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/**
\page doc_script_handle Object handles
An object handle is a type that can hold a reference to an object. With
object handles it is possible to declare more than one variables that refer
to the same physical object.
Not all types allow object handles to be used. None of the primitive
data types, bool, int, float, etc, can have object handles. Object types
registered by the application may or may not allow object handles, depending
on how they have been registered.
\see \ref doc_obj_handle
\section doc_script_handle_1 General usage
An object handle is declared by appending the @ symbol to the data type.
<pre>
object\@ obj_h;
</pre>
This code declares the object handle obj and initializes it to null, i.e.
it doesn't hold a reference to any object.
In expressions variables declared as object handles are used the exact
same way as normal objects. But you should be aware that object handles are
not guaranteed to actually reference an object, and if you try to access the
contents of an object in a handle that is null an exception will be raised.
<pre>
object obj;
object\@ obj_h;
obj.Method();
obj_h.Method();
</pre>
Operators like = or any other operator registered for the object type work
on the actual object that the handle references. These will also throw an
exception if the handle is empty.
<pre>
object obj;
object\@ obj_h;
obj_h = obj;
</pre>
When you need to make an operation on the actual handle, you should prepend
the expression with the \@ symbol. Setting the object handle to point to an
object is for example done like this:
<pre>
object obj;
object\@ obj_h;
\@obj_h = \@obj;
</pre>
Note that the compiler can often implicitly determine that it is the handle of the
object that is needed rather than the actual object itself. In these cases it is not
necessary to explicitly prepend the expression with \@.
An object handle can be compared against another object handle
(of the same type) to verify if they are pointing to the same object or not.
It can also be compared against null, which is a special keyword that
represents an empty handle. This is done using the identity operator, <tt>is</tt>.
<pre>
object\@ obj_a, obj_b;
if( obj_a is obj_b ) {}
if( obj_a !is null ) {}
</pre>
Observe, the == and != operators will do a value comparison on the objects referred to
by the handles using the \ref doc_script_class_cmp_ops "opEquals" or \ref doc_script_class_cmp_ops "opCmp"
operator overloads. Though, if the expressions are prepended with @ the operators will have the
same function as <tt>is</tt> and <tt>!is</tt>.
\section doc_script_handle_2 Object life times
An object's life time is normally for the duration of the scope the
variable was declared in. But if a handle outside the scope is set to
reference the object, the object will live on until all object handles are
released.
<pre>
object\@ obj_h;
{
object obj;
\@obj_h = \@obj;
// The object would normally die when the block ends,
// but the handle is still holding a reference to it
}
// The object still lives on in obj_h ...
obj_h.Method();
// ... until the reference is explicitly released
// or the object handle goes out of scope
\@obj_h = null;
</pre>
\section doc_script_handle_3 Object relations and polymorphing
Object handles can be used to write common code for related types, by means of
inheritance or interfaces. This allows a handle to an interface to store references
to all object types that implement that interface, similarly a handle to a base
class can store references to all object types that derive from that class.
<pre>
interface I {}
class A : I {}
class B : I {}
// Store reference in handle to interface
I \@i1 = A();
I \@i2 = B();
void function(I \@i)
{
// Functions implemented by the interface can be
// called directly on the interface handle. But if
// special treatment is need for a specific type, a
// cast can be used to get a handle to the true type.
A \@a = cast<A>(i);
if( a !is null )
{
// Access A's members directly
...
}
else
{
// The object referenced by i is not of type A
...
}
}
</pre>
\section doc_script_handle_4 Const handles
Sometimes it is necessary to hold handles to objects that shouldn't be allowed to be modified.
This is done by prefixing the type with 'const', e.g.
<pre>
obj \@a; // handle to modifiable object
const obj \@b; // handle to non-modifiable object
</pre>
A handle to a non-modifiable object can refer to both modifiable objects and non-modifiable objects, but
the script will not allow the object to be modified through that handle, nor allow the handle to be passed
to another handle that would allow modifications.
This syntax is not to be confused with handles that are themselves read-only, i.e. the handle cannot be
re-assigned to refer to a different object. Read-only handles like this are declared by adding the 'const' keyword
as a suffix after the '@' symbol.
<pre>
obj \@ const c = obj(); // read-only handle to a modifiable object
const obj \@ const d = obj(); // read-only handle to a non-modifiable object
</pre>
A read-only handle can only be initialized when declared.
*/
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