Implementing the Interface and Private Struct

Oct 13, 2025 by wbrum.org 116 views

Introduction

When working with GObject C, defining a derivable type that implements an interface and has a private struct can be a bit tricky. In this article, we will explore how to achieve this using GObject C.

Understanding GObject C

Before we dive into the implementation, let's quickly understand what GObject C is. GObject C is a C library that provides a way to create objects and classes in C. It is widely used in the GNOME project and other C-based projects. GObject C provides a lot of features such as inheritance, polymorphism, and more.

Defining a Derivable Type

To define a derivable type in GObject C, we need to use the G_TYPE_DERIVABLE flag when registering the type. This flag indicates that the type is a derivable type, meaning it can be inherited by other types.

Here is an example of how to define a derivable type:

GType
my_type_register (void)
{
  static GType type = 0;
if (type == 0) {
static const GTypeInfo my_type_info = {
sizeof (MyTypeClass),
(GBaseInitFunc) NULL,
(GBaseFinalizeFunc) NULL,
(GClassInitFunc) my_class_init,
(GClassFinalizeFunc) NULL,
NULL,
sizeof (MyType),
0,
(GInstanceInitFunc) my_instance_init,
NULL
};
type = g_type_register_static (G_TYPE_OBJECT,
                               &quot;MyType&quot;,
                               &amp;my_type_info,
                               G_TYPE_FLAG_DERIVABLE);

}
return type;
}

In this example, we define a type MyType that is a derivable type using the G_TYPE_FLAG_DERIVABLE flag.

Implementing an Interface

To implement an interface in GObject C, we need to use the G_TYPE_INTERFACE flag when registering the interface. This flag indicates that the type is an interface, meaning it can be implemented by other types.

Here is an example of how to implement an interface:

GType
my_interface_register (void)
{
  static GType type = 0;
if (type == 0) {
static const GTypeInfo my_interface_info = {
sizeof (MyInterfaceClass),
(GBaseInitFunc) NULL,
(GBaseFinalizeFunc) NULL,
(GClassInitFunc) my_interface_class_init,
(GClassFinalizeFunc) NULL,
NULL,
0,
0,
(GInstanceInitFunc) NULL,
NULL
};
type = g_type_register_static (G_TYPE_INTERFACE,
                               &quot;MyInterface&quot;,
                               &amp;my_interface_info,
                               G_TYPE_FLAG_INTERFACE);

}
return type;
}

In this example, we define an interface MyInterface that is implemented by other types using the G_TYPE_FLAG_INTERFACE flag.

Creating a Private Struct

To create a private struct in GObject C, we need to use the G_TYPE_BOXED flag when registering the type. This flag indicates that the type is a boxed type, meaning it is a struct that is not directly accessible.

Here is an example of how to create a private struct:

GType
my_private_struct_register (void)
{
  static GType type = 0;
if (type == 0) {
static const GTypeInfo my_private_struct_info = {
sizeof (MyPrivateStruct),
(GBaseInitFunc) NULL,
(GBaseFinalizeFunc) NULL,
(GClassInitFunc) my_private_struct_class_init,
(GClassFinalizeFunc) NULL,
NULL,
sizeof (MyPrivateStruct),
0,
(GInstanceInitFunc) my_private_struct_instance_init,
NULL
};
type = g_type_register_static (G_TYPE_BOXED,
                               &quot;MyPrivateStruct&quot;,
                               &amp;my_private_struct_info,
                               G_TYPE_FLAG_BOXED);

}
return type;
}

In this example, we define a private struct MyPrivateStruct that is not directly accessible using the G_TYPE_FLAG_BOXED flag.

Implementing the Interface and Private Struct

To implement the interface and private struct in our derivable type, we need to use the g_type_add_interface function to add the interface to our type, and the g_type_add_class_private function to add the private struct to our type.

Here is an example of how to implement the interface and private struct:

void
my_class_init (MyTypeClass *klass)
{
  GType interface_type = my_interface_register ();
  g_type_add_interface (klass, interface_type);
GType private_struct_type = my_private_struct_register ();
g_type_add_class_private (klass, private_struct_type);
}

In this example, we add the interface and private struct to our derivable type using the g_type_add_interface and g_type_add_class_private functions.

Conclusion

In this article, we explored how to define a derivable type that implements an interface and has a private struct in GObject C. We covered the basics of GObject C, including defining a derivable type, implementing an interface, and creating a private struct. We also showed how to implement the interface and private struct in our derivable type using the g_type_add_interface and g_type_add_class_private functions.

