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Filename : Design.Arrays.txt
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Dynamic Arrays
==============

'lib/array.h' and 'lib/array-decl.h' describes Dovecot's type-safe dynamic
arrays. Trying to add wrong typed elements gives a compiler warning.

Declaring
---------

Arrays can be declared in two ways:

 1. Directly: 'ARRAY_DEFINE(array_name, array_type);'. For example:
    'ARRAY_DEFINE(numbers, int);' or 'ARRAY_DEFINE(foos, struct foo);'
 2. Via predefined type: 'ARRAY_DEFINE_TYPE(foo, struct foo); ...
    ARRAY_TYPE(foo) foos;'

The main reason to define a type for an array is to be able to pass the array
as a function parameter, like:

---%<-------------------------------------------------------------------------
void func(ARRAY_TYPE(foo) *foos) { .. }
---%<-------------------------------------------------------------------------

Trying to do the same with 'ARRAY_DEFINE()' will generate a compiler warning.
'lib/array-decl.h' defines several commonly used types.

Initializing
------------

Arrays are typically initialized by calling 'i_array_init()', 'p_array_init()'
or 't_array_init()' depending on where you want to allocate the memory from.
Arrays are internally handled as <buffers> [Design.Buffers.txt], so the initial
size is just multiplied by element size and passed to
'buffer_create_dynamic()'.

Example:

---%<-------------------------------------------------------------------------
ARRAY_DEFINE(foo, struct foo *);

i_array_init(&foo, 32); /* initialize array with 32 elements until it needs to
be grown */
---%<-------------------------------------------------------------------------

Arrays can be freed with 'array_free()', but this isn't necessary if the memory
gets freed by other means (i.e. it was allocated from alloconly-pool or data
stack).

Writing
-------

 * 'array_append(array, data, count)' is the most common way to add data to
   arrays
 * 'array_append_array(dest, src)'
 * 'array_insert(array, idx, data, count)'
 * 'array_delete(array, idx, count)'
 * 'array_idx_set(array, idx, data)' replaces (or adds) data to given index
 * 'array_idx_clear(array, idx)' clears given index by writing NULs to it
 * 'array_append_space(array, count)'

Reading
-------

'array_idx(array, idx)' returns pointer to given index in array. The index must
already exist, otherwise the call assert-crashes. This call adds extra overhead
for accessing arrays though, so usually it's better to just get list of all
elements and access them directly:

---%<-------------------------------------------------------------------------
data = array_get(&array, &count);
---%<-------------------------------------------------------------------------

You can also iterate through the whole array easily:

---%<-------------------------------------------------------------------------
const char *str;

array_foreach(&string_array, str) {
  /* str changes in each iteration */
}
---%<-------------------------------------------------------------------------

There's also 'array_foreach_modifiable()' to get the data without const.

Unsafe Read/Write
-----------------

Functions below have similar problems to [[Design/Buffer|buffer]'s '*_unsafe()'
functions. Memory returned by them must not be accessed after calls to other
'array_*()' modifying functions, because they may reallocate the array
elsewhere in memory.

 * 'array_append_space(array)'
 * 'array_insert_space(array, idx)'
 * 'array_get_modifiable(array, &count)'
 * 'array_idx_modifiable(array, idx)'

Others
------

 * 'array_cmp(array1, array2)' compares two arrays
 * 'array_reverse(array)' reverses all elements in an array
 * 'array_sort(array, cmp_func)' is a wrapper for 'qsort()' adding also type
   safety. The parameters in cmp_func should be the same type as the array,
   instead of 'const void *'.
 * 'array_bsearch(array, key, cmp_func)' is a wrapper for 'bsearch()' also
   adding type safety, just like 'array_sort()'.

(This file was created from the wiki on 2019-06-19 12:42)