GtkContainer

## GtkContainer GtkContainer — Base class for widgets which contain other widgets |

## Includes

    #include <gtk/gtk.h>

## Description

A GTK+ user interface is constructed by nesting widgets inside widgets. Container widgets are the inner nodes in the resulting tree of widgets: they contain other widgets. So, for example, you might have a GtkWindow containing a GtkFrame containing a GtkLabel. If you wanted an image instead of a textual label inside the frame, you might replace the GtkLabel widget with a GtkImage widget.

There are two major kinds of container widgets in GTK+. Both are subclasses of the abstract GtkContainer base class.

The first type of container widget has a single child widget and derives from GtkBin. These containers are decorators, which add some kind of functionality to the child. For example, a GtkButton makes its child into a clickable button; a GtkFrame draws a frame around its child and a GtkWindow places its child widget inside a top-level window.

The second type of container can have more than one child; its purpose is to manage layout. This means that these containers assign sizes and positions to their children. For example, a GtkHBox arranges its children in a horizontal row, and a GtkTable arranges the widgets it contains in a two-dimensional grid.

To fulfill its task, a layout container must negotiate the size requirements with its parent and its children. This negotiation is carried out in two phases, size requisition and size allocation.

### Size Requisition

The size requisition of a widget is it's desired width and height. This is represented by a GtkRequisition.

How a widget determines its desired size depends on the widget. A GtkLabel, for example, requests enough space to display all its text. Container widgets generally base their size request on the requisitions of their children.

The size requisition phase of the widget layout process operates top-down. It starts at a top-level widget, typically a GtkWindow. The top-level widget asks its child for its size requisition by calling `gtk_widget_size_request()`. To determine its requisition, the child asks its own children for their requisitions and so on. Finally, the top-level widget will get a requisition back from its child.

### Size Allocation

When the top-level widget has determined how much space its child would like to have, the second phase of the size negotiation, size allocation, begins. Depending on its configuration (see `gtk_window_set_resizable()`), the top-level widget may be able to expand in order to satisfy the size request or it may have to ignore the size request and keep its fixed size. It then tells its child widget how much space it gets by calling `gtk_widget_size_allocate()`. The child widget divides the space among its children and tells each child how much space it got, and so on. Under normal circumstances, a GtkWindow will always give its child the amount of space the child requested.

A child's size allocation is represented by a GtkAllocation. This struct contains not only a width and height, but also a position (i.e. X and Y coordinates), so that containers can tell their children not only how much space they have gotten, but also where they are positioned inside the space available to the container.

Widgets are required to honor the size allocation they receive; a size request is only a request, and widgets must be able to cope with any size.

### Child properties

GtkContainer introduces child properties - these are object properties that are not specific to either the container or the contained widget, but rather to their relation. Typical examples of child properties are the position or pack-type of a widget which is contained in a GtkBox.

Use `gtk_container_class_install_child_property()` to install child properties for a container class and `gtk_container_class_find_child_property()` or `gtk_container_class_list_child_properties()` to get information about existing child properties.

To set the value of a child property, use `gtk_container_child_set_property()`, `gtk_container_child_set()` or `gtk_container_child_set_valist()`. To obtain the value of a child property, use `gtk_container_child_get_property()`, `gtk_container_child_get()` or `gtk_container_child_get_valist()`. To emit notification about child property changes, use `gtk_widget_child_notify()`.

### GtkContainer as GtkBuildable

The GtkContainer implementation of the GtkBuildable interface supports a <packing> element for children, which can contain multiple <property> elements that specify child properties for the child.

Example 41. Child properties in UI definitions

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    <object class="GtkVBox">
      <child>
        <object class="GtkLabel"/>
        <packing>
          <property name="pack-type">start</property>
        </packing>
      </child>
    </object>

Since 2.16, child properties can also be marked as translatable using the same "translatable", "comments" and "context" attributes that are used for regular properties.

## Functions

## Types and Values