pass by reference - Page 2

Village Idiot · 05-07-2009, 02:16 PM

Its been a long time since I've worked with PHP, I never got too in depth to whichever these are (in PHP). So you may very well be right.

Salathe · 05-07-2009, 03:12 PM

Sketch is along the right lines. References in PHP are not pointers at all, they're just aliased variable names in a symbol table. In PHP, values are kept in a zval container which keeps track of several things about them; not just the value of the variable but its type and some other things.

Variables are stored in symbol tables mapping variable names to their associated values. When talking about references, we're not talking about one variable referencing another.

Instead, multiple variable names being associated with the same zval container (and therefore the same value) which has its is_ref flag set to 1. To perhaps confuse things, multiple variable names can be associated with the same zval container and not be references (is_ref set to 0), merely sharing that same value.

allworknoplay · 05-07-2009, 03:55 PM

Quote:

Originally Posted by Salathe

Sketch is along the right lines. References in PHP are not pointers at all, they're just aliased variable names in a symbol table. In PHP, values are kept in a zval container which keeps track of several things about them; not just the value of the variable but its type and some other things.

Variables are stored in symbol tables mapping variable names to their associated values. When talking about references, we're not talking about one variable referencing another.

Instead, multiple variable names being associated with the same zval container (and therefore the same value) which has its is_ref flag set to 1. To perhaps confuse things, multiple variable names can be associated with the same zval container and not be references (is_ref set to 0), merely sharing that same value.

Even after a cup of coffee, it still doesn't help....

I assume that what you say here is referring to pass-by-ref?

Quote:

multiple variable names being associated with the same zval container (and therefore the same value) which has its is_ref flag set to 1

What is this technique below? How is it done? And is there a name for this?

Quote:

To perhaps confuse things, multiple variable names can be associated with the same zval container and not be references (is_ref set to 0), merely sharing that same value.

Village Idiot · 05-07-2009, 03:55 PM

Oh. For those who now understand pointers, you have grasped the hardest beginner concept for both C and C++.

allworknoplay · 05-07-2009, 03:57 PM

Quote:

Originally Posted by sketchMedia

I was under the impression that PHP's references functioned more like an alias (like a reference in C++) rather than a pointer, in other words they don't point to a memory location but are an alternative name for the target (think of it as a symlink), they more or less achieve the same but are different in terms of what you can do with them, you cant re-seat a reference as you can a pointer (in C++ anyway), you can't have a null reference whereas you can have a null pointer also you can't perform pointer arithmetic on references (which some would consider to be a good thing).

In terms of PHP however, you won't really need an in-depth knowledge of the differences between the too, they will both provide more or less the same functionality (in any case you can't directly fiddle with the memory in PHP like you can in C as its interpreted not compiled).

Just for the sake of illustration, heres a C++ pointer:

c++ Code:

#include <iostream>
using namespace std;

int main()
{
    unsigned short int age = 21; 
    unsigned short int * pAge = 0;
    pAge =& age;

    cout << "Age: " << age << "pointer age: " << *pAge << endl;

    *pAge = 25; 

    cout << "Age: " << age << "pointer age: " << *pAge << endl;
    
    age = 22; 

    cout << "Age: " << age << "pointer age: " << *pAge << endl;

    return 0;
}

and a C++ reference:

c++ Code:

#include <iostream>
using namespace std;

int main()
{
    unsigned short int age = 21; 
    unsigned short int&  rAge = age;
    
    cout << "Age: " << age << "ref age: " << rAge << endl;
    
    age = 25; 

    cout << "Age: " << age << "ref age: " << rAge << endl;

    rAge = 22;

    cout << "Age: " << age << "ref age: " << rAge << endl;
    
    return 0;
}

Heartburn!!!!

I don't have a compiler avaiable, can someone provide the output for these 2 code snippets?

Salathe · 05-07-2009, 04:06 PM

Here's a pretty picture to tell the tale. The diagrams show two similar sets of code, one using "normal" assignments by value and the other using references. The single letters are variable names, the detail boxes represent the zval container (a C struct) which contains the value, the type, an is_ref flag and the count of "references" (in the latter case, the count of variable names associated with this container). The green highlights are what changes.

