Logtalk reference manual
Control construct: []/1
Contents > Reference Manual > Control Constructs > []/1

## []/1

#### Description

    [Object::Message]
    [{Proxy}::Message]

This control construct allows the programmer to send a message to an object while preserving the original sender. It is mainly used in the definition of object handlers for unknown messages. This functionality is usually know as delegation but be aware that this is an overloaded word that can mean different things in different object-oriented programming languages.

To prevent using of this control construct to break object encapsulation, an attempt to delegate a message to same object as the original sender results in an error. The remaining error conditions are the same as the `::/2` control construct.

Note that, despite the correct functor for this control construct being (traditionally) `'.'/2`, we refer to it as `[]/1` simply to emphasize that the syntax is a list with a single element.

#### Template and modes

    [+object_identifier::+callable]
    [{+object_identifier}::+callable]

#### Errors

Object and the original sender are the same object:
  `permission_error(access, object, Sender)`

Either Object or Message is a variable:
  `instantiation_error`
Object is not a valid object identifier:
  `type_error(object_identifier, Object)`
Message is neither a variable nor a callable term:
  `type_error(callable, Message)`
Message, with predicate indicator Functor/Arity, is declared private:
  `permission_error(access, private_predicate, Functor/Arity)`
Message, with predicate indicator Functor/Arity, is declared protected:
  `permission_error(access, protected_predicate, Functor/Arity)`
Message, with predicate indicator Functor/Arity, is not declared:
  `existence_error(predicate_declaration, Functor/Arity)`
Object does not exist:
  `existence_error(object, Object)`

Proxy is a variable:
  `instantiation_error`
Proxy is not a valid object identifier:
  `type_error(object_identifier, Proxy)`
The predicate Proxy does not exist in the user pseudo-object:
  `existence_error(procedure, ProxyFunctor/ProxyArity)`

#### Examples

    forward(Message) :-
        [backup::Message].