==和===的区别是什么?
松==比较究竟是如何工作的? 严格的===比较到底是如何工作的?
有什么有用的例子吗?
当前回答
<?php
/**
* Comparison of two PHP objects == ===
* Checks for
* 1. References yes yes
* 2. Instances with matching attributes and its values yes no
* 3. Instances with different attributes yes no
**/
// There is no need to worry about comparing visibility of property or
// method, because it will be the same whenever an object instance is
// created, however visibility of an object can be modified during run
// time using ReflectionClass()
// http://php.net/manual/en/reflectionproperty.setaccessible.php
//
class Foo
{
public $foobar = 1;
public function createNewProperty($name, $value)
{
$this->{$name} = $value;
}
}
class Bar
{
}
// 1. Object handles or references
// Is an object a reference to itself or a clone or totally a different object?
//
// == true Name of two objects are same, for example, Foo() and Foo()
// == false Name of two objects are different, for example, Foo() and Bar()
// === true ID of two objects are same, for example, 1 and 1
// === false ID of two objects are different, for example, 1 and 2
echo "1. Object handles or references (both == and ===) <br />";
$bar = new Foo(); // New object Foo() created
$bar2 = new Foo(); // New object Foo() created
$baz = clone $bar; // Object Foo() cloned
$qux = $bar; // Object Foo() referenced
$norf = new Bar(); // New object Bar() created
echo "bar";
var_dump($bar);
echo "baz";
var_dump($baz);
echo "qux";
var_dump($qux);
echo "bar2";
var_dump($bar2);
echo "norf";
var_dump($norf);
// Clone: == true and === false
echo '$bar == $bar2';
var_dump($bar == $bar2); // true
echo '$bar === $bar2';
var_dump($bar === $bar2); // false
echo '$bar == $baz';
var_dump($bar == $baz); // true
echo '$bar === $baz';
var_dump($bar === $baz); // false
// Object reference: == true and === true
echo '$bar == $qux';
var_dump($bar == $qux); // true
echo '$bar === $qux';
var_dump($bar === $qux); // true
// Two different objects: == false and === false
echo '$bar == $norf';
var_dump($bar == $norf); // false
echo '$bar === $norf';
var_dump($bar === $norf); // false
// 2. Instances with matching attributes and its values (only ==).
// What happens when objects (even in cloned object) have same
// attributes but varying values?
// $foobar value is different
echo "2. Instances with matching attributes and its values (only ==) <br />";
$baz->foobar = 2;
echo '$foobar' . " value is different <br />";
echo '$bar->foobar = ' . $bar->foobar . "<br />";
echo '$baz->foobar = ' . $baz->foobar . "<br />";
echo '$bar == $baz';
var_dump($bar == $baz); // false
// $foobar's value is the same again
$baz->foobar = 1;
echo '$foobar' . " value is the same again <br />";
echo '$bar->foobar is ' . $bar->foobar . "<br />";
echo '$baz->foobar is ' . $baz->foobar . "<br />";
echo '$bar == $baz';
var_dump($bar == $baz); // true
// Changing values of properties in $qux object will change the property
// value of $bar and evaluates true always, because $qux = &$bar.
$qux->foobar = 2;
echo '$foobar value of both $qux and $bar is 2, because $qux = &$bar' . "<br />";
echo '$qux->foobar is ' . $qux->foobar . "<br />";
echo '$bar->foobar is ' . $bar->foobar . "<br />";
echo '$bar == $qux';
var_dump($bar == $qux); // true
// 3. Instances with different attributes (only ==)
// What happens when objects have different attributes even though
// one of the attributes has same value?
