# Session summary — 2026-09-29 (b): VIRTUAL dispatch + showcase18

Suite **144/144**; self-hosting fixpoint **OK**
(image **2,310,300 bytes**). `master`, tree clean except the user's
uncommitted `compiler/toto.mod`.

This session completed the Clarion OOP story by adding **dynamic
dispatch**, then showcased it.

## Steps, tags and headline commits

| Step | Tag | Commit | What |
| --- | --- | --- | --- |
| VIRTUAL dispatch | `v3-virtual-dispatch` | `98d3067` | per-class vtables + subtype pointer assignment |
| Showcase | `v3-showcase18` | (this commit) | polymorphic Shape hierarchy |

## 1. VIRTUAL dispatch (vtables)

`VIRTUAL` methods now resolve at run time. A base-class pointer
holding a derived object calls the derived override:

```modula-2
TYPE
  CLASS Shape;  VIRTUAL PROCEDURE Area() : INTEGER; END Shape;
  CLASS Circle (Shape); VIRTUAL PROCEDURE Area() : INTEGER; END Circle;
...
VAR b : POINTER TO Shape; c : POINTER TO Circle;
BEGIN
  NEW(c); b := c;            (* subtype pointer assignment *)
  ExitCode := b^.Area()      (* Circle.Area, decided at run time *)
END
```

- **vtable** — each class with virtuals owns a slice of a shared
  pool: the parent's slots are inherited first; a same-named virtual
  method overrides in place; new ones append. Each method node
  records its `vslot`.
- **vptr** — the class introducing virtuals gets an 8-byte vtable
  pointer (offset 0 with no parent, else after the parent's fields);
  subclasses share it, so an object's vptr identifies its dynamic
  class.
- **emission** — `data $vt_<typeindex> = { l $Method_uid, … }`;
  a declared-but-unimplemented method is `l 0`.
- **init** — static class variables and `NEW` store the vptr.
- **dispatch** — a virtual call loads the vptr, fetches the slot, and
  calls indirectly; the receiver is the hidden first argument.
- **subtyping** — `POINTER TO Derived` assigns to `POINTER TO Base`
  (`IsSubclass`), which lets a base pointer hold a derived object.

Docs: `docs/summary_virtual-dispatch.md`.

## 2. Showcase

`compiler/tests/showcase18.mod` (`Showcase18`, exit 42): a `Shape`
hierarchy (`Circle`, `Square`) with a virtual `Area()`; an array of
`POINTER TO Shape` holds objects of both kinds and each area is
resolved at run time; the sum is asserted and printed:

```
showcase18 areas sum=139
```

(The number is built with `Conversions.IntToStr` + CHAR-array `+`,
tying in the earlier stdlib/string work.)

## Tests (144, was 143)

New: `t_virtual` (3) and `Showcase18` (42).

## Resume

```sh
cd .../m2compiler-V3
./bootstrap/fixpoint.sh                       # FIXPOINT OK
cd compiler && ./build.sh && ./run_tests.sh   # 144/144
```

## Remaining polish

- CLASS: bare sibling-method calls inside a method body, a class
  `BEGIN … END` init body, multiple parents.
- `VAL(LONGINT|REAL, x)` and long→int narrowing.
- More stdlib (`SysClock`, richer `Strings`/`Math`).