Unable to access class constant from instance using :: scope statement

Question

Today I ran into some strange problem, and even as a software engineer I don’t understand:

It seems you can access the class constant from an instance of an object, for example:

class a { const abc = 1; } $a = new a(); var_dump($a->abc); 

This will output null instead of the expected 1. I was able to do the following:

 class a { const abc = 1; } $a = new a(); var_dump(a::abc); 

But in the context of a subordinate object that does not really know who the parent is, it is very unpleasant for me to do this:

 class a { const abc = 1; } $a = new a(); $c = get_class($a); var_dump($c::abc); 

Is it me, or is it completely stupid, if not, please enlighten me why this works.

EDIT

Another way to do this, but it is not better:

 class a { const abc = 1; } class b { public function getA(){ return new a(); } } $b = new b(); $c = $b->getA(); var_dump($c::abc); 

This last example is more like what I do and worry about ...

Answer 1

I found a relatively beautiful and clean way to make my task easier. Here is the solution I applied. This is not necessarily the best, but for my purposes it does exactly what I need.

When creating the __get magic method, I intercept the query for the constant name and instance point, and I use quick reflection to see if this constant exists and return its value.

This allows me to actually use an all in one line template that looks like this:

 class a { const abc = 1; public function __get($key){ $r = new ReflectionObject($this); if($r->hasConstant($key)){ return $r->getConstant($key); } } } class b { public function getA(){ return new a(); } } $b = new b(); var_dump($b->getA()->abc); var_dump($b->getA()->def); 

Although I would like to:

 var_dump($b->getA()::abc); var_dump($b->getA()::def); 

I suppose this might be possible later in 5.4+, given that we finally dereferenced the array, we could probably ask them to add static raffles in the near future.

Answer 2

Just use an instance variable with a scope statement:

 $a = new a(); var_dump($a::abc); 

This prints 1 .

Answer 3

The PHP documentation indicates that class constants are accessible via SRO ( :: , and not -> .

 <?php class MyClass { const constant = 'constant value'; function showConstant() { echo self::constant . "\n"; } } echo MyClass::constant . "\n"; 

Answer 4

As I already mentioned, in php constants they are attached to the class definition, they are static by definition and cannot be accessed with the operator →.

If you really want to use it with your coding paradigm, you can try the reflection class in php5.

 class MyClass { const A = "I am A"; } $o = new ReflectionClass( "MyClass" ); echo $o->getconstant("A"); //will print "I am A" 

Also, I think the example in EDIT might not work. I did not run it, but I am not sure that SRO (: :) can be called in anything that is not a reference to the class ..

Answer 5

I know this is an old thread, but for people who want to know the best way to do this, look at the PHP function constant () .

With constant () you can just do this:

 $a = new a(); $value = constant(get_class($a)."::abc"); // $value === 1 

it was available since PHP 4 and still works fine in PHP 5.5