Update virtual table policy diagram.md

Author: thecppzoo

design/virtual table policy diagram.md

@@ -1 +1,27 @@
+# Diagrams of operations in the Virtual Table Policy and Affordance Framework.
+
+## Note: Some of the identifiers in the framework come from very old times, when the concepts where more primitive.
+
+## The base value manager
+
+The *policy* tells `AnyContainer` what its local storage is going to be, via the member `MemoryLayout`.  You may think of the "Memory Layout" as the base ***value manager***.
+
+The ownership operations (destruction, moving, copying) require direct support from the framework, that's why they must be provided by the "value managers", via functions like `copyOp`, `move`, `destructor`.  Everything else is provided directly by the ***affordances***.  The value managers implement `copyOp`, `move`, `destructor` deferring to the affordances, this allows the user to choose any mechanism.  For example, you can implement and select an affordance of destruction that performs *tracing* of the destruction of some opt-in objects.
+
+The most important feature of this flavor of the framework is to use virtual tables as the *type switching*.  The base value manager inherits from a "virtual table holder" class that controls access to the virtual table:
+
+1. The `VTHolder` is a base class to guarantee the virtual table pointer is the first member in the memory layout of the base value manager, for performance.
+2. The type of the virtual table is the aggregation of the `VTableEntry` types of each affordance: [`struct VTable: AffordanceSpecifications::VTableEntry... {`](https://github.com/thecppzoo/zoo/blob/d6435fc984ee0bde31979f7908a73473f61ac4bd/inc/zoo/Any/VTablePolicy.h#L247C5-L247C60)
+3. The `VTHolder` allows every affordance to access its part of the virtual table via the template member `vTable`, if `Affordance` is the type representing an affordance, `baseManager->template vTable<Affordance>()` returns that section of the virtual table.
+4. The base value manager is the provider of the implementations for the default constructed `AnyContainer`.
+    1. Implementations are either `constexpr` data, or more typically, the pointers to the functions that implement the behaviors.  The base manager must defer to the affordances for these values, in particular to the `Default` class-members of the affordances, which are assumed to be of their `VTableEntry` type [^default implementations]
+6. The so-called "concrete value managers" provide the implementations for the value they manage.
+
+[^default implementations]:
+```c++
+        constexpr static inline VTable Default = {
+            AffordanceSpecifications::template Default<Container>...
+        };
+```
+[from here](https://github.com/thecppzoo/zoo/blob/d6435fc984ee0bde31979f7908a73473f61ac4bd/inc/zoo/Any/VTablePolicy.h#L274-L276).