Why is it not possible to do something equivalent to this in PHP:
(Array(0))[0];
This is just for sake of argument, but it seems strange it does not allow access of anonymous objects. I would have to do something like the following:
$array = Array(0);
$array[0];
Any ideas why this is the behavior of PHP?
I read something somewhat detailed about this once and I regret not bookmarking it because it was quite insightful. However, it's something along the lines of
"Because the array does not exist in memory until the current statement (line) executes in full (a semicolon is reached)"
So, basically, you're only defining the array - it's not actually created and readable/accessible until the next line.
I hope this somewhat accurately sums up what I only vaguely remember reading many months ago.
This language feature hasn’t been inplemented yet but will come in PHP 6.
I guess the short answer is: nobody has coded it yet. I've used (and loved) that syntax in both Python and Javascript, but still we wait for PHP.
The main reason is because unlike some languages like Python and JavaScript, Array() (or in fact array()) is not an object, but an language construct which creates an inbuilt data type.
Inbuilt datatypes themselves aren't objects either, and the array() construct doesn't return a reference to the "object" but the actual value itself when can then be assigned to a variable.
Related
Is there any way a variable can be assigned from multiple procedures in one line?
For example:
$class_splits = explode("\\", $class_name);
$short_class_name = $class_splits[count($class_splits) - 1] ?? null;
Translated to this pseudo code:
$short_class_name = explode("\\", $class_name) => prev_result(count(prev_result) -1);
I don't have big expectations on this as I know it looks too "high-level" but not sure if newer versions of PHP can handle this.
Thanks.
You can use an assignment as an expression, then refer to the variable that was assigned later in the containing expression.
$short_class_name = ($class_splits = explode("\\", $class_name))[count($class_splits) - 1] ?? null;
That said, I don't recommend coding like this. If the goal was to avoid creating another variable, it doesn't succeed at that. It just makes the whole thing more complicated and confusing.
I believe you have an "X/Y Problem": your actual requirement seems to be "how to split a string and return just the last element", but you've got stuck thinking about a particular solution to that.
As such, we can look at the answers to "How to get the last element of an array without deleting it?" To make it a one-line statement, we need something that a) does not require an argument by reference, and b) does not require the array to be mentioned twice.
A good candidate looks like array_slice, which can return a single-element array with just the last element, from which we can then extract the string with [0]:
$short_class_name = array_slice(explode("\\", $class_name), -1)[0];
Since we no longer need to call count(), we can avoid the problem of needing the same intermediate value in two places.
Whether the result is actually more readable than using two lines of code is a matter of taste - remember that a program is as much for human use as for machine use.
I was marvelled when I tested the following code today:
$star = "Aquarius";
$star = 11;
While debugging, I observed that $star simply changes from string type to integer type. I was amazed by this functionality. In C++ for instance, this is just impossible, but in c# I considered the var variable but it's not the same.
For instance you can't do:
var dynamic = "Hello";
dynamic = 3;
I began to wonder what exactly happens at the point when I basically say $star = 11. My guess is that $star is simply reinitialized since it's being directly assigned to (but this seems weird since the interpreter already knows that a variable $star has been declared earlier). Can anyone help with some clear or official source-backed explanation?
Thanks.
In C/C++ the type is defined at compile time because of the kinds of optimization that can occur based on it.
In C# the compiler infers the type based on the context and in the compilers brain it substitutes the var keyword for the type. This is why you can not change the type after the compiler made the initial inference.
In scripting languages like PHP a variable is an entry into a Hash Map (Associative Array, a Symbol Table). This defines the namespace (and scope). The actual value part is a generic object type that stores both the value and the type.
PHP is a dynamic language, similar in spirit to Perl, Ruby, Python, or many others. C++, on the other hand, is compiled and statically typed, requiring each variable to have a type defined at compile time.
Check the PHP docs for some great insight to PHP's dynamic typing implementation:
http://php.net/manual/en/language.types.type-juggling.php
PHP is a loosely typed language. PHP converts the variable to the correct data type, according to the value.
