glib/docs/reference/gobject/tmpl/generic_values.sgml

<!-- ##### SECTION Title ##### -->
Generic Values

<!-- ##### SECTION Short_Description ##### -->
A polymorphic type that can hold values of any other type.

<!-- ##### SECTION Long_Description ##### -->
<para>
The #GValue structure is basically a variable container that consists
of a type identifier and a specific value of that type.
The type identifier within a #GValue structure always determines the
type of the associated value.
To create a undefined #GValue structure, simply create a zero-filled
#GValue structure. To initialize the #GValue, use the g_value_init()
function. A #GValue cannot be used until it is initialized.
The basic type operations (such as freeing and copying) are determined
by the #GTypeValueTable associated with the type ID stored in the #GValue.
Other #GValue operations (such as converting values between types) are
provided by this interface.
</para>

<!-- ##### SECTION See_Also ##### -->
<para>
The fundamental types which all support #GValue operations and thus
can be used as a type initializer for g_value_init() are defined by
a separate interface.  See the Standard Values API for details.
</para>

<!-- ##### MACRO G_VALUE_HOLDS ##### -->
<para>
Returns #TRUE if @value holds (or contains) a value of @type.
This macro will also check for @value != #NULL and issue a
warning if the check fails.
</para>

@value: 
@type: 


<!-- ##### MACRO G_VALUE_TYPE ##### -->
<para>
Returns the type identifier of @value.
</para>

@value: A #GValue structure.


<!-- ##### MACRO G_VALUE_TYPE_NAME ##### -->
<para>
Returns the type name of @value.
</para>

@value: A #GValue structure.


<!-- ##### MACRO G_TYPE_IS_VALUE ##### -->
<para>
Return whether the passed in type ID can be used for g_value_init().
That is, this macro checks whether this type provides an implementation
of the #GTypeValueTable functions required for a type to create a #GValue of.
</para>

@type: 	  A #GType value.
@Returns: Whether @type is suitable as a #GValue type.


<!-- ##### MACRO G_IS_VALUE ##### -->
<para>
Returns #TRUE if @value is a valid and initialized #GValue structure.
</para>

@value: A #GValue structure.


<!-- ##### USER_FUNCTION GValueExchange ##### -->
<para>

</para>

@value1: 
@value2: 


<!-- ##### STRUCT GValue ##### -->
<para>
A mostly opaque structure used to hold a #GValue object.  Mostly because
the data within the structure has protected scope: it is accessible only
to functions within a #GTypeValueTable structure, or implementations of
the g_value_*() API.
</para>


<!-- ##### FUNCTION g_value_init ##### -->
<para>
Initializes @value with the default value of @type.
</para>

@value: 		A zero-filled (uninitialized) #GValue structure.
@g_type: 	Type the #GValue should hold values of.
@Returns: 


<!-- ##### FUNCTION g_value_copy ##### -->
<para>
Copies the value of @src_value into @dest_value.
</para>

@src_value: 	An initialized #GValue structure.
@dest_value: 	An initialized #GValue structure of the same type as @src_value.


<!-- ##### FUNCTION g_value_reset ##### -->
<para>
Clears the current value in @value and resets it to the default value
(as if the value had just been initialized).
</para>

@value: 	An initialized #GValue structure.
@Returns: 


<!-- ##### FUNCTION g_value_unset ##### -->
<para>
Clears the current value in @value and "unsets" the type,
this releases all resources associated with this GValue.
An unset value is the same as an uninitialized (zero-filled)
#GValue structure.
</para>

@value: 	An initialized #GValue structure.


<!-- ##### FUNCTION g_value_fits_pointer ##### -->
<para>
Determines if @value will fit inside the size of a pointer value.
This is an internal function introduced mainly for C marshallers.
</para>

@value: 		An initialized #GValue structure.
@Returns: 	#TRUE if @value will fit inside a pointer value.


