How do I update Labels in GTK+-3.0 in C

问题

I can't seem to be able to use the function

gtk_label_set_text();

this is what I write:

#include <gtk/gtk.h>

int main(int argc, char *argv[] )
{
  gtk_init(&argc, &argv);

  GtkWidget *label;

  //label
  label = gtk_label_new("This is my label");

  gtk_label_set_text(label, "I cannot use this func");

  if(GTK_IS_LABEL(label)){
    g_print("IT IS A LABEL\n");
  }else if (GTK_IS_WIDGET(label)){
    g_print("well at least its a Widget\n");
  }else {
    g_print("why is it not a label?! T_T\n");
  }

  gtk_main();
  return 0;
}

it says

passing argument 1 of ‘gtk_label_set_text’ from incompatible pointer type
expected ‘struct GtkLabel *’ but argument is of type ‘struct GtkWidget *’

when I try to compile it. OK. I change it to

GtkLabel *label;

but then it doesn't let me use gtk_label_new(); because the compiler complains with

assignment from incompatible pointer type

then I give up and comment //gtk_label_set_text to ask the program GTK_IS_LABEL(label); and it prints out IT IS A LABEL. I give up for real now and ask you, dear community of Stack Overflow.

回答1:

As it is C code, you have apply a "cast" your pointer since gtk_label_new returns the "base class" or structure:

╰── GtkWidget
    ╰── GtkMisc
        ╰── GtkLabel
            ╰── GtkAccelLabel

The cast is done through a macro (see here) code:

GtkWidget *label;

//label
label = gtk_label_new("This is my label");

gtk_label_set_text(GTK_LABEL (label), "I cannot use this func");

Note: in C++ no need to cast since GtkLabel structure inherits from GtkWidget, you could store label in a GtkLabel directly, but there's no such thing as inheritance in C.

回答2:

As per the [ prototype ] , gtk_label_set_text expects GtkLabel* as its first parameter, so change

gtk_label_set_text(label, "I cannot use this func");

gtk_label_set_text((GtkLabel*)label, "I cannot use this func");

or more conveniently

gtk_label_set_text(GTK_LABEL(label), "I cannot use this func");

In fact GTK_LABEL() macro expands to :

#define GTK_LABEL(obj) (G_TYPE_CHECK_INSTANCE_CAST ((obj), GTK_TYPE_LABEL, GtkLabel))

in the header gtklabel.h.

来源：https://stackoverflow.com/questions/39155120/how-do-i-update-labels-in-gtk-3-0-in-c

标签

gtk

label