/C++/GTK+/newtab `pkg-config gtk+-2.0 --cflags --libs`
/home/phongcao/C++/GTK+/newtab.cc: In function ‘int main(int, char**)’:
/home/phongcao/C++/GTK+/newtab.cc:51:3: error: void value not ignored as it ought to be

And here is the code:

#include <gtk/gtk.h>

void destroy(GtkWidget *window, gpointer data) {
  gtk_main_quit();
}

void new_tab(GtkNotebook *notebook, GtkWidget *content, GtkWidget *hbox) {
  gtk_notebook_append_page(notebook, content, hbox);
}

int main(int argc, char *argv[]) 
{
  GtkWidget *window, *label, *content;
  GtkWidget *button, *hbox, *notebook;

  gtk_init(&argc, &argv);

  window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
  gtk_container_set_border_width(GTK_CONTAINER(window), 10);
  gtk_window_set_title(GTK_WINDOW(window), "New Tab");
  gtk_widget_set_size_request(window, 300, 200);

  notebook = gtk_notebook_new();

  button = gtk_button_new_with_label("New");
  label = gtk_label_new("Tab");
  hbox = gtk_hbox_new(FALSE, 5);
  content = gtk_label_new("This is a tab");
  gint a = 0;

  gtk_box_pack_start_defaults(GTK_BOX(hbox), label);
  gtk_box_pack_start_defaults(GTK_BOX(hbox), button);

  gtk_notebook_append_page(GTK_NOTEBOOK(notebook), content, hbox);
  gtk_notebook_set_tab_pos(GTK_NOTEBOOK(notebook), GTK_POS_TOP);

  //This following line is where the error is from:
  g_signal_connect(G_OBJECT(button), "new_tab", G_CALLBACK(new_tab(GTK_NOTEBOOK(notebook), content, hbox)), NULL); 

  g_signal_connect(G_OBJECT(window), "destroy", G_CALLBACK(destroy), NULL);

  gtk_container_add(GTK_CONTAINER(window), notebook);

  gtk_widget_show_all(hbox); 
  gtk_widget_show_all(window);

  gtk_main();
  return 0;
}

Thanks for your help!! I really appreciate it!!

It seems like you try to use the return value from a void function. I can't count to 53, so can't say on what line. — Bo Persson
Can you add a comment to the line the compiler is complaining about? Your example is only 48 lines :) — hammar
I already add comment on the line that the error occurs... Sorry guys!! — phongvcao

mu is too short mu is too short · Accepted Answer · 2011-04-22T06:30:29

The line number doesn't make much sense to me but I think your problem is right here (reformatted for illustrative purposes):

g_signal_connect(G_OBJECT(button), "new_tab",
    G_CALLBACK(new_tab(GTK_NOTEBOOK(notebook), content, hbox)), /* <== Badness */
    NULL
);

You're calling the new_tab function inside the G_CALLBACK macro. The new_tab function returns void (i.e. no return value) but it is being called in a context that needs a value and hence the "void value not ignored" error. I think you mean to say something more like this:

g_signal_connect(G_OBJECT(button), "new_tab", G_CALLBACK(new_tab), NULL);

error: void value not ignored as it ought to be - C/GTK+

2 Answers