## Caches

Caches — caches allow sharing of complex data structures to save resources

## Synopsis

    #include <glib.h>

                        GCache;
    GCache*             g_cache_new                         (GCacheNewFunc value_new_func,
                                                             GCacheDestroyFunc value_destroy_func,
                                                             GCacheDupFunc key_dup_func,
                                                             GCacheDestroyFunc key_destroy_func,
                                                             GHashFunc hash_key_func,
                                                             GHashFunc hash_value_func,
                                                             GEqualFunc key_equal_func);
    gpointer            g_cache_insert                      (GCache *cache,
                                                             gpointer key);
    void                g_cache_remove                      (GCache *cache,
                                                             gconstpointer value);
    void                g_cache_destroy                     (GCache *cache);

    void                g_cache_key_foreach                 (GCache *cache,
                                                             GHFunc func,
                                                             gpointer user_data);
    void                g_cache_value_foreach               (GCache *cache,
                                                             GHFunc func,
                                                             gpointer user_data);

    void                (*GCacheDestroyFunc)                (gpointer value);
    gpointer            (*GCacheDupFunc)                    (gpointer value);
    gpointer            (*GCacheNewFunc)                    (gpointer key);

## Description

A GCache allows sharing of complex data structures, in order to save system resources.

GTK+ uses caches for GtkStyles and GdkGCs. These consume a lot of resources, so a GCache is used to see if a GtkStyle or GdkGC with the required properties already exists. If it does, then the existing object is used instead of creating a new one.

GCache uses keys and values. A GCache key describes the properties of a particular resource. A GCache value is the actual resource.

## Details

### GCache

    typedef struct _GCache GCache;

The GCache struct is an opaque data structure containing information about a GCache. It should only be accessed via the following functions.

### g_cache_new ()

    GCache*             g_cache_new                         (GCacheNewFunc value_new_func,
                                                             GCacheDestroyFunc value_destroy_func,
                                                             GCacheDupFunc key_dup_func,
                                                             GCacheDestroyFunc key_destroy_func,
                                                             GHashFunc hash_key_func,
                                                             GHashFunc hash_value_func,
                                                             GEqualFunc key_equal_func);

Creates a new GCache.

`value_new_func` : | a function to create a new object given a key. This is called by `g_cache_insert()` if an object with the given key does not already exist.
`value_destroy_func` : | a function to destroy an object. It is called by `g_cache_remove()` when the object is no longer needed (i.e. its reference count drops to 0).
`key_dup_func` : | a function to copy a key. It is called by `g_cache_insert()` if the key does not already exist in the GCache.
`key_destroy_func` : | a function to destroy a key. It is called by `g_cache_remove()` when the object is no longer needed (i.e. its reference count drops to 0).
`hash_key_func` : | a function to create a hash value from a key.
`hash_value_func` : | a function to create a hash value from a value.
`key_equal_func` : | a function to compare two keys. It should return `TRUE` if the two keys are equivalent.
Returns : | a new GCache.

### g_cache_insert ()

    gpointer            g_cache_insert                      (GCache *cache,
                                                             gpointer key);

Gets the value corresponding to the given key, creating it if necessary. It first checks if the value already exists in the GCache, by using the `key_equal_func` function passed to `g_cache_new()`. If it does already exist it is returned, and its reference count is increased by one. If the value does not currently exist, if is created by calling the `value_new_func`. The key is duplicated by calling `key_dup_func` and the duplicated key and value are inserted into the GCache.

`cache` : | a GCache.
`key` : | a key describing a GCache object.
Returns : | a pointer to a GCache value.

### g_cache_remove ()

    void                g_cache_remove                      (GCache *cache,
                                                             gconstpointer value);

Decreases the reference count of the given value. If it drops to 0 then the value and its corresponding key are destroyed, using the `value_destroy_func` and `key_destroy_func` passed to `g_cache_new()`.

`cache` : | a GCache.
`value` : | the value to remove.

### g_cache_destroy ()

    void                g_cache_destroy                     (GCache *cache);

Frees the memory allocated for the GCache.

Note that it does not destroy the keys and values which were contained in the GCache.

`cache` : | a GCache.

### g_cache_key_foreach ()

    void                g_cache_key_foreach                 (GCache *cache,
                                                             GHFunc func,
                                                             gpointer user_data);

Calls the given function for each of the keys in the GCache.

NOTE `func` is passed three parameters, the value and key of a cache entry and the `user_data`. The order of value and key is different from the order in which `g_hash_table_foreach()` passes key-value pairs to its callback function !

`cache` : | a GCache.
`func` : | the function to call with each GCache key.
`user_data` : | user data to pass to the function.

### g_cache_value_foreach ()

    void                g_cache_value_foreach               (GCache *cache,
                                                             GHFunc func,
                                                             gpointer user_data);

### Warning

`g_cache_value_foreach` has been deprecated since version 2.10 and should not be used in newly-written code. The reason is that it passes pointers to internal data structures to `func`; use `g_cache_key_foreach()` instead

Calls the given function for each of the values in the GCache.

`cache` : | a GCache.
`func` : | the function to call with each GCache value.
`user_data` : | user data to pass to the function.

### GCacheDestroyFunc ()

    void                (*GCacheDestroyFunc)                (gpointer value);

Specifies the type of the `value_destroy_func` and `key_destroy_func` functions passed to `g_cache_new()`. The functions are passed a pointer to the GCache key or GCache value and should free any memory and other resources associated with it.

`value` : | the GCache value to destroy.

### GCacheDupFunc ()

    gpointer            (*GCacheDupFunc)                    (gpointer value);

Specifies the type of the `key_dup_func` function passed to `g_cache_new()`. The function is passed a key (not a value as the prototype implies) and should return a duplicate of the key.

`value` : | the GCache key to destroy (not a GCache value as it seems).
Returns : | a copy of the GCache key.

### GCacheNewFunc ()

    gpointer            (*GCacheNewFunc)                    (gpointer key);

Specifies the type of the `value_new_func` function passed to `g_cache_new()`. It is passed a GCache key and should create the value corresponding to the key.

`key` : | a GCache key.
Returns : | a new GCache value corresponding to the key.