class A{
static $var = true;
function f(){
}
}
vs
class A{
function f(){
static $var = true;
}
}
There doesn't seem to be any difference. Are there any advantages of using one over the other?
Note that $var would be used in the f() function only. I understand that declaring it in the class header is required if the variable needs to be used in multiple functions
If you only use the static variable in the f function, there's only a scope difference, that means no difference as long as you don't try to use it elsewhere.
When used in local scope, the static variable value is kept between each function call. See Static variables part of this page.
Thanks insertusernamehere for pointing that out.
In the later example you may only use the var inside the f function. The other is accessible anywhere from inside the class A::var or outside A::var
Related
I'm having a method inside a class, in which I want to initialize a static variable.
1/ when I initialize the variable, then affect it to some other value using the $this keyword, it works. E.g.:
class Test {
// ...
function test($input_variable)
{
static $my_static_variable = null;
if (!isset($my_static_variable))
$my_static_variable = $this->someFunction($input_variable);
// ... some further processing
}
}
2/ However, when I try to directly initialize / construct the variable with the $this keyword, there is a syntax error : unexpected '$this' (T_VARIABLE):
class Test {
// ...
function test($input_variable)
{
static $my_static_variable = $this->someFunction($input_variable); // *** syntax error, unexpected '$this' (T_VARIABLE)
// ... some further processing
}
}
Is 1/ a good way to initialize a static variable?
Why is 2/ not allowed, as it is supposed to do the exact same thing than in 1/?
I'm using PHP 5.5.21 (cli) (built: Jul 22 2016 08:31:09).
Thanks
You can't use $this on a static variable. You can use self with scope resolution operator (::) instead.
Here is the example:
class Foo {
public static $my_stat;
static function init() {
self::$my_stat= 'My val';
}
}
Static variable and function can be accessed without instantiation of the class. You can not use $this to access the variable or function declared as static.
You have to use scope resolution operator:: to access variable and functions declared as static.
For variables:-
class A
{
static $my_static = 'foo';
public function staticValue() {
return self::$my_static;// Try to use $this here insted of self:: you will get error
}
}
class B extends A
{
public function fooStatic() {
return parent::$my_static;
}
}
Use to the following to access the variable:-
print A::$my_static
For functions:-
class A {
public static function aStaticMethod() {
// ...
}
}
You can call the function as follows:-
A::aStaticMethod();
I think I have the answer. In the php documentation, it is stated the following:
Static variables may be declared as seen in the examples above. From
PHP 5.6 you can assign values to these variables which are the result
of expressions, but you can't use any function here, what will cause a
parse error.
So I guess this also holds for PHP 5.5.
As #MagnusEriksson pointed out, I could have used a class property as well. However, I don't want my variable to be accessed somewhere else apart from the test() method.
BTW, somehow the same thing is stated for static properties in the doc:
Static properties cannot be accessed through the object using the
arrow operator ->.
Like any other PHP static variable, static properties may only be
initialized using a literal or constant before PHP 5.6; expressions
are not allowed. In PHP 5.6 and later, the same rules apply as const
expressions: some limited expressions are possible, provided they can
be evaluated at compile time.
I've been looking at some code and am having a hard time working out variable declaration in php classes. Specifically it appears that the code i'm looking at doesn't declare the class variables before it uses them. Now this may be expected but I can't find any info that states that it is possible. So would you expect this:
class Example
{
public function __construct()
{
$this->data = array();
$this->var = 'something';
}
}
to work? and does this create these variables on the class instance to be used hereafter?
This works the same as a normal variable declaration would work:
$foo = 'bar'; // Created a new variable
class Foo {
function __construct() {
$this->foo = 'bar'; // Created a new variable
}
}
PHP classes are not quite the same as in other languages, where member variables need to be specified as part of the class declaration. PHP class members can be created at any time.
Having said that, you should declare the variable like public $foo = null; in the class declaration, if it's supposed to be a permanent member of the class, to clearly express the intent.
So would you expect this: (code sample) to work?