Check this out - http://php.net/manual/en/language.types.type-juggling.php
Why don't the function handling functions like call_user_func() support passing parameters by reference?
The docs say terse things like "Note that the parameters for call_user_func() are not passed by reference." I assume the PHP devs had some kind of reason for disabling that capability in this case.
Were they facing a technical limitation? Was it a language design choice? How did this come about?
EDIT:
In order to clarify this, here is an example.
<?php
function more(&$var){ $var++; }
$count = 0;
print "The count is $count.\n";
more($count);
print "The count is $count.\n";
call_user_func('more', $count);
print "The count is $count.\n";
// Output:
// The count is 0.
// The count is 1.
// The count is 1.
This is functioning normally; call_user_func does not pass $count by reference, even though more() declared it as a referenced variable. The call_user_func documentation clearly says that this is the way it's supposed to work.
I am well aware that I can get the effect I need by using call_user_func_array('more', array(&$count)).
The question is: why was call_user_func designed to work this way? The passing by reference documentation says that "Function definitions alone are enough to correctly pass the argument by reference." The behavior of call_user_func is an exception to that. Why?
The answer is embedded deep down in the way references work in PHP's model - not necessarily the implementation, because that can vary a lot, particularly in the 5.x versions. I'm sure you've heard the lines, they're not like C pointers, or C++ references, etc etc... Basically when a variable is assigned or bound, it can happen in two ways - either by value (in which case the new variable is bound to a new 'box' containing a copy of the old value), or by reference (in which case the new variable is bound to the same value box as the old value). This is true whether we're talking about variables, or function arguments, or cells in arrays.
Things start to get a bit hairy when you start passing references into functions - obviously the intent is to be able to modify the original variables. Quite some time ago, call-time pass-by-reference (the ability to pass a reference into a function that wasn't expecting one) got deprecated, because a function that wasn't aware it was dealing with a reference might 'accidentally' modify the input. Taking it to another level, if that function calls a second function, that itself wasn't expecting a reference... then everything ends up getting disconnected. It might work, but it's not guaranteed, and may break in some PHP version.
This is where call_user_func() comes in. Suppose you pass a reference into it (and get the associated the call-time pass-by-reference warning). Then your reference gets bound to a new variable - the parameters of call_user_func() itself. Then when your target function is called, its parameters are not bound where you expect. They're not bound to the original parameters at all. They're bound to the local variables that are in the call_user_func() declaration. call_user_func_array() requires caution too. Putting a reference in an array cell could be trouble - since PHP passes that array with "copy-on-write" semantics, you can't be sure if the array won't get modified underneath you, and the copy won't get detached from the original reference.
The most insightful explanation I've seen (which helped me get my head around references) was in a comment on the PHP 'passing by reference' manual:
http://ca.php.net/manual/en/language.references.pass.php#99549
Basically the logic goes like this. How would you write your own version of call_user_func() ? - and then explain how that breaks with references, and how it fails when you avoid call-time pass-by-reference. In other words, the right way to call functions (specify the value, and let PHP decide from the function declaration whether to pass value or reference) isn't going to work when you use call_user_func() - you're calling two functions deep, the first by value, and the second by reference to the values in the first.
Get your head around this, and you'll have a much deeper understanding of PHP references (and a much greater motivation to steer clear if you can).
See this:
http://hakre.wordpress.com/2011/03/09/call_user_func_array-php-5-3-and-passing-by-reference/
Is it possible to pass parameters by reference using call_user_func_array()?
http://bugs.php.net/bug.php?id=17309&edit=1
Passing references in an array works correctly.
Updated Answer:
You can use:
call_user_func('more', &$count)
to achieve the same effect as:
call_user_func_array('more', array(&$count))
For this reason I believe (unfoundedly) that call_user_func is just a compiler time short cut. (i.e. it gets replaced with the later at compile time)
To give my view on you actual question "Why was call_user_func designed to work this way?":
It probably falls under the same lines as "Why is some methods strstr and other str_replace?, why is array functions haystack, needle and string functions needle, haystack?
