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

Question

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

gtk_label_set_text();

this is what I write:

#include 

int main(         


        

Answer 1

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");

to

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.

Answer 2

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.