<!-- ##### FUNCTION g_value_peek_pointer ##### -->
<para>
Return the value contents as pointer. This function asserts that
g_value_fits_pointer() returned #TRUE for the passed in value.
This is an internal function introduced mainly for C marshallers.
</para>

@value: 		An initialized #GValue structure.
@Returns: 	#TRUE if @value will fit inside a pointer value.


<!-- ##### STRUCT GTypeValueTable ##### -->
<para>
The #GTypeValueTable provides the functions required by the #GValue implementation,
to serve as a container for values of a type.
</para>

@value_init: 		Default initialize @values contents by poking values
			directly into the value-&gt;data array. The data array of
			the #GValue passed into this function was zero-filled
			with memset, so no care has to be taken to free any
			old contents. E.g. for the implementation of a string
			value that may never be NULL, the implementation might
			look like:
<msgtext><programlisting>
{
  value-&gt;data[0].v_pointer = g_strdup ("");
}
</programlisting></msgtext>
@value_free: 		Free any old contents that might be left in the
			data array of the passed in @value. No resources may
			remain allocated through the #GValue contents after
			this function returns. E.g. for our above string type:
<msgtext><programlisting>
{
  /* only free strings without a specific flag for static storage */
  if (!(value-&gt;data[1].v_uint & G_VALUE_NOCOPY_CONTENTS))
    g_free (value-&gt;data[0].v_pointer);
}
</programlisting></msgtext>
@value_copy: 		@dest_value is a #GValue with zero-filled data section
			and @src_value is a properly setup #GValue of same or
			derived type.
			The purpose of this function is to copy the contents of
			@src_value into @dest_value in a way, that even after
			@src_value has been freed, the contents of @dest_value
			remain valid. String type example:
<msgtext><programlisting>
{
  dest_value-&gt;data[0].v_pointer = g_strdup (src_value-&gt;data[0].v_pointer);
}
</programlisting></msgtext>
@value_peek_pointer: 	If the value contents fit into a pointer, such as objects
			or strings, return this pointer, so the caller can peek at
			the current contents. To extend on our above string example:
<msgtext><programlisting>
{
  return value-&gt;data[0].v_pointer;
}
</programlisting></msgtext>
@collect_format: 	A string format describing how to collect the contents of
			this value, bit-by-bit. Each character in the format represents
			an argument to be collected, the characters themselves indicate
			the type of the argument. Currently supported arguments are:
<msgtext><variablelist>
  <varlistentry><term></term><listitem><para>
        'i' - Integers. passed as collect_values[].v_int.
  </para></listitem></varlistentry>
  <varlistentry><term></term><listitem><para>
        'l' - Longs. passed as collect_values[].v_long.
  </para></listitem></varlistentry>
  <varlistentry><term></term><listitem><para>
        'd' - Doubles. passed as collect_values[].v_double.
  </para></listitem></varlistentry>
  <varlistentry><term></term><listitem><para>
        'p' - Pointers. passed as collect_values[].v_pointer.
  </para></listitem></varlistentry>
</variablelist></msgtext>
			It should be noted, that for variable argument list construction,
			ANSI C promotes every type smaller than an integer to an int, and
			floats to doubles. So for collection of short int or char, 'i'
			needs to be used, and for collection of floats 'd'.
@collect_value: 	The collect_value() function is responsible for converting the
			values collected from a variable argument list into contents
			suitable for storage in a GValue. This function should setup
			@value similar to value_init(), e.g. for a string value that
			does not allow NULL pointers, it needs to either spew an error,
			or do an implicit conversion by storing an empty string.
			The @value passed in to this function has a zero-filled data
			array, so just like for @value_init it is guaranteed to not
			contain any old contents that might need freeing.
			@n_collect_values is exactly the string length of @collect_format,
			and @collect_values is an array of unions #GTypeCValue with
			length @n_collect_values, containing the collected values
			according to @collect_format.
			@collect_flags is an argument provided as a hint by the caller,
			which may contain the flag #G_VALUE_NOCOPY_CONTENTS indicating,
			that the collected value contents may be considered "static"
			for the duration of the #@value lifetime.
			Thus an extra copy of the contents stored in @collect_values is
			not required for assignment to @value.
			For our above string example, we continue with:
<msgtext><programlisting>
{
  if (!collect_values[0].v_pointer)
    value->data[0].v_pointer = g_strdup ("");
  else if (collect_flags & G_VALUE_NOCOPY_CONTENTS)
    {
      value-&gt;data[0].v_pointer = collect_values[0].v_pointer;
      /* keep a flag for the value_free() implementation to not free this string */
      value-&gt;data[1].v_uint = G_VALUE_NOCOPY_CONTENTS;
    }
  else
    value-&gt;data[0].v_pointer = g_strdup (collect_values[0].v_pointer);

