############################################################################# ## ## 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. ##
### but this cannot be expressed via an explicit implication;
java.lang.StringIndexOutOfBoundsException: Index 5 out of bounds for length 2
IsWPObj );
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