Man modifiers

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Revision as of 13:35, 11 June 2007

Description

A modifier changes the syntactic and/or semantic behaviour of an object-global variable or a function in an object. The existing modifiers are described below. To use a modifier just prepend it to the declaration. If several modifiers are to be used their order does not matter:

 private int bar;                         // example for a variable
 protected nomask int foo() { return 3; } // example for a function

For functions:

  • private -- such functions can only be called with an internal
            call from within this file. Not even inheriting
            objects can call these functions. You can nevertheless
            build an lfun-closure with #' out of a private function.
  • protected -- such functions can be called from within the object,
            or from inheriting objects; but in neither case
            with call_other(). It is possible to create #' closures
            or use symbol_function() from within the object.
            Its use is preferred over the older "static".
  • static -- such functions can be called from within the object
            in either way (internal call or with call_other()).
            Inheriting objects can call such functions.
            But it is not possible to call static functions from
            other objects via call_other().
            Note that an add_action() is treated like a call
            from within the object except the player who got the
            add_action() was forced (thus it is a simple way to
            secure an add_action() against forces, although this
            method has the severe disadvantages of raising an error
            at the force so better use the security system).
            Also efuns like call_out() or input_to() can call
            these functions if given as a string.
  • public -- this is the default type. Such functions can be called
            from within the file as well as from inheriting objects
            and other objects via call_other().
            To declare a function public only results in the
            impossibility to change the accessibility at the
            inherit statement (see below). No error will occur,
            only the type will not be modified by the inherit
            statement.
  • nomask -- such functions cannot be overridden by inheriting
            objects. If you have the fun foo() defined in your
            object and inherit an object which also has declared
            a function foo() and this nomask, you will get an
            compile error if you try to load your object.
            Furthermore a shadow will fail if it tries to shadow
            a nomask declared function.
  • varargs -- this changes the syntax of the function in a way that
            not all of the arguments in the declaration must be
            given at the call. This is often very usefull if some
            of the arguments shall be omitable (the omitted
            arguments are set to 0 if the function is called with
            fewer arguments than specified).
            This is mainly within the object really necessary;
            call_other()s usually (that is if they do not have a
            certain pragma ('man pragma')) do not need the called
            function to be declared varargs to omit any arguments,
            but it is good style to use this modifier to document
            the code by this.

For object-global variables:

  • private -- such variables can only be accessed from within the
            same object. Not even inheriting objects can access
            private variables.
            It is a good style to declare all internal variables
            private to prevent inheriting objects from accessing
            the variables directly without using functions.
  • nosave -- such variables are neither stored with save_object()
            nor restored with restore_object(). This can be very
            useful if you want a room to use save_object() and
            restore_object() to save your own defined variables
            but not the hundreds of variables inherited from a
            room-class (e.g. /complex/room). You then use the modifier
            at the inherit statement (see below).
            Note that nosave and private do not overlap in any
            way. They are absolutely independant.
            The driver may be compiled to not recognize this
            keyword ('static' is then to be used). If 'nosave'
            is available, the macro __LPC_NOSAVE__ is defined.
  • static -- the old name for 'nosave'. Its use is deprecated.
  • public -- declares the variable public. It cannot be declared
            private or static by inheriting. No error will occur,
            only the type will not be modified by the inherit
            statement.

It is not good style to let inheriting objects have access to internal variables so declare them as private and offer functions to query and change the variables if possible.

It is also possible to redeclare all variables and/or functions of an inherited object for the own object at the inheriting statement:

private functions nosave variables inherit "complex/room";
public variables inherit "complex/room";
private functions inherit "complex/room";

To redeclare a function or a variable declared public in the inherited object to be private or protected is not possible.

There also exists a modifier explicitly for the inherit statement:

  • virtual -- inherits the given object virtually. This only makes
            sense in a complex inherit tree.
            If an object is inherited normally (not virtually)
            twice somewhere in the inherit tree the intern
            variables exist twice. If inherited virtually they
            exist only once.
            Example:
            A inherits B and C.
            B inherits D.
            C inherits D.
            If the inheritance of D is virtual in B and C
            D's variables exist only once in A. If A changes
            D's variables via functions of B this also changes
            the variables of D as known by C.

virtual: non-virtual: A A / \ / \ B C B C \ / | | D D D

To simplify the adoption of existing code, LPC allows to specify a default visibility for functions and variables, using a syntax similar to the inherit syntax:

 default private;

All variables and functions are by default private.

 default private variables public functions;

All variables are by default private, but functions are public.

Only the modifiers 'private', 'public' and 'protected' (and 'static' for functions only) are allowed here.

The default visibility thus set affects only variables/functions with no explicite visibility:

 default private;
 int private_var;
 public int public_var;

The definition is valid from the point of the 'default' statement until the end of the file, or until the next 'default' statement:

 default private;
 int private_var;
 default public;
 int public_var;

Note that this default visibility does not affect inherits.


History

The modifier 'static' for variables was renamed to 'nosave' with LDMud 3.2.8. 'static' is still recognized as an alias.

The default visibility was added in LDMud 3.2.9 as experimental feature.

See Also

closures(LPC), inheritance(LPC), functions(LPC), types(LPC)

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