echo "3. Instances with different attributes (only ==) <br />";
// Dynamically create a property with the name in $name and value
// in $value for baz object
$name = 'newproperty';
$value = null;
$baz->createNewProperty($name, $value);
echo '$baz->newproperty is ' . $baz->{$name};
var_dump($baz);
$baz->foobar = 2;
echo '$foobar' . " value is same again <br />";
echo '$bar->foobar is ' . $bar->foobar . "<br />";
echo '$baz->foobar is ' . $baz->foobar . "<br />";
echo '$bar == $baz';
var_dump($bar == $baz); // false
var_dump($bar);
var_dump($baz);
?>
其他回答
在PHP数组和对象中，==和===之间有两个没有人提到的区别:两个具有不同键排序的数组和对象。
两个具有不同键排序的数组
如果你有两个数组，它们的键排序不同，但有相同的键值映射，它们是严格不同的(即使用===)。这可能会导致问题，如果你对一个数组进行键排序，并试图将排序后的数组与原始数组进行比较。
例如:
$arrayUnsorted = [
"you" => "you",
"I" => "we",
];
$arraySorted = $arrayUnsorted;
ksort($arraySorted);
$arrayUnsorted == $arraySorted; // true
$arrayUnsorted === $arraySorted; // false
对象
请记住，主要规则是两个不同的对象永远不会严格相等。请看下面的例子:
$stdClass1 = new stdClass();
$stdClass2 = new stdClass();
$clonedStdClass1 = clone $stdClass1;
$stdClass1 == $stdClass2; // true
$stdClass1 === $stdClass2; // false
$stdClass1 == $clonedStdClass1; // true
$stdClass1 === $clonedStdClass1; // false
注意:将一个对象赋值给另一个变量并不会创建一个副本——相反，它创建了对同一对象的引用。在这里看到的。
注意:从PHP7开始，引入了匿名类。在上面的测试中，新类{}和新stdClass()之间没有区别。
运算符==在两个不同类型之间进行类型强制转换，而===运算符执行“类型安全比较”。这意味着只有当两个操作数具有相同的类型和相同的值时，它才返回true。
例子:
1 === 1: true
1 == 1: true
1 === "1": false // 1 is an integer, "1" is a string
1 == "1": true // "1" gets casted to an integer, which is 1
"foo" === "foo": true // both operands are strings and have the same value警告:具有等价成员的同一个类的两个实例不匹配===操作符。例子:
$a = new stdClass();
$a->foo = "bar";
$b = clone $a;
var_dump($a === $b); // bool(false)
您可以使用===来测试函数或变量是否为假，而不仅仅是等于假(零或空字符串)。
$needle = 'a';
$haystack = 'abc';
$pos = strpos($haystack, $needle);
if ($pos === false) {
echo $needle . ' was not found in ' . $haystack;
} else {
echo $needle . ' was found in ' . $haystack . ' at location ' . $pos;
}
在这种情况下，strpos将返回0，这在测试中等同于false
if ($pos == false)
or
if (!$pos)
这不是你想要的。
<?php
/**
* Comparison of two PHP objects == ===
* Checks for
* 1. References yes yes
* 2. Instances with matching attributes and its values yes no
* 3. Instances with different attributes yes no
**/
// There is no need to worry about comparing visibility of property or
// method, because it will be the same whenever an object instance is
// created, however visibility of an object can be modified during run
// time using ReflectionClass()
// http://php.net/manual/en/reflectionproperty.setaccessible.php
//
class Foo
{
public $foobar = 1;
public function createNewProperty($name, $value)
{
$this->{$name} = $value;
}
}
class Bar
{
}
// 1. Object handles or references
// Is an object a reference to itself or a clone or totally a different object?
//
// == true Name of two objects are same, for example, Foo() and Foo()
// == false Name of two objects are different, for example, Foo() and Bar()
// === true ID of two objects are same, for example, 1 and 1
// === false ID of two objects are different, for example, 1 and 2
echo "1. Object handles or references (both == and ===) <br />";
$bar = new Foo(); // New object Foo() created
$bar2 = new Foo(); // New object Foo() created
$baz = clone $bar; // Object Foo() cloned
$qux = $bar; // Object Foo() referenced
$norf = new Bar(); // New object Bar() created
echo "bar";
var_dump($bar);
echo "baz";
var_dump($baz);
echo "qux";
var_dump($qux);
echo "bar2";
var_dump($bar2);
echo "norf";
var_dump($norf);
// Clone: == true and === false
echo '$bar == $bar2';
var_dump($bar == $bar2); // true
echo '$bar === $bar2';
var_dump($bar === $bar2); // false
echo '$bar == $baz';
var_dump($bar == $baz); // true
echo '$bar === $baz';
var_dump($bar === $baz); // false
// Object reference: == true and === true
echo '$bar == $qux';
var_dump($bar == $qux); // true
echo '$bar === $qux';
var_dump($bar === $qux); // true
// Two different objects: == false and === false
echo '$bar == $norf';
var_dump($bar == $norf); // false
echo '$bar === $norf';
var_dump($bar === $norf); // false
// 2. Instances with matching attributes and its values (only ==).