Example Use Case

Here is an example use case of our derivable type:

MyType *my_type = g_object_new (MY_TYPE_TYPE, NULL);
MyInterface *my_interface = MY_TYPE_GET_INTERFACE (my_type);
MyPrivateStruct *my_private_struct = g_object_get_data (G_OBJECT (my_type), "my-private-struct");
g_print ("MyType: %p\n", my_type);
g_print ("MyInterface: %p\n", my_interface);
g_print ("MyPrivateStruct: %p\n", my_private_struct);

In this example, we create an instance of our derivable type MyType, and then get the interface and private struct from the instance using the MY_TYPE_GET_INTERFACE and g_object_get_data functions.

Code Snippets

Here are some code snippets that demonstrate the concepts covered in this article:

Defining a Derivable Type

GType my_type_register (void) { static GType type = 0;

if (type == 0) { static const GTypeInfo my_type_info = { sizeof (MyTypeClass), (GBaseInitFunc) NULL, (GBaseFinalizeFunc) NULL, (GClassInitFunc) my_class_init, (GClassFinalizeFunc) NULL, NULL, sizeof (MyType), 0, (GInstanceInitFunc) my_instance_init, NULL };

type = g_type_register_static (G_TYPE_OBJECT,
                               "MyType",
                               &my_type_info,
                               G_TYPE_FLAG_DERIVABLE);

}

return type; }

*   **Implementing an Interface**
    ```c
GType
my_interface_register (void)
{
  static GType type = 0;
if (type == 0) {
static const GTypeInfo my_interface_info = {
sizeof (MyInterfaceClass),
(GBaseInitFunc) NULL,
(GBaseFinalizeFunc) NULL,
(GClassInitFunc) my_interface_class_init,
(GClassFinalizeFunc) NULL,
NULL,
0,
0,
(GInstanceInitFunc) NULL,
NULL
};
type = g_type_register_static (G_TYPE_INTERFACE,
                               &quot;MyInterface&quot;,
                               &amp;my_interface_info,
                               G_TYPE_FLAG_INTERFACE);

}
return type;
}

Creating a Private Struct

GType my_private_struct_register (void) { static GType type = 0;

if (type == 0) { static const GTypeInfo my_private_struct_info = { sizeof (MyPrivateStruct), (GBaseInitFunc) NULL, (GBaseFinalizeFunc) NULL, (GClassInitFunc) my_private_struct_class_init, (GClassFinalizeFunc) NULL, NULL, sizeof (MyPrivateStruct), 0, (GInstanceInitFunc) my_private_struct_instance_init, NULL };

type = g_type_register_static (G_TYPE_BOXED,
                               "MyPrivateStruct",
                               &my_private_struct_info,
                               G_TYPE_FLAG_BOXED);

}

return type; }

*   **Implementing the Interface and Private Struct**
    ```c
void
my_class_init (MyTypeClass *klass)
{
  GType interface_type = my_interface_register ();
  g_type_add_interface (klass, interface_type);
GType private_struct_type = my_private_struct_register ();
g_type_add_class_private (klass, private_struct_type);
}

Example Use Case

MyType *my_type = g_object_new (MY_TYPE_TYPE, NULL); MyInterface *my_interface = MY_TYPE_GET_INTERFACE (my_type); MyPrivateStruct *my_private_struct = g_object_get_data (G_OBJECT (my_type), "my-private-struct");

g_print ("MyType: %p\n", my_type); g_print ("MyInterface: %p\n", my_interface); g_print ("MyPrivateStruct: %p\n", my_private_struct);

**Q&A: Defining a Derivable Type with Interface Implementation and Private Struct in GObject C**
====================================================================================
Q: What is the difference between a derivable type and a non-derivable type in GObject C?
A: In GObject C, a derivable type is a type that can be inherited by other types, whereas a non-derivable type is a type that cannot be inherited. Derivable types are created using the G_TYPE_FLAG_DERIVABLE flag when registering the type.
Q: How do I define a derivable type in GObject C?
A: To define a derivable type in GObject C, you need to use the G_TYPE_FLAG_DERIVABLE flag when registering the type. Here is an example of how to define a derivable type:
GType
my_type_register (void)
{
  static GType type = 0;

