This repo captures the finished product in Sean Parent's Inheritance is the Base Class of Evil and Value Semantics and Concepts-based Polymorphism talks.
renamed classes -------------I renamed classes to allow for multiple uses of what Sean called a concept. The new names describe what it is rather than describe it's role in the architecture:
| Sean's code | Now | Description |
|---|---|---|
| document_t | document_t | a container of objects that conform to the draw API |
| history_t | history_t | a container of documents |
| object_t | drawable_ptr | pointer to a vtable which invokes the draw API |
| concept_t | draw_API_vtable | vtable of functions which invoke the draw API |
| model_t | draw_API_binding | templated binding of (inferred) type to draw API |
drawable_ptr, draw_API_vtable, draw_API_binding are all scoped to the document_t. To aid in comprehension, everything that can be private is.
draw_ is called call_draw ('cause that's what it does).
The original demo code called draw(current(h), cout, 0). It now calls draw(h, cout, 0); which captures the entire history object.
The member-functions branch treats draw function as an object method. This simplifies the library at the cost of requiring more ceremony for PODs. (The presumption is that, in normal use, most concept-based polymorphism will be applied to more complex structures.) See the difference to compare the syntax.
The document_t object holds a vector of pointers to things it needs to be able to draw (drawable_ptr). These point to an object with an interface contract defined by draw_API_vtable. When invoked in the calling code, the templated constructror for drawable_ptr invokes the appropriate (templated) draw_API_binding for the calling type. This barfs if the calling type doesn't have an associated draw function (a method in the member-functions branch or a global function in master). If it doesn't barf, the compiler has ensured the appropriate binding and plugged it into the vtable.