This chapter describes the use of the kernel feature of weak pointers. This feature is primarily intended for use only in GAP internals, and should be used extremely carefully otherwise.
The garbage collector (see Section 7.12-1) is the part of the kernel that manages memory in the user's workspace. It will normally only reclaim the storage used by an object when the object cannot be reached as a subobject of any GAP variable, or from any reference in the kernel. We say that any link to object \(a\) from object \(b\) keeps object \(a\) alive
, as long as \(b\) is alive. It is occasionally convenient, however, to have a link to an object which does not keep it alive, and this is a weak pointer. The most common use is in caches, and similar structures, where it is only necessary to remember how to solve problem \(x\) as long as some other link to \(x\) exists.
The following section 86.1 describes the semantics of the objects that contain weak pointers. Following sections describe the functions available to manipulate them.
A weak pointer object is similar to a mutable plain list, except that it does not keep its subobjects alive during a garbage collection. From the GAP viewpoint this means that its entries may become unbound, apparently spontaneously, at any time. Considerable care is therefore needed in programming with such an object.
‣ WeakPointerObj( list ) | ( function ) |
WeakPointerObj returns a weak pointer object which contains the same subobjects as the list list, that is it returns a shallow weak copy of list.
gap> w := WeakPointerObj( [ 1, , [2,3], fail, rec( a := 1) ] ); WeakPointerObj( [ 1, , [ 2, 3 ], fail, rec( a := 1 ) ] )
After some computations involving garbage collections (but not necessarily in the first garbage collection after the above assignment), GAP will notice that the list and the record stored in w are not referenced by other objects than w, and that therefore these entries may disappear.
gap> CollectGarbage( true ); ... (perhaps more computations and garbage collections) ... gap> CollectGarbage( true ); gap> w; WeakPointerObj( [ 1, , , fail ] )
Note that w has failed to keep its list and record subobjects alive during the garbage collections. Certain subobjects, such as small integers and elements of small finite fields, are not stored in the workspace, and so are not subject to garbage collection, while certain other objects, such as the boolean values, are always reachable from global variables or the kernel and so are never garbage collected.
Subobjects reachable without going through a weak pointer object do not evaporate, as in:
gap> w := WeakPointerObj( [ 1, , , fail ] ); WeakPointerObj( [ 1, , , fail ] ) gap> l := [1,2,3];; gap> w[1] := l;; gap> w; WeakPointerObj( [ [ 1, 2, 3 ], , , fail ] ) gap> CollectGarbage( true ); gap> w; WeakPointerObj( [ [ 1, 2, 3 ], , , fail ] )
Note also that the global variables last, last2 and last3 will keep things alive –this can be confusing when debugging.
‣ SetElmWPObj( wp, pos, val ) | ( function ) |
‣ UnbindElmWPObj( wp, pos ) | ( function ) |
‣ ElmWPObj( wp, pos ) | ( function ) |
‣ IsBoundElmWPObj( wp, pos ) | ( function ) |
‣ LengthWPObj( wp ) | ( function ) |
The functions SetElmWPObj and UnbindElmWPObj set and unbind entries in a weak pointer object.
The function ElmWPObj returns the element at position pos of the weak pointer object wp, if there is one, and fail otherwise. A return value of fail can thus arise either because (a) the value fail is stored at position pos, or (b) no value is stored at position pos. Since fail cannot vanish in a garbage collection, these two cases can safely be distinguished by a subsequent call to IsBoundElmWPObj, which returns true if there is currently a value bound at position pos of wp and false otherwise.
Note that it is not safe to write:
if IsBoundElmWPObj(w,i) then x:= ElmWPObj(w,i); fi;
and treat x as reliably containing a value taken from w, as a badly timed garbage collection could leave x containing fail. Instead use
x := ElmWPObj(w,i); if x <> fail or IsBoundElmWPObj(w,i) then . . ..
Here is an example.
gap> w := WeakPointerObj( [ 1, , [2,3], fail, rec() ] ); WeakPointerObj( [ 1, , [ 2, 3 ], fail, rec( ) ] ) gap> SetElmWPObj(w,5,[]); gap> w; WeakPointerObj( [ 1, , [ 2, 3 ], fail, [ ] ] ) gap> UnbindElmWPObj(w,1); gap> w; WeakPointerObj( [ , , [ 2, 3 ], fail, [ ] ] ) gap> ElmWPObj(w,3); [ 2, 3 ] gap> ElmWPObj(w,1); fail
Now after some computations and garbage collections \(\ldots\)
gap> 2;; 3;; 4;; CollectGarbage( true ); # clear last, last2, last3
\(\ldots\) we get the following.
gap> ElmWPObj(w,3); fail gap> w; WeakPointerObj( [ , , , fail ] ) gap> ElmWPObj(w,4); fail gap> IsBoundElmWPObj(w,3); false gap> IsBoundElmWPObj(w,4); true
Weak pointer objects are members of ListsFamily and the categories IsList (21.1-1) and IsMutable (12.6-2). Methods based on the low-level functions in the previous section, are installed for the list access operations, enabling them to be used as lists. However, it is not recommended that these be used in programming. They are supplied mainly as a convenience for interactive working, and may not be safe, since functions and methods for lists may assume that after IsBound(w[i]) returns true, access to w[i] is safe.
A ShallowCopy (12.7-1) method is installed, which makes a new weak pointer object containing the same objects as the original.
It is possible to apply StructuralCopy (12.7-2) to a weak pointer object, obtaining a new weak pointer object containing copies of the objects in the original. This may not be safe if a badly timed garbage collection occurs during copying.
Applying Immutable (12.6-3) to a weak pointer object produces an immutable plain list containing immutable copies of the objects contained in the weak pointer object. An immutable weak pointer object is a contradiction in terms.
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