Would it be possible to write a class that is virtually indistinguishable from an actual PHP array by implementing all the necessary SPL interfaces? Are they missing anything that would be critical?
I'd like to build a more advanced Array object, but I want to make sure I wouldn't break an existing app that uses arrays everywhere if I substituted them with a custom Array class.
The only problems i can think of are the gettype() and the is_array() functions.
Check your code for
gettype($FakeArray) == 'array'
is_array($FakeArray)
Because although you can use the object just like an array, it will still be identified as an object.
In addition to the points made above, you would not be able to make user-space array type hints work with instances of your class. For example:
<?php
function f(array $a) { /*...*/ }
$ao = new ArrayObject();
f($ao); //error
?>
Output:
Catchable fatal error: Argument 1 passed to f() must be an array, object given
Other differences include the '+' operator for arrays (merging) and the failure of the entire array_* functions, including the commonly used array_merge and array_shift.
Related
I have this class:
class ResultSet
implements ArrayAccess, Countable, Iterator {
/// Rest of implementation ...
}
I'm running into a problem using usort() and passing my object as the first parameter. usort() expects an array instead of an object, but given my implementation of the ArrayAccess interface I don't know what else it could be needing.
The exact error returned by php is:
Warning: usort() expects parameter 1 to be array, object given.
How would usort know how you've implemented ArrayAccess? There is no defined place where the values are kept -- that flexibility is the whole point of the interface.
If you are storing the elements in an array that is a private member of the object, you could proxy the usort operation. For example:
public function usort($callback) {
usort($this->container, $callback);
}
If memory serves, there's a big warning sign on the ArrayAccess page, or in one of the comments on it (probably the latter, in fact). It basically says something in the order of: the interface is somewhat useless, because PHP's array functions don't recognize any of its members as arrays.
I've seen alot of coders implement SimpleXML_load_string() instead of the SimpleXMLElement() class. Are there any benefits to using the former over the latter? I've read the PHP manual on simplexml. I couldn't manage to get a grasp on what it is.
Any help and guidance (perhaps through examples) would be much appreciated. Thanks in advance.
simplexml_load_string() (as the name suggest) load xml from a string and returns an object of SimepleXMLElement. There is no difference between this and using just the usual constructor of the class.
Update:
SimpleXML::__construct() has an additional parameter (the third one) bool $data_is_url = false. If this is true the behavior is the same as simplexml_load_file() (in conjunction with the common stream wrappers). Thus both simplexml_load_*()-functions cover the same functionality as SimpleXML::__construct().
Additional the functions simplexml_load_*() has a second parameter string $class_name = "SimpleXMLElement" to specify the class of the object to get returned. Thats not a specific feature of the functions, because you can "use" something very similar with usual instanciation too
class MyXML extends SimpleXMLElement {}
$a = new MyXML($xml);
$b = simplexml_load_string($xml, 'MyXML');
A difference between the OOP- and the procedural approach is, that the functions return false on error, but the constructor throws an Exception.
It's mostly a convenience wrapper. With constructing the base element yourself, you need at least two lines of code to accomplish anything. With simplexml_load_string() a single expression might suffice:
print simplexml_load_string($xml)->title;
Is shorter than:
$e = new SimpleXMLElement($xml);
print $e->title;
And then of course, there is also the slight variation in the function signature.
Update: And exactly the same length as of
print(new SimpleXMLElement($xml))->title;
simplexml_load_string can return a different object:
class_name
You may use this optional parameter so that
simplexml_load_string() will return an
object of the specified class. That
class should extend the
SimpleXMLElement class.
This question already has answers here:
How to use class methods as callbacks
(5 answers)
Closed 10 months ago.
I'm trying to understand some code I found the open source oauth-php library. The relevant code snippet is:
protected function sql_printf ( $args )
{
$sql = array_shift($args);
if (count($args) == 1 && is_array($args[0]))
{
$args = $args[0];
}
$args = array_map(array($this, 'sql_escape_string'), $args);
return vsprintf($sql, $args);
}
Where $args is an array of arguments that contain variables intended for use in a formatted printing operation. I looked at the docs for array_map:
http://php.net/manual/en/function.array-map.php
and the user comments and I did not see any use case where the first parameter in a call to array_map() was an array itself. In all the use cases I saw, the first parameter was either NULL or a (callback) function. It seems pretty obvious to me that the code takes the $args array and then builds a new array with the arguments sanitized by $this->sql_escape_string().
But the statement "array($this, 'sql_escape_string')" is throwing me since I would have expected simply '$this->sql_escape_string', or is that not a valid syntax? If so, how does wrapping $this and 'sql_escape_string' in an array create a valid callback function for array_map() to use?
It is actually passing the sql_escape_string method from the class itself as a callback. It is a way of clarifying ambiguous method calls. For example:
array_map('sql_escape_string', $args);
of course applies sql_escape_string() to each value in $args, whereas:
array_map(array($someClass, 'sql_escape_string'), $args);
applies the sql_escape_string() method from $someClass to each value in $args.
The first parameter is a callback. It can be either a string or an array.
since I would have expected simply '$this->sql_escape_string'
You would if it were just one scalar value. But you have an array and you need to apply that escape function to each item of the $args array. So you need to implement foreach and apply that function or use one-liner with array_map.
