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It appears that in GTK you hold a reference to an object, for example like an GtkEntry, but you hold it with a pointer to a GtkWidget. For example

GtkWidget* pointer = gtk_entry_new();

Then when you want to do something like set the text of that entry, you have to call a function and do something with that pointer. 

gtk_entry_set_text(GTK_ENTRY(pointer), "hello");

A few questions:

  1. What is the "GTK_ENTRY()" thing? Is it a function call or typecast?
  2. Why is it in all Capital letters, why not do something like (GtkEntry*)pointer?
  3. Why even do this? Why not return a GtkEntry pointer when you create a new Entry?
Faruk
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Matthew
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1 Answers1

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GTK_ENTRY is a macro, behaving like a function. It casts its argument to GtkEntry*, but it may - depending on macros like NDEBUG - do additional check to verify that provided argument may be cast.

It is in all CAPITAL because that is the general convention in C for macros.

The function gtk_entry_new returns GtkWidget* and not GtkEntry* more functions in GTK operates on GtkWidget*, and C language does not provide inheritance so the conversion cannot be automatic.

Wojtek Surowka
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  • What is better to use, a macro rather than a conversion like (GtkEntry*)pointer – Matthew Aug 26 '14 at 20:07
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    Macro is better because it is safer - it performs the check. Just the cast would perform the conversion blindly and some errors would not be visible immediately. This is how GTK emulates classes and inheritance. – Wojtek Surowka Aug 26 '14 at 20:11
  • How come GTK says it has ancestry but there is no inheritance in C? – Matthew Aug 26 '14 at 22:15
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    GTK emulates it. More details at http://www.gtk.org/tutorial1.2/gtk_tut-22.html#ss22.2 – Wojtek Surowka Aug 26 '14 at 22:51