11 Object visiting

Visiting the containing objects of DGS nodes in the eviction queues

Consider the following xc++ code:

    $class X isa ceda::IObject
    {
        $indep int x;
        $dep   int y;
        $cache int z(int a) const { return a; }
    };

This generates code in which independent or dependent DGS nodes are "embedded" in X - either directly as members (for $indeps, $deps and nullary $cache functions) or as the mapped value type of std::maps for non-nullary $cache functions.

For all these embedded independent/dependent DGS nodes the object instance X is called the containing object. This typically implements IObject.

The CSpace GC visits the DGSystem object which visits its eviction queues which in turn calls the virtual function

    virtual void VisitContainingObject(IObjectVisitor& v) const = 0;

on every independent and dependent node in the eviction queue.

Since the eviction queue is just a linear list it is not necessary to trace the edges of the dependency graph to visit all DGS nodes.

The DGS nodes tend to be embedded in IObjects - i.e. the "containing object" implements IObject. The above virtual function should be implemented in order to visit the containing IObject.

VisitContainingObject is a bit of a misnomer because it is also used to visit $indep, $dep variables and also the mapped variables in $cache functions.

When a visit{...} block is specified on an $indep or $dep variable, an implementation of VisitContainingObject is generated which calls the provided code. WARNING: it doesn't perform the normal visiting of the containing object or the $indep or $dep variable.

Visiting the pointers to IObjects in the calculated values

Xcpp generates code to automatically visit the mapped values in $cache functions.

The visiting of the mapped values of cache functions can be disabled using <<--visit>>. For example:

    $cache-^ <<-visit>> void Foo(X& x) const
    {
        CalculateX(x);
    };

    $dep^ <<-visit>> X x
        invalidate { ... }
        calc { ... };

visit {...} blocks on DGS nodes

In the xc++ language a visit handler can be defined on $indep and $dep variables and $cache functions. For example:

    $mixin M
    {
        $dep- int y
            visit { }
            calc { y = foo(); };
    };

Although supported these are not very useful anymore. E.g. CedaJigsaw doesn't use them at all!