5.23.3.11 objects.fs Implementation

An object is a piece of memory, like one of the data structures described with struct...end-struct. It has a field object-map that points to the method map for the object's class.

The method map¹ is an array that contains the execution tokens (xts) of the methods for the object's class. Each selector contains an offset into a method map.

selector is a defining word that uses CREATE and DOES>. The body of the selector contains the offset; the DOES> action for a class selector is, basically:

     ( object addr ) @ over object-map @ + @ execute

Since object-map is the first field of the object, it does not generate any code. As you can see, calling a selector has a small, constant cost.

A class is basically a struct combined with a method map. During the class definition the alignment and size of the class are passed on the stack, just as with structs, so field can also be used for defining class fields. However, passing more items on the stack would be inconvenient, so class builds a data structure in memory, which is accessed through the variable current-interface. After its definition is complete, the class is represented on the stack by a pointer (e.g., as parameter for a child class definition).

A new class starts off with the alignment and size of its parent, and a copy of the parent's method map. Defining new fields extends the size and alignment; likewise, defining new selectors extends the method map. overrides just stores a new xt in the method map at the offset given by the selector.

Class binding just gets the xt at the offset given by the selector from the class's method map and compile,s (in the case of [bind]) it.

I implemented this as a value. At the start of an m:...;m method the old this is stored to the return stack and restored at the end; and the object on the TOS is stored TO this. This technique has one disadvantage: If the user does not leave the method via ;m, but via throw or exit, this is not restored (and exit may crash). To deal with the throw problem, I have redefined catch to save and restore this; the same should be done with any word that can catch an exception. As for exit, I simply forbid it (as a replacement, there is exitm).

inst-var is just the same as field, with a different DOES> action:

     @ this +

Similar for inst-value.

Each class also has a word list that contains the words defined with inst-var and inst-value, and its protected words. It also has a pointer to its parent. class pushes the word lists of the class and all its ancestors onto the search order stack, and end-class drops them.

An interface is like a class without fields, parent and protected words; i.e., it just has a method map. If a class implements an interface, its method map contains a pointer to the method map of the interface. The positive offsets in the map are reserved for class methods, therefore interface map pointers have negative offsets. Interfaces have offsets that are unique throughout the system, unlike class selectors, whose offsets are only unique for the classes where the selector is available (invokable).

This structure means that interface selectors have to perform one indirection more than class selectors to find their method. Their body contains the interface map pointer offset in the class method map, and the method offset in the interface method map. The does> action for an interface selector is, basically:

     ( object selector-body )
     2dup selector-interface @ ( object selector-body object interface-offset )
     swap object-map @ + @ ( object selector-body map )
     swap selector-offset @ + @ execute

where object-map and selector-offset are first fields and generate no code.

As a concrete example, consider the following code:

     interface
       selector if1sel1
       selector if1sel2
     end-interface if1
     
     object class
       if1 implementation
       selector cl1sel1
       cell% inst-var cl1iv1
     
     ' m1 overrides construct
     ' m2 overrides if1sel1
     ' m3 overrides if1sel2
     ' m4 overrides cl1sel2
     end-class cl1
     
     create obj1 object dict-new drop
     create obj2 cl1    dict-new drop

The data structure created by this code (including the data structure for object) is shown in the figure, assuming a cell size of 4.