Yes. It's pretty bad practice (at least it makes my C++ skin crawl), but it wouldn't surprise me in the slightest. See example 2 in the following page for an example of using another class without declaring it beforehand. http://www.php.net/manual/en/language.oop5.basic.php It will throw an error if E_STRICT is enabled.
And does this create these variables on the class instance to be used hereafter?
Yep. Ain't PHP Fun? Coming from a C++/C# background, PHP took a while to grow on me with its very loose typing, but it has its advantages.
That's completely functional, though opinions will differ. Since the creation of the class member variables are in the constructor, they will exist in every instance of the object unless deleted.
It's conventional to declare class member variables with informative comments:
class Example
{
private $data; // array of example data
private $var; // main state variable
public function __construct()
{
$this->data = array();
$this->var = 'something';
}
}
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Access global variable from within a class
I want to fortify my knowledge in PHP. But most of the time I have trouble understanding with scopes. I am not sure when to use $this and global declaration keyword on my functions.
This is just an example class I just omitted the __construct()
class myClass{
public myVariable;
public function1() {
$this->myVariable=1;
}
public function2(){
global $myVariable;
$myVariable=1;
}
}
Which one is the best approach to use when assigning pre-declared variables inside a function? I am confused somehow by the different books by major publishers in PHP. I am not sure if I am asking the correct question or somehow relevant.
First off, thats not valid PHP, as Jared Farrish already said. Its public $myvar instead of public myvar. Variable names always begin with $.
When you declare a variable in a class:
<?php
class A
{
private $var;
}
That variable will be automatically available in all methods if accessed through $this (unless the method static, but that is another story). So this would work:
<?php
class A
{
private $var;
public function foo () {
// This works
$this->var = 1;
// This is a completely different thing and does not change
// the contents of A::$var
$var = 1;
}
}
Now global is a different thing altogether. In PHP, you cant do this:
<?php
$global_a = 123;
function myfunc ()
{
// You wont actually change the value of $global_a with this
$global_a = 1234;
}
echo $global_a; // still 123
You'd think this would work, but global variables aren't automatically available to functions. This is where global comes in:
<?php
$global_a = 123;
function myfunc ()
{
global $global_a;
$global_a = 1234;
}
echo $global_a; // now it will be 1234
I suggest you read about variable scope in PHP and then you can go on to OOP in PHP.
PHP is a very quirky language. Just because something works in most languages in a certain way doesn't mean it will work in PHP. Most of the times, it wont. So its best to educate yourself as much as possible.
Hope this helps
Which one is best often depends on the situation. In general it's best to avoid global variables as they bring risks with them of not knowing exactly where they're used.
Using a class variable is a good option if the variable is specific to that instance of the class. (You will have to prefix it with a $, though. I.e., public $myVariable;).
If the variable is only relevant to the function and instances outside the class, you'd do best to pass it as an reference. This is done by adding an & before the $ in the parameter space. I.e.:
public function3(&$myVariable) {
$myVariable = 1;
}
Or alternatively just return the value you wish and set it outside the function. I.e.,:
class MyClass {
public function3() {
return 1;
}
}
$myObject = new MyClass();
$myVariable = $myObject->function4();
try to avoid passing global variable,as there are many good reasons for that:
Reusing parts of the script is impossible :
If a certain function relies on global variables, it becomes almost impossible to use that function in a different context. Another problem is that you can’t take that function, and use it in another script.
Solving bugs is much harder :
Tracking a global variable is much harder than a non-global variable.
Understanding the code in a year will be much more difficult :
Globals make it difficult to see where a variable is coming from and what it does.
Now your questions:
Which one is the best approach to use when assigning pre-declared variables inside a function?
You can pass them by value or reference inside a function
By value
$firstVar = 1;
function abc($firstVar){
echo $firstVar ; // will give you 1
}
By reference
function abc(&$firstVar){
$firstVar = 3; // this will give utility to change that variable even
echo $firstVar ; // will give you 3
}
Now where to use $this variable:
$this is used to refer to the current object.
example:
class abc{
private $firstVar;
function abc(){
$this->firstVar = 2;
}
}
I noticed two ways in PHP to do the same thing. Can you tell me which way is a better programming practice?
