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#############################################################################
##
## This file is part of GAP, a system for computational discrete algebra.
## This file's authors include Steve Linton.
##
## Copyright of GAP belongs to its developers, whose names are too numerous
## to list here. Please refer to the COPYRIGHT file for details.
##
## SPDX-License-Identifier: GPL-2.0-or-later
##
## This file contains the definition of operations and functions for
## weak pointers.
##
## WP objects behave in most respects like mutable plain lists, except that
## they do not keep their subobjects alive, so that one sees things like.
##
## gap> w := WeakPointerObj([[1,2]]);;
## gap> IsBound(w[1]);
## true
## gap> CollectGarbage( true );
## gap> w;
## WeakPointerObj([ [ ] ]);
##
## for this reason the common idiom
##
## if IsBound(w[i]) then
## DoSomethigWith(w[i]);
## fi;
##
## is not really safe.
##
## A solution is provided by the kernel function ElmWPObj (weakptr.c), which
## returns fail if the entry is (1) unbound or (2) bound to the value fail.
## Since fail will never be collected as garbage, a subsequent call to IsBound
## can safely be used to distinguish these two cases, as in:
##
## x := ElmWPObj(w,i);
## if x <> fail or IsBound(w[i]) then
## DoSomethingWith(x);
## else
## DoSomethingElse();
## fi;
##
#############################################################################
##
##
#C IsWeakPointerObject( <obj> ) . . . . . . . . . . category of WP objects
##
## All WP objects have to be mutable (a stronger term like volatile would
## be appropriate),
## but this cannot be expressed via an explicit implication;
## note that `Immutable' is handled by the kernel.
##
DeclareCategoryKernel( "IsWeakPointerObject",
IsList and IsSmallList,
IsWPObj );
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