deferred

#deferred

##Syntax A deferredDeclaration is:

deferred subprogramHeader

##Description A procedure or function is declared to be deferred when you want to be able to override the subprogram in an expansion. The procedure or function must be in a module, monitor or class.

##Example The display procedure is deferred in this class of stacks to allow various ways of graphically displaying the stack on the screen:

An expansion to the stack class can give a body for display, as in:

The following creates a stack that can be displayed and displays it:

    class stack
        export push, pop
        � local declarations �
        � declarations of the push and pop procedures �
        deferred procedure display (howbig : int)
    end stack        class stackWithSimpleDisplay
        body procedure display      % (howbig : int)
               � graphically display the stack on the screen �
        end display
    end stackWithSimpleDisplay        var p : ^stackWithSimpleDisplay
    new p
    �
    p -> display (25)       % Display the stack on the screen

##Details A deferred procedure is resolved by giving its body. This can be done in the scope (module, monitor or class) containing the deferred declaration (following the deferred declaration) or in any expansion of that scope. Only one resolution per scope is allowed. Unresolved subprograms can be called, but they immediately abort.

All exported subprograms are implicitly deferred and can be overridden in expansions.

During initialization of a module, monitor or object of a class, deferred subprograms (including exported subprograms) cannot be called. This restriction prevents accessing an object before it is fully initialized.

A deferred declaration must not appear in the main program.

##See also module.html, monitor.html and class.html. See also export.html list, import.html list, inherit.html list, implement.html list and [implement_by.html](implement by) list.