In today’s PSA, mistakes with `g_value_set_boxed()`

. A mistake that’s been made several times by yours truly, and only realised today thanks to Xavier.

At some point in the GLib 2.22 cycle, types such as `GArray`

, `GPtrArray`

, `GByteArray`

and `GHashTable`

^{1} gained `ref()`

and `unref()`

methods, which allowed things like `g_boxed_copy()`

and `g_boxed_free()`

to be a stack faster for those types. This was mid-2008.

Danni, who didn’t get the memo, had been occasionally writing bits of code like this:

GArray *array = g_array_new (...);</p> <p>g_value_set_boxed (value, array);<br /> g_array_free (array, TRUE);

Which, if you read the code, will keep the wrapper alive, so your code won’t crash, but will release all the data the array contains. Similarly for `g_ptr_array_free()`

etc. It nicely zeros out the length of the array, so when you come to read it in your GValue, it looks empty.

The correct thing to do (of course) is unref your data, which will then free the memory and the wrapper when the refcount reaches zero.

GArray *array = g_array_new (...);</p> <p>g_value_set_boxed (value, array);<br /> g_array_unref (array);

**Update:** As pointed out in this simple example, you can replace this with the following pattern:

GArray *array = g_array_new (...);</p> <p>g_value_take_boxed (value, array);

Which passes your ownership to the `GValue`

. Where this falls down is for more complex values built for dbus-glib, where the ownership isn’t clear-cut, and so you need to free the components separately afterwards.

- You can discover what
`g_boxed_copy()`

and`g_boxed_free()`

will do for a type by looking for its`G_DEFINE_BOXED_TYPE`

in GLib, which is probably in`gobject/gboxed.c`. [↩]