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| <div class="sect1" title="Non-instantiable non-classed fundamental types"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="gtype-non-instantiable"></a>Non-instantiable non-classed fundamental types</h2></div></div></div> |
| <p> |
| A lot of types are not instantiable by the type system and do not have |
| a class. Most of these types are fundamental trivial types such as <span class="emphasis"><em>gchar</em></span>, |
| registered in <code class="function">g_value_types_init</code> (in <code class="filename">gvaluetypes.c</code>). |
| </p> |
| <p> |
| To register such a type in the type system, you just need to fill the |
| <span class="type"><a class="link" href="gobject-Type-Information.html#GTypeInfo" title="GTypeInfo">GTypeInfo</a></span> structure with zeros since these types are also most of the time |
| fundamental: |
| </p> |
| <pre class="programlisting"> |
| GTypeInfo info = { |
| 0, /* class_size */ |
| NULL, /* base_init */ |
| NULL, /* base_destroy */ |
| NULL, /* class_init */ |
| NULL, /* class_destroy */ |
| NULL, /* class_data */ |
| 0, /* instance_size */ |
| 0, /* n_preallocs */ |
| NULL, /* instance_init */ |
| NULL, /* value_table */ |
| }; |
| static const GTypeValueTable value_table = { |
| value_init_long0, /* value_init */ |
| NULL, /* value_free */ |
| value_copy_long0, /* value_copy */ |
| NULL, /* value_peek_pointer */ |
| "i", /* collect_format */ |
| value_collect_int, /* collect_value */ |
| "p", /* lcopy_format */ |
| value_lcopy_char, /* lcopy_value */ |
| }; |
| info.value_table = &value_table; |
| type = g_type_register_fundamental (G_TYPE_CHAR, "gchar", &info, &finfo, 0); |
| </pre> |
| <p> |
| </p> |
| <p> |
| Having non-instantiable types might seem a bit useless: what good is a type |
| if you cannot instantiate an instance of that type ? Most of these types |
| are used in conjunction with <span class="type"><a class="link" href="gobject-Generic-values.html#GValue" title="GValue">GValue</a></span>s: a GValue is initialized |
| with an integer or a string and it is passed around by using the registered |
| type's value_table. <span class="type"><a class="link" href="gobject-Generic-values.html#GValue" title="GValue">GValue</a></span>s (and by extension these trivial fundamental |
| types) are most useful when used in conjunction with object properties and signals. |
| </p> |
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