glib/docs/reference/glib/tmpl/hash_tables.sgml

<!-- ##### SECTION Title ##### -->
Hash Tables

<!-- ##### SECTION Short_Description ##### -->
associations between keys and values so that given a key the value
can be found quickly.

<!-- ##### SECTION Long_Description ##### -->
<para>
A #GHashTable provides associations between keys and values which
is optimized so that given a key, the associated value can be found
very quickly.
</para>
<para>
Note that neither keys nor values are copied when inserted into the
#GHashTable, so they must exist for the lifetime of the #GHashTable.
This means that the use of static strings is OK, but temporary
strings (i.e. those created in buffers and those returned by GTK widgets)
should be copied with g_strdup() before being inserted.
</para>
<para>
If keys or values are dynamically allocated, you must be careful to ensure
that they are freed when they are removed from the #GHashTable, and also
when they are overwritten by new insertions into the #GHashTable.
It is also not advisable to mix static strings and dynamically-allocated
strings in a #GHashTable, because it then becomes difficult to determine
whether the string should be freed.
</para>
<para>
To create a #GHashTable, use g_hash_table_new().
</para>
<para>
To insert a key and value into a #GHashTable, use g_hash_table_insert().
</para>
<para>
To lookup a value corresponding to a given key, use g_hash_table_lookup()
and g_hash_table_lookup_extended().
</para>
<para>
To remove a key and value, use g_hash_table_remove().
</para>
<para>
To call a function for each key and value pair use g_hash_table_foreach().
</para>
<para>
To destroy a #GHashTable use g_hash_table_destroy().
</para>

<!-- ##### SECTION See_Also ##### -->
<para>

</para>

<!-- ##### STRUCT GHashTable ##### -->
<para>
The #GHashTable struct is an opaque data structure to represent a
<link linkend="glib-Hash-Tables">Hash Table</link>.
It should only be accessed via the following functions.
</para>


<!-- ##### FUNCTION g_hash_table_new ##### -->
<para>
Creates a new #GHashTable.
</para>

@hash_func: a function to create a hash value from a key.
Hash values are used to determine where keys are stored within the
#GHashTable data structure.
The g_direct_hash(), g_int_hash() and g_str_hash() functions are provided for
some common types of keys. If hash_func is NULL, g_direct_hash() is used.
@key_compare_func: a function to compare two keys to see if they are equal.
This is used when looking up keys in the #GHashTable.
The g_direct_equal(), g_int_equal() and g_str_equal() functions are provided
for the most common types of keys. If compare_func is NULL, keys are compared
directly in a similar fashion to g_direct_equal(), but without the overhead
of a function call.
@Returns: a new #GHashTable.


<!-- ##### USER_FUNCTION GHashFunc ##### -->
<para>
Specifies the type of the hash function which is passed to
g_hash_table_new() when a #GHashTable is created.
</para>
<para>
The function is passed a key and should return a guint hash value.
The functions g_direct_hash(), g_int_hash() and g_str_hash() provide
hash functions which can be used when the key is a #gpointer, #gint, and 
#gchar* respectively.
</para>
<para>
FIXME: Need more here.
The hash values should be evenly distributed over a fairly large range?
The modulus is taken with the hash table size (a prime number)
to find the 'bucket' to place each key into.
The function should also be very fast, since it is called for each key
lookup.
</para>

@key: a key.
@Returns: the hash value corresponding to the key.


<!-- ##### USER_FUNCTION GCompareFunc ##### -->
<para>
Specifies the type of a comparison function used to compare two values.
The value which should be returned depends on the context in which the
#GCompareFunc is used.
</para>
<para>
In g_hash_table_new(), g_cache_new(), and g_relation_index() the function
should return TRUE if the two parameters are equal, or FALSE if they are not.
</para>
<para>
In g_list_find_custom() and g_slist_find_custom() the function should return
0 if the two parameters are equal.
</para>
<para>
In g_list_insert_sorted(), g_list_sort(), g_slist_insert_sorted(),
g_slist_sort() and g_tree_new()  the function should return a negative integer
if the first value comes before the second, 0 if they are equal, or a positive
integer if the first value comes after the second.
</para>

@a: a value.
@b: a value to compare with.
@Returns: TRUE if the two values are equivalent.


<!-- ##### FUNCTION g_hash_table_insert ##### -->
<para>
Inserts a new key and value into a #GHashTable.
If the key already exists in the #GHashTable its current value is replaced
with the new value.
</para>
<note>
<para>
If the keys or values use dynamically allocated memory, then you should
first check if the key already exists in the GHashTable. If it does,
you should free the existing key and/or value before inserting the
new key and value.
</para>
</note>

@hash_table: a #GHashTable.
@key: a key to insert.
@value: the value to associate with the key.


<!-- ##### FUNCTION g_hash_table_size ##### -->
<para>
Returns the number of key/value pairs in a #GHashTable.
</para>

@hash_table: a #GHashTable.
@Returns: the number of key/value pairs in the #GHashTable.


<!-- ##### FUNCTION g_hash_table_lookup ##### -->
<para>
Looks up a key in the #GHashTable, returning the associated value or NULL
if the key is not found.
</para>

@hash_table: a #GHashTable.
@key: the key to look up.
@Returns: the associated value, or NULL if the key is not found.