// What happens when objects (even in cloned object) have same
// attributes but varying values?
// $foobar value is different
echo "2. Instances with matching attributes and its values (only ==) <br />";
$baz->foobar = 2;
echo '$foobar' . " value is different <br />";
echo '$bar->foobar = ' . $bar->foobar . "<br />";
echo '$baz->foobar = ' . $baz->foobar . "<br />";
echo '$bar == $baz';
var_dump($bar == $baz); // false
// $foobar's value is the same again
$baz->foobar = 1;
echo '$foobar' . " value is the same again <br />";
echo '$bar->foobar is ' . $bar->foobar . "<br />";
echo '$baz->foobar is ' . $baz->foobar . "<br />";
echo '$bar == $baz';
var_dump($bar == $baz); // true
// Changing values of properties in $qux object will change the property
// value of $bar and evaluates true always, because $qux = &$bar.
$qux->foobar = 2;
echo '$foobar value of both $qux and $bar is 2, because $qux = &$bar' . "<br />";
echo '$qux->foobar is ' . $qux->foobar . "<br />";
echo '$bar->foobar is ' . $bar->foobar . "<br />";
echo '$bar == $qux';
var_dump($bar == $qux); // true
// 3. Instances with different attributes (only ==)
// What happens when objects have different attributes even though
// one of the attributes has same value?
echo "3. Instances with different attributes (only ==) <br />";
// Dynamically create a property with the name in $name and value
// in $value for baz object
$name = 'newproperty';
$value = null;
$baz->createNewProperty($name, $value);
echo '$baz->newproperty is ' . $baz->{$name};
var_dump($baz);
$baz->foobar = 2;
echo '$foobar' . " value is same again <br />";
echo '$bar->foobar is ' . $bar->foobar . "<br />";
echo '$baz->foobar is ' . $baz->foobar . "<br />";
echo '$bar == $baz';
var_dump($bar == $baz); // false
var_dump($bar);
var_dump($baz);
?>
关于对象比较的其他答案的补充:
==使用对象的名称和它们的值来比较对象。如果两个对象具有相同的类型并且具有相同的成员值，$a == $b的结果为true。
===比较对象的内部对象id。即使成员是相等的，如果它们不是完全相同的对象，$a !== $b。
class TestClassA {
public $a;
}
class TestClassB {
public $a;
}
$a1 = new TestClassA();
$a2 = new TestClassA();
$b = new TestClassB();
$a1->a = 10;
$a2->a = 10;
$b->a = 10;
$a1 == $a1;
$a1 == $a2; // Same members
$a1 != $b; // Different classes
$a1 === $a1;
$a1 !== $a2; // Not the same object
推荐文章
- 原则-如何打印出真正的sql，而不仅仅是准备好的语句?
- 如何从关联PHP数组中获得第一项?
- PHP/MySQL插入一行然后获取id
- 我如何排序一个多维数组在PHP
- 如何在PHP中截断字符串最接近于一定数量的字符?
- PHP错误:“zip扩展名和unzip命令都没有，跳过。”
- Nginx提供下载。php文件，而不是执行它们
- Json_encode()转义正斜杠
- 如何在PHP中捕获cURL错误
- 如何要求一个分叉与作曲家?
- 如何在php中创建可选参数?
- 在文本文件中创建或写入/追加
- 为什么PHP的json_encode函数转换UTF-8字符串为十六进制实体?
- 如何从一个查询插入多行使用雄辩/流利
- 在PHP单元测试执行期间，如何在CLI中输出?