P.S. The term "pass" is generally reserved for feeding arguments into function calls. Stick with "assign" for assignment operations.

allworknoplay · 05-07-2009, 04:07 PM

Quote:

Originally Posted by sketchMedia

you can't have a null reference whereas you can have a null pointer also you can't perform pointer arithmetic on references

Ok, I wanted to test this, how come I didn't get any error?

PHP Code:


			
<?php





$var100 = NULL;



$var200 =& $var100;



$get_type = gettype($var100);

$get_type2 = gettype($var200);



echo "Type: " . $get_type . "<br>";

echo "Type2: " . $get_type2 . "<br>";



?>

Output is:

Type: NULL
Type2: NULL

Shouldn't there be an error for Type2? Like, "Error, reference cannot be null"

allworknoplay · 05-07-2009, 05:40 PM

Quote:

Originally Posted by Salathe

Here's a pretty picture to tell the tale. The diagrams show two similar sets of code, one using "normal" assignments by value and the other using references. The single letters are variable names, the detail boxes represent the zval container (a C struct) which contains the value, the type, an is_ref flag and the count of "references" (in the latter case, the count of variable names associated with this container). The green highlights are what changes.

P.S. The term "pass" is generally reserved for feeding arguments into function calls. Stick with "assign" for assignment operations.

Lovely! Ok, some quick questions...

is_ref is a flag? It only takes 0 or 1 right?

0 = no pass-by-ref..so value is pass-by-value?
1 = yes, someone is using pass-by-ref on this value

refcount = just some kind of simple counter of who is "accessing" this data. Doesn't matter if it's by ref or by value....

So like I've mentioned before, it seems that by-ref is more than just a looking glass to the data, but actually provides a medium to modify that data at that stored location.

And like the Borg, any other variables accessing this value also receives the new changes...

My question is, "Is PHP not using the correct terminology?"

Are they confusing individuals like myself by using pass-by-ref when it should be assign-by-ref?

sketchMedia · 05-07-2009, 06:20 PM

Quote:

Oh. For those who now understand pointers, you have grasped the hardest beginner concept for both C and C++.

Indeed! took me a few attempts to crack this egg myself.

Quote:

I don't have a compiler avaiable, can someone provide the output for these 2 code snippets?

oops, sorry i forgot.

1st:

Code:

Age: 21 pointer age: 21

Age: 25 pointer age: 25

Age: 22 pointer age: 22

2nd:

Code:

Age: 21 ref age: 21

Age: 25 ref age: 25

Age: 22 ref age: 22

I'll explain the null reference (which in C++ can cause some arguments) when i get home, unless ofc someone else has stepped into the fray!

Salathe · 05-07-2009, 07:30 PM

Quote:

Originally Posted by allworknoplay

is_ref is a flag? It only takes 0 or 1 right?

0 = no pass-by-ref..so value is pass-by-value?
1 = yes, someone is using pass-by-ref on this value

refcount = just some kind of simple counter of who is "accessing" this data. Doesn't matter if it's by ref or by value....

Yes, that's right.

Quote:

Originally Posted by allworknoplay

My question is, "Is PHP not using the correct terminology?"

Are they confusing individuals like myself by using pass-by-ref when it should be assign-by-ref?

I'm not really sure where you are getting this from, perhaps there are places in the documentation which might be unclear (or wrong), the docs are written (mostly) by humans after all. Where have you been reading the confusing terms? The difference between passing by reference and assigning by reference is the same as passing and assigning by value.

PHP Code:


			
function byref(&$foo){}

function byval($foo){}



$byref =& $blah; // Assign by reference

$byval =  $blah; // Assign by value

byref($blah);    // Pass by reference

byval($blah);    // Pass by value



// Also

byval(&$blah); // Call-time pass by reference, deprecated

allworknoplay · 05-07-2009, 08:04 PM

Quote:

Originally Posted by Salathe

Yes, that's right.

The difference between passing by reference and assigning by reference is the same as passing and assigning by value.