Its because PHP was designed, by many different people, over a long period of time, and with no strict standards in place at the time.
Original Answer:
You must make sure you set the variable inside the array to a reference as well.
Try this and take note of the array(&$t) part:
function test(&$t) {
$t++;
echo '$t is '.$t.' inside function'.PHP_EOL;
}
$t = 0;
echo '$t is '.$t.' in global scope'.PHP_EOL;
test($t);
$t++;
echo '$t is '.$t.' in global scope'.PHP_EOL;
call_user_func_array('test', array(&$t));
$t++;
echo '$t is '.$t.' in global scope'.PHP_EOL;
Should output:
$t is 0 in global scope
$t is 1 inside function
$t is 2 in global scope
$t is 3 inside function
$t is 4 in global scope
Another possible way - the by-reference syntax stays the 'right' way:
$data = 'some data';
$func = 'more';
$func($more);
function more(&$data) {
// Do something with $data here...
}
I will always be in confusion whether to create pass/call by reference functions. It would be great if someone could explain when exactly I should use it and some realistic examples.
A common reason for calling by reference (or pointers) in other languages is to save on space - but PHP is smart enough to implement copy-on-write for arguments which are declared as passed-by-value (copies). There are also some hidden semantic oddities - although PHP5 introduced the practice of always passing objects by reference, array values are always stored as references, call_user_func() always calls by value - never by reference (because it itself is a function - not a construct).
But this is additional to the original question asked.
In general its good practice to always declare your code as passing by value (copy) unless you explicitly want the value to be different after the invoked functionality returns. The reason being that you should know how the invoked functionality changes the state of the code you are currently writing. These concepts are generally referred to as isolation and separation of concerns.
Since PHP 5 there is no real reason to pass values by reference.
One exception is if you want to modify arrays in-place. Take for example the sort function. You can see that the array is passed by reference, which means that the array is sorted in place (no new array is returned).
Or consider a recursive function where each call needs to have access to the same datum (which is often an array too).
In php4 it was used for large variables. If you passed an array in a function the array was copied for use in the function, using a lot of memory and cpu. The solution was this:
function foo(&$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.
when I was looking for some MVC framework, I got the website:
http://www.phppatterns.com/docs/design/archive/model_view_controller_pattern
however, like the code listed there makes me confused about references.
For example:
$dao=& new DataAccess ('localhost','user','pass','dbname');
$productModel=& new ProductModel($dao);
each instance it makes, it adds '&' before the new operator, what does it exactly mean? the reference to the instance? Actually, I removed all the '&' before all these kind of instances and the code still works perfectly.
Another, codes like :
function ProductView (&$model) {
$this->model=& $model;
}
I really think it could be revised like:
function ProductView (&$model) {
$this->model=$model;
}
Am I right? what's the differences between these two? Actually, like the MVC code example listed above? if you revise the code like I did, the code still works.
Then, I got this post somewhere else:
http://schlueters.de/blog/archives/125-Do-not-use-PHP-references.html
does it make sense? any suggestions about this would be helpful.
Prior to PHP version 5 objects were passed by value and you had to explicitly specify the ampersand to get the object by reference.*
In PHP 5+, all objects are passed by reference and thus using ampersand is redundant.
Furthermore, as of PHP 5.3.0, code like the above will generate a STRICT error of "Assigning the return value of new by reference is deprecated".
If you're curious about the historical use (PHP 4 or before) of "$o =& new Object()" code see php-by-reference (in particular, the accepted answer there provides a good explanation).
To summarise:
in PHP 5 or above, it makes no difference. The code will work as expected with no memory or other differences.
In PHP 5.3 you might get some STRICT notices complaining about this usage (assuming you have STRICT notices turned on).
In PHP 4.x (or earlier) this method was used to prevent unnecessary duplication of objects.
(*) Passing references around is a good thing - no need to create copies of objects when you only need the one instance.