rfc:instance_counter

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PHP RFC: instance counter

Introduction

Coming to Object-Orientated Programming, PHP offers a variety of possibilities to obtain information about classes and their instances (objects). In this regard, the classes/objects functions (http://www.php.net/manual/en/ref.classobj.php) as well as the reflection API (http://www.php.net/manual/en/book.reflection.php) do a great job. But still there is no function to obtain a particular information: the number of instances of a class.

One might say, simply implement a static counter in each class. But what about built-in classes like SPL? A wrapper would be required. Implementing it these ways, it does not only sound, but it actually is cumbersome. The purpose of this RFC is to address this issue.

Proposal

The proposal is to add a new functionality dealing with the number of instances of either a specific class name/object or set of class names/objects or all classes.

The function name should fit in with the current names of class/object functions. Therefore, the name get_objects_count() seems to be reasonable, but is still up for discussion (see Function name).

  • If no argument is provided, the number of all objects in the object store as an associative array ('class name' ⇒ count) will be returned.
print_r (get_objects_count());
// Array ()
 
$foo = new stdClass;
 
print_r (get_objects_count());
// Array ( [stdClass] => 1 )
 
$bar = new stdClass;
 
print_r (get_objects_count());
// Array ( [stdClass] => 2 )
 
$bar = null;
 
print_r (get_objects_count());
// Array ( [stdClass] => 1 )
 
$foo = null;
 
print_r (get_objects_count());
// Array ()
  • If a class name is provided, the number of objects of the specified class in the object store will be returned.
print get_objects_count('stdClass');
// 0
 
$foo = new stdClass;
 
print get_objects_count('stdClass');
// 1
 
$bar = new stdClass;
 
print get_objects_count('stdClass');
// 2
 
$bar = null;
 
print get_objects_count('stdClass');
// 1
 
$foo = null;
 
print get_objects_count('stdClass');
// 0
  • If an object is provided, the number of objects of the specifiied objects class in the object store will be returned. The return value is always ≥ 1.
$foo = new stdClass;
 
print get_objects_count($foo);
// 1
 
$bar = new stdClass;
 
print get_objects_count($bar);
// 2
 
$bar = null;
 
print get_objects_count($foo);
// 1
  • If an an array is provided, it will be the treated as an inclusive indexed array of class names. An associative array ('class name' ⇒ count) will be returned.
print_r (get_objects_count(array('stdClass')));
// Array ( [stdClass] => 0 )
 
$foo = new stdClass;
 
print_r (get_objects_count(array('stdClass')));
// Array ( [stdClass] => 1 )
 
$bar = new stdClass;
 
print_r (get_objects_count(array('stdClass')));
// Array ( [stdClass] => 2 )
 
$bar = null;
 
print_r (get_objects_count(array('stdClass')));
// Array ( [stdClass] => 1 )
 
$foo = null;
 
print_r (get_objects_count(array('stdClass')));
// Array ( [stdClass] => 0 )

General questions & answers

Inheritance

On internals list there was the question, if only the “direct” instances of a class or also the instances of subclasses are counted? The answer is: only direct instances. See the following code:

class A {}
class B extends A {}
 
print get_objects_count('A');
// 0
 
$b = new B;
 
var_dump($b instanceof A);
// bool(true)
 
print get_objects_count('A');
// 0
 
print get_objects_count('B');
// 1

Use cases:

  • Debugging
  • Implementation of design patterns (f.ex. flyweight pattern)
  • Extended control: limit the number of objects
  • Teach people about assignment of objects
  • to be continued…

Function name

Further suggestions for the function name in example #1:

// Proposed
get_objects_count()

// Alternatives
get_instances_counts()
get_instances_count()
get_instance_counts()
get_object_store_count()
get_class_counts() 

Backward Incompatible Changes

No BC breaks.

Proposed PHP Version(s)

next PHP 5.4.x or PHP 5.5.x

SAPIs Impacted

None.

Impact to Existing Extensions

None.

New Constants

None.

php.ini Defaults

None.

Open Issues

  • Decision on function name.

Patches and Tests

Implementation: https://gist.github.com/krakjoe/5275773

Tests need to be done.

References

Rejected Features

- None.

rfc/instance_counter.1365681187.txt.gz · Last modified: 2013/04/11 13:53 by frank