In the first example, I use a private variable on the class. On the second example, I use a static variable in a class method.
class Test {
private $_myvar;
public function getVar(){
if (!isset($this->_myvar)) {
$this->_myvar = "test\n";
}
return $this->_myvar;
}
}
$oTest = new Test();
echo $oTest->getVar(); // sets var first time and returns it
echo $oTest->getvar(); // pulls from cache
Or:
class Test {
public function getVar(){
static $myvar;
if (!isset($myvar)) {
$myvar = "test\n";
}
return $myvar;
}
}
$oTest = new Test();
echo $oTest->getVar(); // sets var first time and returns it
echo $oTest->getvar(); // pulls from cache
That is like saying which room is better the Kitchen or the Bathroom, they are both rooms, but they have different functions.
A static variable is the same in multiple objects.
An instance variable, declared via private above is particular to a given object.
Note that private is an access modifier, static is not, a variable can be both.
In the location you have your static variable, within the function, it is not a class/object variable at all, but a traditional function-level static variable, which will be single-instanced across all calls to the function, making it similar to a class-level static variable, but only accessible within the method it is defined within.
With the class property (be it public, private or protected), the variable is accessible to other parts of the class.
With the static variable, it is only visible to that method of the class.
I would suggest using the class property (but probably not private, which I generally don't find much use for; protected is normally a better idea) as it's easier for testing later; you can't do anything to unset, alter or check the static variable.
I see some possible confusion in the other answers here between static variables and static class properties. PHP uses the same modifier, but the behaviour is quite different; an example follows.
<?php
class Foo {
// Static class property
public static $bar = 'bar';
public function blarg() {
static $bar;
if (empty($bar)) {
$bar = 'blarg';
}
return $bar;
}
}
In the above example the static class property can be accessed using Foo::$bar (or self::$bar within the class and static::$bar in PHP 5.3); the static variable cannot and is only visible inside the function blarg().
Neither is "better". It would be like asking whether a screwdriver is better than a tenon saw.
The private variable in your first example will only be available to that instance of the class (although only to its methods). Create a new instance, and the variable can and will have a different value.
The static variable in your second example will be available to all instances of that class, admittedly only in that method. But set it one instance, and a second instance will see the same value.
I've been looking at some code and am having a hard time working out variable declaration in php classes. Specifically it appears that the code i'm looking at doesn't declare the class variables before it uses them. Now this may be expected but I can't find any info that states that it is possible. So would you expect this:
class Example
{
public function __construct()
{
$this->data = array();
$this->var = 'something';
}
}
to work? and does this create these variables on the class instance to be used hereafter?
This works the same as a normal variable declaration would work:
$foo = 'bar'; // Created a new variable
class Foo {
function __construct() {
$this->foo = 'bar'; // Created a new variable
}
}
PHP classes are not quite the same as in other languages, where member variables need to be specified as part of the class declaration. PHP class members can be created at any time.
Having said that, you should declare the variable like public $foo = null; in the class declaration, if it's supposed to be a permanent member of the class, to clearly express the intent.
So would you expect this: (code sample) to work?
Yes. It's pretty bad practice (at least it makes my C++ skin crawl), but it wouldn't surprise me in the slightest. See example 2 in the following page for an example of using another class without declaring it beforehand. http://www.php.net/manual/en/language.oop5.basic.php It will throw an error if E_STRICT is enabled.
And does this create these variables on the class instance to be used hereafter?
Yep. Ain't PHP Fun? Coming from a C++/C# background, PHP took a while to grow on me with its very loose typing, but it has its advantages.
That's completely functional, though opinions will differ. Since the creation of the class member variables are in the constructor, they will exist in every instance of the object unless deleted.
It's conventional to declare class member variables with informative comments:
class Example
{
private $data; // array of example data
private $var; // main state variable
public function __construct()
{
$this->data = array();
$this->var = 'something';
}
}