The SplFixedArray class

(PHP 5 >= 5.3.0, PHP 7)

简介

The SplFixedArray class provides the main functionalities of array. The main differences between a SplFixedArray and a normal PHP array is that the SplFixedArray is of fixed length and allows only integers within the range as indexes. The advantage is that it uses less memory than a standard array.

类摘要

SplFixedArray implements Iterator , ArrayAccess , Countable {
/* 方法 */
public __construct ([ int $size = 0 ] )
public count ( void ) : int
public current ( void ) : mixed
public static fromArray ( array $array [, bool $save_indexes = TRUE ] ) : SplFixedArray
public getSize ( void ) : int
public key ( void ) : int
public next ( void ) : void
public offsetExists ( int $index ) : bool
public offsetGet ( int $index ) : mixed
public offsetSet ( int $index , mixed $newval ) : void
public offsetUnset ( int $index ) : void
public rewind ( void ) : void
public setSize ( int $size ) : bool
public toArray ( void ) : array
public valid ( void ) : bool
public __wakeup ( void ) : void
}

范例

Example #1 SplFixedArray usage example

<?php
// Initialize the array with a fixed length
$array = new SplFixedArray(5);

$array[1] = 2;
$array[4] = "foo";

var_dump($array[0]); // NULL
var_dump($array[1]); // int(2)

var_dump($array["4"]); // string(3) "foo"

// Increase the size of the array to 10
$array->setSize(10);

$array[9] = "asdf";

// Shrink the array to a size of 2
$array->setSize(2);

// The following lines throw a RuntimeException: Index invalid or out of range
try {
    var_dump($array["non-numeric"]);
} catch(RuntimeException $re) {
    echo "RuntimeException: ".$re->getMessage()."\n";
}

try {
    var_dump($array[-1]);
} catch(RuntimeException $re) {
    echo "RuntimeException: ".$re->getMessage()."\n";
}

try {
    var_dump($array[5]);
} catch(RuntimeException $re) {
    echo "RuntimeException: ".$re->getMessage()."\n";
}
?>

以上例程会输出：

NULL
int(2)
string(3) "foo"
RuntimeException: Index invalid or out of range
RuntimeException: Index invalid or out of range
RuntimeException: Index invalid or out of range

Table of Contents

User Contributed Notes

herhor67 at interia dot pl 31-Dec-2019 06:23
Memory usage for arrays of 1132766 ints (data derived from some 1kx1k img):
Regular:  76453160B (67.5B/int)
SplFixed: 18898744B (16.7B/int)

In my application, SFA uses 75% less RAM, which is a life-saver.

Speed comparison:
Regular: 449ms
SplFixed (resized before every element): 791ms
SplFixed (fully preallocated): 392ms
SplFixed (preall-d to 1M and then resized): 547ms

Pros and cons:
+ much more efficient RAM-wise
+ a bit faster if max size is known
~ a bit slower if max size is only approximated
- much slower if max size is not known
- cannot be used with most array functions

To sum up:
SplFixedArray is a very good choice for storing giant amount of data, though only as long as you at least roughly know the size and can work without array functions.
Anonymous 08-Jan-2018 01:04
getSize() and count() return the same value
php at stij dot nospam dot npeete dot nospam dot rs dot nl 28-Dec-2017 06:55
Note that (as of PHP 7.1) SplFixedArray uses 16 bytes (!) per array item. This can be checked easily via:

<?php
ini_set('memory_limit', '4M');
$array = new SplFixedArray(1000000);
//PHP Fatal error:  Allowed memory size of 4194304 bytes exhausted (tried to allocate 16000000 bytes)
?>

So if you're considering SplFixedArray because you need to use large arrays with very large numbers of items, make sure you have enough memory to support them.
Anonymous 03-Nov-2017 12:04
PHP Arrays are very fast, and faster overall, especially in PHP7. You almost never need this. There are some cases where using this is better than using arrays but beware that they aren't that obvious.
artaxerxes 13-Nov-2012 04:41
As the documentation says, SplFixedArray is meant to be *faster* than array. Do not blindly believe other people's benchmarks, and beextra careful with the user comments on php.net. For instance, nairbv's benchmark code is completely wrong. Among other errors, it intends to increase the size of the arrays, but always initialize a 20 elements SplFixedArray.

On a PHP 5.4 64 bits linux server, I found SplFixedArray to be always faster than array().
* small data (1,000):
    * write: SplFixedArray is 15 % faster
    * read:  SplFixedArray is  5 % faster
* larger data (512,000):
    * write: SplFixedArray is 33 % faster
    * read:  SplFixedArray is 10 % faster
alex dot andrienko at gmail dot com 15-Nov-2010 02:13
Memory footprint of splFixedArray is about 37% of a regular "array" of the same size.
I was hoping for more, but that's also significant, and that's where you should expect to see difference, not in "performance".
chrisstocktonaz at gmail dot com 14-Jul-2009 11:07
Note, that this is considerably faster and should be used when the size of the array is known. Here are some very basic bench marks:

<?php
for($size = 1000; $size < 50000000; $size *= 2) {
    echo PHP_EOL . "Testing size: $size" . PHP_EOL;
    for($s = microtime(true), $container = Array(), $i = 0; $i < $size; $i++) $container[$i] = NULL;
    echo "Array(): " . (microtime(true) - $s) . PHP_EOL;

    for($s = microtime(true), $container = new SplFixedArray($size), $i = 0; $i < $size; $i++) $container[$i] = NULL;
    echo "SplArray(): " . (microtime(true) - $s) . PHP_EOL;
}
?>

OUTPUT
Testing size: 1000
Array(): 0.00046396255493164
SplArray(): 0.00023293495178223

Testing size: 2000
Array(): 0.00057101249694824
SplArray(): 0.0003058910369873

Testing size: 4000
Array(): 0.0015869140625
SplArray(): 0.00086307525634766

Testing size: 8000
Array(): 0.0024251937866211
SplArray(): 0.00211501121521

Testing size: 16000
Array(): 0.0057680606842041
SplArray(): 0.0041120052337646

Testing size: 32000
Array(): 0.011334896087646
SplArray(): 0.007631778717041

Testing size: 64000
Array(): 0.021990060806274
SplArray(): 0.013560056686401

Testing size: 128000
Array(): 0.053267002105713
SplArray(): 0.030976057052612

Testing size: 256000
Array(): 0.10280108451843
SplArray(): 0.056283950805664

Testing size: 512000
Array(): 0.20657992362976
SplArray(): 0.11510300636292

Testing size: 1024000
Array(): 0.4138810634613
SplArray(): 0.21826505661011

Testing size: 2048000
Array(): 0.85640096664429
SplArray(): 0.46247816085815

Testing size: 4096000
Array(): 1.7242450714111
SplArray(): 0.95304894447327

Testing size: 8192000
Array(): 3.448086977005
SplArray(): 1.96746301651