  return NULL;
}
</programlisting></msgtext>
			It should be noted, that it is generally a bad idea to follow the
			#G_VALUE_NOCOPY_CONTENTS hint for reference counted types. Due to
			reentrancy requirements and reference count assertions performed
			by the GSignal code, reference counts should always be incremented
			for reference counted contents stored in the value-&gt;data array.
			To deviate from our string example for a moment, and taking a look
			at an exemplary implementation for collect_value() of #GObject:
<msgtext><programlisting>
{
  if (collect_values[0].v_pointer)
    {
      GObject *object = G_OBJECT (collect_values[0].v_pointer);

      /* never honour G_VALUE_NOCOPY_CONTENTS for ref-counted types */
      value-&gt;data[0].v_pointer = g_object_ref (object);
      return NULL;
    }
  else
    return g_strdup_printf ("Object passed as invalid NULL pointer");
}
</programlisting></msgtext>
			The reference count for valid objects is always incremented,
			regardless of @collect_flags. For invalid objects, the example
			returns a newly allocated string without altering @value.
			Upon success, collect_value() needs to return NULL, if however
			a malicious condition occurred, collect_value() may spew an
			error by returning a newly allocated non-NULL string, giving
			a suitable description of the error condition.
			The calling code makes no assumptions about the @value
			contents being valid upon error returns, @value
			is simply thrown away without further freeing. As such, it is
			a good idea to not allocate #GValue contents, prior to returning
			an error, however, collect_values() is not obliged to return
			a correctly setup @value for error returns, simply because
			any non-NULL return is considered a fatal condition so further
			program behaviour is undefined.
@lcopy_format: 		Format description of the arguments to collect for @lcopy_value,
			analogous to @collect_format. Usually, @lcopy_format string consists
			only of 'p's to provide lcopy_value() with pointers to storage locations.
@lcopy_value: 		This function is responsible for storing the @value contents into
			arguments passed through a variable argument list which got
			collected into @collect_values according to @lcopy_format.
			@n_collect_values equals the string length of @lcopy_format,
			and @collect_flags may contain #G_VALUE_NOCOPY_CONTENTS.
			In contrast to collect_value(), lcopy_value() is obliged to
			always properly support #G_VALUE_NOCOPY_CONTENTS.
			Similar to collect_value() the function may prematurely abort
			by returning a newly allocated string describing an error condition.
			To complete the string example:
<msgtext><programlisting>
{
  gchar **string_p = collect_values[0].v_pointer;

  if (!string_p)
    return g_strdup_printf ("string location passed as NULL");

  if (collect_flags & G_VALUE_NOCOPY_CONTENTS)
    *string_p = value-&gt;data[0].v_pointer;
  else
    *string_p = g_strdup (value-&gt;data[0].v_pointer);

}
</programlisting></msgtext>
			And an exemplary version of lcopy_value() for
			reference-counted types:
<msgtext><programlisting>
{
  GObject **object_p = collect_values[0].v_pointer;

  if (!object_p)
    return g_strdup_printf ("object location passed as NULL");
  if (!value-&gt;data[0].v_pointer)
    *object_p = NULL;
  else if (collect_flags & G_VALUE_NOCOPY_CONTENTS) /* always honour */
    *object_p = value-&gt;data[0].v_pointer;
  else
    *object_p = g_object_ref (value-&gt;data[0].v_pointer);
  return NULL;
}
</programlisting></msgtext>