<!-- ##### FUNCTION g_hash_table_lookup_extended ##### -->
<para>
Looks up a key in the #GHashTable, returning the original key and the
associated value and a gboolean which is TRUE if the key was found.
This is useful if you need to free the memory allocated for the
original key, for example before calling g_hash_table_remove().
</para>

@hash_table: a #GHashTable.
@lookup_key: the key to look up.
@orig_key: returns the original key.
@value: returns the value associated with the key.
@Returns: TRUE if the key was found in the #GHashTable.


<!-- ##### FUNCTION g_hash_table_foreach ##### -->
<para>
Calls the given function for each of the key/value pairs in the #GHashTable.
The function is passed the key and value of each pair, and the given
@user_data parameter.
</para>

@hash_table: a #GHashTable.
@func: the function to call for each key/value pair.
@user_data: use data to pass to the function.


<!-- ##### USER_FUNCTION GHFunc ##### -->
<para>
Specifies the type of the function passed to g_hash_table_foreach().
It is called with each key/value pair, together with the @user_data parameter
which is passed to g_hash_table_foreach().
</para>

@key: a key.
@value: the value corresponding to the key.
@user_data: user data passed to g_hash_table_foreach().


<!-- ##### FUNCTION g_hash_table_remove ##### -->
<para>
Removes a key and its associated value from a #GHashTable.
</para>
<note>
<para>
As with g_hash_table_insert(), you should make sure that any dynamically
allocated values are freed yourself.
</para>
</note>

@hash_table: a #GHashTable.
@key: the key to remove.


<!-- ##### FUNCTION g_hash_table_foreach_remove ##### -->
<para>
Calls the given function for each key/value pair in the #GHashTable.
If the function returns TRUE, then the key/value pair is removed from the
#GHashTable.
</para>

@hash_table: a #GHashTable.
@func: the function to call for each key/value pair.
@user_data: user data to pass to the function.
@Returns: the number of key/value paris removed.


<!-- ##### USER_FUNCTION GHRFunc ##### -->
<para>
Specifies the type of the function passed to g_hash_table_foreach_remove().
It is called with each key/value pair, together with the @user_data parameter
passed to g_hash_table_foreach_remove().
It should return TRUE if the key/value pair should be removed from the
#GHashTable.
</para>

@key: a key.
@value: the value associated with the key.
@user_data: user data passed to g_hash_table_remove().
@Returns: TRUE if the key/value pair should be removed from the #GHashTable.


<!-- ##### FUNCTION g_hash_table_freeze ##### -->
<para>
Disable resizing of a #GHashTable.
</para>
<para>
This should be used if you need to make a lot of changes to a #GHashTable
at once, as it reduces the number of times that the #GHashTable is rebuilt.
You should call g_hash_table_thaw() after updating the #GHashTable to
enable resizing again.
</para>

@hash_table: a #GHashTable.


<!-- ##### FUNCTION g_hash_table_thaw ##### -->
<para>
Enables resizing of a #GHashTable.
</para>

@hash_table: a #GHashTable.


<!-- ##### FUNCTION g_hash_table_destroy ##### -->
<para>
Destroys the #GHashTable.
</para>
<note>
<para>
If keys and/or values are dynamically allocated, you should free them
first.
</para>
</note>

@hash_table: a #GHashTable.


<!-- ##### FUNCTION g_direct_equal ##### -->
<para>
Compares two #gpointer arguments and returns TRUE if they are equal.
It can be passed to g_hash_table_new() as the @key_compare_func
parameter, when using pointers as keys in a #GHashTable.
</para>

@v: a key.
@v2: a key to compare with @v.
@Returns: TRUE if the two keys match.


<!-- ##### FUNCTION g_direct_hash ##### -->
<para>
Converts a gpointer to a hash value.
It can be passed to g_hash_table_new() as the @hash_func parameter, when
using gpointer values as keys in a #GHashTable.
</para>

@v: a gpointer key.
@Returns: a hash value corresponding to the key.


<!-- ##### FUNCTION g_int_equal ##### -->
<para>
Compares the two #gint values being pointed to and returns TRUE if they are
equal.
It can be passed to g_hash_table_new() as the @key_compare_func
parameter, when using pointers to integers as keys in a #GHashTable.
</para>

@v: a pointer to a #gint key.
@v2: a pointer to a #gint key to compare with @v.
@Returns: TRUE if the two keys match.


<!-- ##### FUNCTION g_int_hash ##### -->
<para>
Converts a pointer to a #gint to a hash value.
It can be passed to g_hash_table_new() as the @hash_func parameter, when
using pointers to gint values as keys in a #GHashTable.
</para>

@v: a pointer to a #gint key.
@Returns: a hash value corresponding to the key.


<!-- ##### FUNCTION g_str_equal ##### -->
<para>
Compares two strings and returns TRUE if they are equal.
It can be passed to g_hash_table_new() as the @key_compare_func
parameter, when using strings as keys in a #GHashTable.
</para>

@v: a key.
@v2: a key to compare with @v.
@Returns: TRUE if the two keys match.


<!-- ##### FUNCTION g_str_hash ##### -->
<para>
Converts a string to a hash value.
It can be passed to g_hash_table_new() as the @hash_func parameter, when
using strings as keys in a #GHashTable.
</para>

@v: a string key.
@Returns: a hash value corresponding to the key.