Generic value container

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 an 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.

Once you have finished using a GValue, you must call g_value_unset() to ensure that all the resources associated with the GValue are freed.

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.

The code in the example program below demonstrates GValue‘s features:

#include <glib-object.h>

static void
int2string (const GValue *src_value,
            GValue       *dest_value)
{
  if (g_value_get_int (src_value) == 42)
    g_value_set_static_string (dest_value, "An important number");
  else
    g_value_set_static_string (dest_value, "What's that?");
}

int
main (int   argc,
      char *argv[])
{
  // GValues must be initialized
  GValue a = G_VALUE_INIT;
  GValue b = G_VALUE_INIT;
  const char *message;

  // The GValue starts empty
  g_assert (!G_VALUE_HOLDS_STRING (&a));

  // Put a string in it
  g_value_init (&a, G_TYPE_STRING);
  g_assert (G_VALUE_HOLDS_STRING (&a));
  g_value_set_static_string (&a, "Hello, world!");
  g_printf ("%s\n", g_value_get_string (&a));

  // Reset it to its pristine state
  g_value_unset (&a);

  // It can then be reused for another type
  g_value_init (&a, G_TYPE_INT);
  g_value_set_int (&a, 42);

  // Attempt to transform it into a GValue of type STRING
  g_value_init (&b, G_TYPE_STRING);

  // An INT is transformable to a STRING
  g_assert (g_value_type_transformable (G_TYPE_INT, G_TYPE_STRING));

  g_value_transform (&a, &b);
  g_printf ("%s\n", g_value_get_string (&b));

  // Attempt to transform it again using a custom transform function
  g_value_register_transform_func (G_TYPE_INT, G_TYPE_STRING, int2string);
  g_value_transform (&a, &b);
  g_printf ("%s\n", g_value_get_string (&b));

  g_value_unset (&b);
  g_value_unset (&a);

  return 0;
}