That's where I was confused. I thought they were 2 different beasts when I was reading Sketch's post about how PHP isn't really using a pointer but more of an alias(like symbolic links)..

So I thought what he was trying to imply was pass-by-ref had a different fundamental meaning than assign-by-ref.

But thanks for clearing that up, I get it now...

sketchMedia · 05-07-2009, 11:54 PM

Once again another great and informative post from Salathe

By null reference I am referring to a memory reference that is null (either null or 0), in your example the memory location (or whatever PHP does) will be a non-null one as your variable is defined (thus memory set aside with a correct memory location thats non null) then a value of null is assigned (does it get cast to 0 internally?).

For a null reference (in C++ anyway, i know i said you can't but i had my strict standards hat on at the time, i should say you can but you shouldnt, i believe its allowed for certain embedded hardware stuff where you need a specific memory location for things) you dereference a null pointer (meaning it has a memory location of null/0) and try to reference it, thus this will die:

c++ Code:

#include <iostream>
using namespace std;
typedef unsigned short int USHORT;

int main()
{
    USHORT * p = 0;
    USHORT & ref = *p;
    cout << ref;
    return 0;
}

This seg faults (or crashes depending on the machne and OS) as it tries to access a null memory location (when cout is called), which doesn't exist.

for null in C++ you can either include stddef.h which has a macro defined for null, or you could cast it:

c++ Code:

USHORT * p = (USHORT *) NULL;

I dunno if you can replicate it in PHP.

Sorry for any confusion, guess I got carried away with C++, it happens sometimes.

If you have any more questions, I will try to answer them ( promise I'll try NOT to throw a load of C++ junk at you

, I just thought I'd explain pointers and references, as VI said, the one of the hardest concepts to understand)

Hope that helps.

allworknoplay · 05-08-2009, 02:00 AM

Quote:

Originally Posted by sketchMedia

Once again another great and informative post from Salathe

By null reference I am referring to a memory reference that is null (either null or 0), in your example the memory location (or whatever PHP does) will be a non-null one as your variable is defined (thus memory set aside with a correct memory location thats non null) then a value of null is assigned (does it get cast to 0 internally?).

For a null reference (in C++ anyway, i know i said you can't but i had my strict standards hat on at the time, i should say you can but you shouldnt, i believe its allowed for certain embedded hardware stuff where you need a specific memory location for things) you dereference a null pointer (meaning it has a memory location of null/0) and try to reference it, thus this will die:

c++ Code:

#include <iostream>
using namespace std;
typedef unsigned short int USHORT;

int main()
{
    USHORT * p = 0;
    USHORT & ref = *p;
    cout << ref;
    return 0;
}

This seg faults (or crashes depending on the machne and OS) as it tries to access a null memory location (when cout is called), which doesn't exist.

for null in C++ you can either include stddef.h which has a macro defined for null, or you could cast it:

c++ Code:

USHORT * p = (USHORT *) NULL;

I dunno if you can replicate it in PHP.

Sorry for any confusion, guess I got carried away with C++, it happens sometimes.

If you have any more questions, I will try to answer them ( promise I'll try NOT to throw a load of C++ junk at you

, I just thought I'd explain pointers and references, as VI said, the one of the hardest concepts to understand)

Hope that helps.

No need to apologize. You're just bringing back memories from my C++ days in college...

I really appreciate the time all of you guys are taking to explain the basics of PHP. Just when you think you know something, you find out there's more to it...

No matter where I find myself in PHP land, I am never afraid to go back to basics and making sure I get the fundamentals right.

It all clicks now for me, I hope I don't get blind sided by anything else pass-by-ref or pass-by-val related!!!

sketchMedia · 05-08-2009, 10:41 AM

Quote:

You're just bringing back memories from my C++ days in college...

Yea I had to dig deep myself, its along time since my last C++ program. Hopefully i got my facts right.

Quote:

I really appreciate the time all of you guys are taking to explain the basics of PHP. Just when you think you know something, you find out there's more to it...

No probs m8, In fact it helps me also as it acts as a refresher of stuff I have known at some point but have forgotten (Got to the stage where new stuff forces the old stuff out of my memory)

Glad you found it useful