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Parameter passing: by value and by reference

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• Review: parameter passing mechanism
  • 2 commonly used parameter passing mechanisms:

    Pass-by-value: A copy of the variable is passed (= given) to the function When the function updates the parameter variable, the original copy of the variable is not changed Pass-by-reference: The address (= memory location/reference of the variable is passed (= given) to the function The function will visit the memory location (address) to use the value stored in the variable !!! When the function updates the parameter variable, the original copy of the variable is changed !!!!!!!!

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• Parameter passing in PHP     (if you know C++, it's exactly like C++)
  • Passing a variable by value:

    A parameter is passed (by-default) by value

    Example:

    <?php $a = 1; print("a before calling f( ) = " . $a . "\n"); f($a); print("a after calling f( ) = " . $a . "\n"); function f( $x ) # $x is passed by value { $x = 4444; } ?>

    Output:

    a before calling f( ) = 1 a after calling f( ) = 1

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  • Example Program: (Demo above code)                                                

    How to run the program:

    ssh -X netid@holland php /home/cs377001/PHP/intro-PHP/pass-by-value.php

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  • Passing a variable by reference:

    Use a "&" symbol before a parameter to specify the parameter is passed-by-reference

    Example:

    <?php $a = 1; print("a = " . $a . "\n"); f($a); print("a = " . $a . "\n"); function f (& $x ) # $x is passed by REFERENCE !!!! { $x = 4444; } ?>

    Output:

    Before f(a): a = 1 After f(a): a = 4444 <--- Changed !!!

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  • Example Program: (Demo above code)                                                

    How to run the program:

    ssh -X netid@holland php /home/cs377001/PHP/intro-PHP/pass-by-ref.php

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• Array parameters are passed by default by copying all values in the array !!
  • Passing arrays to a function in PHP:

    By default (= if you do not specify anything), arrays in PHP are passed by value: <?php $a[0] = 1; $a[1] = 2; printf('Before f($a): a[0] = %d a[1] = %d'."\n", $a[0], $a[1]); f($a); printf('After f($a): a[0] = %d a[1] = %d'."\n", $a[0], $a[1]); function f( $x ) # array is passed by value !! use & to pass by reference { $x[0] = 4444; $x[1] = 4445; } ?> Will print: php /home/cs377001/PHP/intro-PHP/pass-array.php Before f($a): a[0] = 1 a[1] = 2 After f($a): a[0] = 1 a[1] = 2

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  • How to pass an array variable by reference: use & in the function definition

    function f( & $x ) # How to pass array by reference { $x[0] = 4444; $x[1] = 4445; }

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• Object parameters are passed by default by copying the object identifier in PHP 5 and higher
  • Passing objects to PHP functions:

    Objects in PHP 5 and higher are passed to a function by copying the object identifier The effect of this is "similar" to passing the object by reference <?php class myObject { public $x; public $y; } $a = new myObject(); $a->x = 1; $a->y = 2; printf('Before f($a): $a->x = %d $a->y = %d'."\n", $a->x, $a->y); f($a); printf('After f($a): $a->x = %d $a->y = %d'."\n", $a->x, $a->y); function f( $x ) # Object is passed by value !! use & to pass by reference { $x->x = 4444; $x->y = 4445; } ?> Will print: php /home/cs377001/PHP/intro-PHP/pass-object.php Before f($a): $a->x = 1 $a->y = 2 After f($a): $a->x = 4444 $a->y = 4445

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  • How to pass an object by value (= a copy): clone( $obj )

    $a = new myObject(); $a->x = 1; $a->y = 2; f( clone($a) ); // Pass a clone (= copy) of object to function php /home/cs377001/PHP/intro-PHP/pass-object2.php