问题
When trying to change it,throw an exception.
回答1:
I suppose a solution, for class properties, would be to :
- not define a property with the name that interests you
- use the magic
__get
method to access that property, using the "fake" name - define the
__set
method so it throws an exception when trying to set that property. - See Overloading, for more informations on magic methods.
For variables, I don't think it's possible to have a read-only variable for which PHP will throw an exception when you're trying to write to it.
For instance, consider this little class :
class MyClass {
protected $_data = array(
'myVar' => 'test'
);
public function __get($name) {
if (isset($this->_data[$name])) {
return $this->_data[$name];
} else {
// non-existant property
// => up to you to decide what to do
}
}
public function __set($name, $value) {
if ($name === 'myVar') {
throw new Exception("not allowed : $name");
} else {
// => up to you to decide what to do
}
}
}
Instanciating the class and trying to read the property :
$a = new MyClass();
echo $a->myVar . '<br />';
Will get you the expected output :
test
While trying to write to the property :
$a->myVar = 10;
Will get you an Exception :
Exception: not allowed : myVar in /.../temp.php on line 19
回答2:
class test {
const CANT_CHANGE_ME = 1;
}
and you refer it as test::CANT_CHANGE_ME
回答3:
Use a constant. Keyword const
回答4:
I know this is an old question, but PASCAL's answer really helped me and I wanted to add to it a bit.
__get() fires not only on nonexistent properties, but "inaccessible" ones as well, e.g. protected ones. This makes it easy to make read-only properties!
class MyClass {
protected $this;
protected $that;
protected $theOther;
public function __get( $name ) {
if ( isset( $this->$name ) ) {
return $this->$name;
} else {
throw new Exception( "Call to nonexistent '$name' property of MyClass class" );
return false;
}
}
public function __set( $name ) {
if ( isset( $this->$name ) ) {
throw new Exception( "Tried to set nonexistent '$name' property of MyClass class" );
return false;
} else {
throw new Exception( "Tried to set read-only '$name' property of MyClass class" );
return false;
}
}
}
回答5:
The short answer is you can't create a read-only object member variable in PHP.
In fact, most object-oriented languages consider it poor form to expose member variables publicly anyway... (C# being the big, ugly exception with its property-constructs).
If you want a class variable, use the const
keyword:
class MyClass {
public const myVariable = 'x';
}
This variable can be accessed:
echo MyClass::myVariable;
This variable will exist in exactly one version regardless of how many different objects of type MyClass
you create, and in most object-oriented scenarios it has little to no use.
If, however, you want a read-only variable that can have different values per object, you should use a private member variable and an accessor method (a k a getter):
class MyClass {
private $myVariable;
public function getMyVariable() {
return $this->myVariable;
}
public function __construct($myVar) {
$this->myVariable = $myVar;
}
}
The variable is set in the constructor, and it's being made read-only by not having a setter. But each instance of MyClass
can have its own value for myVariable
.
$a = new MyClass(1);
$b = new MyClass(2);
echo $a->getMyVariable(); // 1
echo $b->getMyVariable(); // 2
$a->setMyVariable(3); // causes an error - the method doesn't exist
$a->myVariable = 3; // also error - the variable is private
来源:https://stackoverflow.com/questions/2343790/how-to-implement-a-read-only-member-variable-in-php