But the statement "array($this, 'sql_escape_string')" is throwing me since I would have expected simply '$this->sql_escape_string', or is that not a valid syntax?
It's valid, but doesn't refer to what you think it refers to. Consider free functions, constants, class names and variables: each exists in different environments (or "namespaces" if you prefer, but that's easily confused with PHP namespaces). The different environment for variables is made explicit by the use of "$" as a sigil: the variable $foo versus the function foo(), constant foo and class Foo. This is also why constants and variables are case-sensitive, but functions and class names aren't: the different environments allow for different name resolution rules.
Similarly, object methods and properties exist in different environments. As a consequence, $this->sql_escape_string refers to a property, not a method. To confuse matters, that property could contain a callable, though such a callable couldn't be invoked directly:
class Foo {
function frob() {return 23.0 / 42;}
}
$foo = new Foo;
$foo->frob = function () {return 0 / 0;};
$foo->frob(); # calls method, not closure function
$frob = $foo->frob;
$frob(); # oops: division by zero
As with constants and functions, properties and methods are distinguished by the absence or presence of an argument list.
If so, how does wrapping $this and 'sql_escape_string' in an array create a valid callback function for array_map() to use?
PHP's syntax for callable references goes beyond strings.
Free functions (functions not associated with a class or object; contrast with "bound functions") can unambiguously be referred to by their names. Static methods are bound to a class, but can be referred to with a string if it includes the class name (the syntax is "Class::method"). A string cannot contain enough information for an object method, however, since the method must be bound to a particular object, and PHP doesn't have a way to refer to an object using a string. The solution PHP's developers settled on was to use array syntax (as shown in the question sample code). They also included support for array syntax for static methods (array('Class', 'method')).
Besides callable references, callables can be closures. These offer an alternative way of passing object methods, but are more verbose and complex.
$self = $this; # workaround: $this not accessible in closures before 5.4
$args = array_map(
function ($value) use($self) {
return $self->sql_escape_string($value);
}, $args);
Closures aren't so useful when a callable reference will do, but are more powerful overall.
I will always be in confusion whether to create pass/call by reference functions. It would be great if someone could explain when exactly I should use it and some realistic examples.
A common reason for calling by reference (or pointers) in other languages is to save on space - but PHP is smart enough to implement copy-on-write for arguments which are declared as passed-by-value (copies). There are also some hidden semantic oddities - although PHP5 introduced the practice of always passing objects by reference, array values are always stored as references, call_user_func() always calls by value - never by reference (because it itself is a function - not a construct).
But this is additional to the original question asked.
In general its good practice to always declare your code as passing by value (copy) unless you explicitly want the value to be different after the invoked functionality returns. The reason being that you should know how the invoked functionality changes the state of the code you are currently writing. These concepts are generally referred to as isolation and separation of concerns.
Since PHP 5 there is no real reason to pass values by reference.
One exception is if you want to modify arrays in-place. Take for example the sort function. You can see that the array is passed by reference, which means that the array is sorted in place (no new array is returned).
Or consider a recursive function where each call needs to have access to the same datum (which is often an array too).
In php4 it was used for large variables. If you passed an array in a function the array was copied for use in the function, using a lot of memory and cpu. The solution was this:
function foo(&$arr)
{
echo $arr['value'];
}
$arr = new array();
foo($arr);
This way you only passed the reference, a link to the array and save memory and cpu. Since php5 every object and array (not sure of scalars like int) are passed by reference internally so there isn't any need to do it yourself.
This is best when your function will always return a modified version of the variable that is passed to it to the same variable
$var = modify($var);
function modify($var)
{
return $var.'ret';
}
If you will always return to the passed variable, using reference is great.
Also, when dealing with large variables and especially arrays, it is good to pass by reference wherever feasible. This helps save on memory.
Usually, I pass by reference when dealing with arrays since I usually return to the modified array to the original array.
With PHP5 using "copy on write" and passing by reference causing more of a performance penalty than a gain, why should I use pass-by-reference? Other than call-back functions that would return more than one value or classes who's attributes you want to be alterable without calling a set function later(bad practice, I know), is there a use for it that I am missing?
You use pass-by-reference when you want to modify the result and that's all there is to it.
Remember as well that in PHP objects are always pass-by-reference.
Personally I find PHP's system of copying values implicitly (I guess to defend against accidental modification) cumbersome and unintuitive but then again I started in strongly typed languages, which probably explains that. But I find it interesting that objects differ from PHP's normal operation and I take it as evidence that PHP"s implicit copying mechanism really isn't a good system.
A recursive function that fills an array? Remember writing something like that, once.
There's no point in having hundreds of copies of a partially filled array and copying, splicing and joining parts at every turn.
Even when passing objects there is a difference.
Try this example:
class Penguin { }
$a = new Penguin();
function one($a)
{
$a = null;
}
function two(&$a)
{
$a = null;
}
var_dump($a);
one($a);
var_dump($a);
two($a);
var_dump($a);
The result will be:
object(Penguin)#1 (0) {}
object(Penguin)#1 (0) {}
NULL
When you pass a variable containing a reference to an object by reference, you are able to modify the reference to the object.