  if (type == 0) {
    static const GTypeInfo my_type_info = {
      sizeof (MyTypeClass),
      (GBaseInitFunc) NULL,
      (GBaseFinalizeFunc) NULL,
      (GClassInitFunc) my_class_init,
      (GClassFinalizeFunc) NULL,
      NULL,
      sizeof (MyType),
      0,
      (GInstanceInitFunc) my_instance_init,
      NULL
    };

    type = g_type_register_static (G_TYPE_OBJECT,
                                   &quot;MyType&quot;,
                                   &amp;my_type_info,
                                   G_TYPE_FLAG_DERIVABLE);
  }

  return type;
}
</code></pre>
<h3>Q: How do I implement an interface in a derivable type in GObject C?</h3>
<p>A: To implement an interface in a derivable type in GObject C, you need to use the <code>g_type_add_interface</code> function to add the interface to the type. Here is an example of how to implement an interface:</p>
<pre><code class="hljs">void
my_class_init (MyTypeClass *klass)
{
  GType interface_type = my_interface_register ();
  g_type_add_interface (klass, interface_type);
}
</code></pre>
<h3>Q: How do I create a private struct in GObject C?</h3>
<p>A: To create a private struct in GObject C, you need to use the <code>G_TYPE_FLAG_BOXED</code> flag when registering the type. Here is an example of how to create a private struct:</p>
<pre><code class="hljs">GType
my_private_struct_register (void)
{
  static GType type = 0;

  if (type == 0) {
    static const GTypeInfo my_private_struct_info = {
      sizeof (MyPrivateStruct),
      (GBaseInitFunc) NULL,
      (GBaseFinalizeFunc) NULL,
      (GClassInitFunc) my_private_struct_class_init,
      (GClassFinalizeFunc) NULL,
      NULL,
      sizeof (MyPrivateStruct),
      0,
      (GInstanceInitFunc) my_private_struct_instance_init,
      NULL
    };

    type = g_type_register_static (G_TYPE_BOXED,
                                   &quot;MyPrivateStruct&quot;,
                                   &amp;my_private_struct_info,
                                   G_TYPE_FLAG_BOXED);
  }

