Countable 接口

(PHP 5 >= 5.1.0, PHP 7, PHP 8)

简介

类实现 Countable 可被用于 count() 函数.

接口摘要

interface Countable {

/* 方法 */

public count(): int

}

I just want to point out that your class has to actually implement the Countable interface, not just define a count method, to be able to use count($object) and get the expected results. I.e. the first example below won't work as expected, the second will. (The normal arrow function accessor ($object->count()) will work fine, but that's not the kewl part :) )



<?php

//Example One, BAD :(



class CountMe

{



    protected $_myCount = 3;



    public function count()

    {

        return $this->_myCount;

    }



}



$countable = new CountMe();

echo count($countable); //result is "1", not as expected



//Example Two, GOOD :)



class CountMe implements Countable

{



    protected $_myCount = 3;



    public function count()

    {

        return $this->_myCount;

    }



}



$countable = new CountMe();

echo count($countable); //result is "3" as expected

?>

down

adam at adamhahn dot com ¶

7 years ago

When using GMP/BC/Floating-Point Numbers to work with integers larger than PHP_INT_MAX, be aware that using the count() function will typecast the returned value to an integer.

<?php
class counter implements Countable {
    public function count() {
        // Number of IPv6 addresses in a single /32 IPv6 allocation (2^96)
        return "18446744073709551616"; // assume generated/exported by big-int library(GMP/BC/etc.)
    }
}

$obj = new counter();

echo $obj->count(); // prints string "18446744073709551616"
echo count($obj);    // prints int PHP_INT_MAX

// This is because of the typecasting
echo (int) "18446744073709551616"; // prints int PHP_INT_MAX
?>

This will also cause problems for floating-point values.

<?php
class counter implements Countable {
    public function count() {
        // Number of IPv6 addresses in a single /32 IPv6 allocation (2^96)
        return 18446744073709551616;
    }
}

$obj = new counter();

echo $obj->count(); // prints float 18446744073709551616.000000
echo count($obj);    // prints int 0

// This is because of the typecasting
echo (int) 18446744073709551616; // prints int 0
?>

This is only problematic when counting higher than PHP_INT_MAX.

down

Anonymous ¶

13 years ago

Note that arrays don't implement countable. Therefore you can't force a countable parameter for a function if you want it also to work with native arrays.

Countable 接口

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