  return type;
}
</code></pre>
<h3>Q: How do I add a private struct to a type in GObject C?</h3>
<p>A: To add a private struct to a type in GObject C, you need to use the <code>g_type_add_class_private</code> function to add the private struct to the type. Here is an example of how to add a private struct:</p>
<pre><code class="hljs">void
my_class_init (MyTypeClass *klass)
{
  GType private_struct_type = my_private_struct_register ();
  g_type_add_class_private (klass, private_struct_type);
}
</code></pre>
<h3>Q: What is the difference between a boxed type and a non-boxed type in GObject C?</h3>
<p>A: In GObject C, a boxed type is a type that is a struct that is not directly accessible, whereas a non-boxed type is a type that is directly accessible. Boxed types are created using the <code>G_TYPE_FLAG_BOXED</code> flag when registering the type.</p>
<h3>Q: How do I get the interface and private struct from an instance of a type in GObject C?</h3>
<p>A: To get the interface and private struct from an instance of a type in GObject C, you need to use the <code>MY_TYPE_GET_INTERFACE</code> and <code>g_object_get_data</code> functions. Here is an example of how to get the interface and private struct:</p>
<pre><code class="hljs">MyType *my_type = g_object_new (MY_TYPE_TYPE, NULL);
MyInterface *my_interface = MY_TYPE_GET_INTERFACE (my_type);
MyPrivateStruct *my_private_struct = g_object_get_data (G_OBJECT (my_type), &quot;my-private-struct&quot;);
</code></pre>
<h3>Q: What is the purpose of the <code>G_TYPE_FLAG_DERIVABLE</code> flag in GObject C?</h3>
<p>A: The <code>G_TYPE_FLAG_DERIVABLE</code> flag in GObject C is used to indicate that a type is a derivable type, meaning it can be inherited by other types.</p>
<h3>Q: What is the purpose of the <code>G_TYPE_FLAG_INTERFACE</code> flag in GObject C?</h3>
<p>A: The <code>G_TYPE_FLAG_INTERFACE</code> flag in GObject C is used to indicate that a type is an interface, meaning it can be implemented by other types.</p>
<h3>Q: What is the purpose of the <code>G_TYPE_FLAG_BOXED</code> flag in GObject C?</h3>
<p>A: The <code>G_TYPE_FLAG_BOXED</code> flag in GObject C is used to indicate that a type is a boxed type, meaning it is a struct that is not directly accessible.</p>
<h3>Q: What is the purpose of the <code>g_type_add_interface</code> function in GObject C?</h3>
<p>A: The <code>g_type_add_interface</code> function in GObject C is used to add an interface to a type.</p>
<h3>Q: What is the purpose of the <code>g_type_add_class_private</code> function in GObject C?</h3>
<p>A: The <code>g_type_add_class_private</code> function in GObject C is used to add a private struct to a type.</p>
<h3>Q: What is the purpose of the <code>g_object_get_data</code> function in GObject C?</h3>
<p>A: The <code>g_object_get_data</code> function in GObject C is used to get the data associated with an object.</p>
<h3>Q: What is the purpose of the <code>MY_TYPE_GET_INTERFACE</code> macro in GObject C?</h3>
<p>A: The <code>MY_TYPE_GET_INTERFACE</code> macro in GObject C is used to get the interface from an instance of a type.</p>
<h3>Q: What is the purpose of the <code>MY_TYPE_GET_PRIVATE</code> macro in GObject C?</h3>
<p>A: The <code>MY_TYPE_GET_PRIVATE</code> macro in GObject C is used to get the private struct from an instance of a type.</p>
<h3>Q: What is the purpose of the <code>g_object_new</code> function in GObject C?</h3>
<p>A: The <code>g_object_new</code> function in GObject C is used to create a new instance of a type.</p>
<h3>Q: What is the purpose of the <code>g_object_ref</code> function in GObject C?</h3>
<p>A: The <code>g_object_ref</code> function in GObject C is used to increase the reference count of an object.</p>
<h3>Q: What is the purpose of the <code>g_object_unref</code> function in GObject C?</h3>
<p>A: The <code>g_object_unref</code> function in GObject C is used to decrease the reference count of an object.</p>
<h3>Q: What is the purpose of the <code>g_object_set_data</code> function in GObject C?</h3>
<p>A: The <code>g_object_set_data</code> function in GObject C is used to set the data associated with an object.</p>
<h3>Q: What is the purpose of the <code>g_object_get_data</code> function in GObject C?</h3>
<p>A: The <code>g_object_get_data</code> function in GObject C is used to get the data associated with an object.</p>
<h3>Q: What is the purpose of the <code>g_object_steal_data</code> function in GObject C?</h3>
<p>A: The <code>g_object_steal_data</code> function in GObject C is used to steal the data associated with an object.</p>
<h3>Q: What is the purpose of the <code>g_object_ref_sink</code> function in GObject C?</h3>
<p>A: The <code>g_object_ref_sink</code> function in GObject C is used to increase the reference count of an object and mark it as not being a sink object.</p>
<h3>Q: What is the purpose of the <code>g_object_unref_sink</code> function in GObject C?</h3>
<p>A: The <code>g_object_unref_sink</code> function in GObject C is used to decrease the reference count of an object and mark it as being a sink object.</p>
<h3>Q: What is the purpose of the <code>g_object_set_property</code> function in GObject C?</h3>
<p>A: The <code>g_object_set_property</code> function in GObject C is used to set the value of a property on an object.</p>
<h3>Q: What is the purpose of the <code>g_object_get_property</code> function in GObject C?</h3>
<p>A: The <code>g_object_get_property</code> function in GObject C is used to get the value of a property on an object.</p>
<h3>Q: What is the purpose of the <code>g_object_notify</code> function in GObject C?</h3>
<p>A: The <code>g_object_notify</code> function in GObject C is used to notify that a property on an object has changed.</p>
<h3>Q: What is the purpose of the <code>g_object_freeze_notify</code> function in GObject C?</h3>
<p>A: The <code>g_object_freeze_notify</code> function in GObject C is used to freeze the notification of a property on an object.</p>
<h3>Q: What is the purpose of the <code>g_object_thaw_notify</code> function in GObject C?</h3>
<p>A: The <code>g_object_thaw_notify</code> function in GObject C is used to thaw the notification of a property on an object.</p>
<h3>Q: What is the purpose of the <code>g_object_watch_closure</code> function in GObject C?</h3>
<p>A: The <code>g_object_watch_closure</code> function in GObject C is used to watch a closure for changes to an object.</p>
<h3>Q: What is the purpose of the <code>g_object_unwatch_closure</code> function in GObject C?</h3>
<p>A: The <code>g_object_unwatch_closure</code> function in GObject C is used to unwatch a closure for changes to</p>


                
                    
                        

                    
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