source: doc/proposals/virtual.txt @ e1e4aa9

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Last change on this file since e1e4aa9 was e1e4aa9, checked in by Rob Schluntz <rschlunt@…>, 4 years ago

More cleanup on revised virtual proposal

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1Proposal for virtual functionality
2
3There are two types of virtual inheritance in this proposal, relaxed
4(implicit) and strict (explicit). Relaxed is the simpler case that uses the
5existing trait system with the addition of trait references and vtables.
6Strict adds some constraints and requires some additional notation but allows
7for down-casting.
8
9Relaxed Virtual Inheritance:
10
11Imagine the following code :
12
13trait drawable(otype T) {
14      void draw(T* );
15};
16
17struct text {
18      char* text;
19};
20
21void draw(text*);
22
23struct line{
24      vec2 start;
25      vec2 end;
26};
27
28void draw(line*);
29
30While all the members of this simple UI support drawing, creating a UI that
31easily supports both these UI requires some tedious boiler-plate code:
32
33enum type_t { text, line };
34
35struct widget {
36      type_t type;
37      union {
38            text t;
39            line l;
40      };
41};
42
43void draw(widget* w) {
44      switch(w->type) {
45            case text : draw(&w->text); break;
46            case line : draw(&w->line); break;
47            default : handle_error(); break;
48      }
49}
50
51While this code will work as implemented, adding any new widgets or any new
52widget behaviors requires changing existing code to add the desired
53functionality. To ease this maintenance effort required CFA introduces the
54concept of trait references.
55
56Using trait references to implement the above gives the following :
57
58trait drawable objects[10];
59fill_objects(objects);
60
61while(running) {
62      for(drawable object : objects) {
63            draw(object);
64      }
65}
66
67The keyword trait is optional (by the same rules as the struct keyword). This
68is not currently supported in CFA and the lookup is not possible to implement
69statically. Therefore we need to add a new feature to handle having dynamic
70lookups like this.
71
72What we really want to do is express the fact that calling draw() on a trait
73reference should find the underlying type of the given parameter and find how
74it implements the routine, as in the example with the enumeration and union.
75
76For instance specifying that the drawable trait reference looks up the type
77of the first argument to find the implementation would be :
78
79trait drawable(otype T) {
80      void draw(virtual T* );
81};
82
83This could be implied in simple cases like this one (single parameter on the
84trait and single generic parameter on the function). In more complex cases it
85would have to be explicitly given, or a strong convention would have to be
86enforced (e.g. implementation of trait functions is always drawn from the
87first polymorphic parameter).
88
89Once a function in a trait has been marked as virtual it defines a new
90function that takes in that trait's reference and then dynamically calls the
91underlying type implementation. Hence a trait reference becomes a kind of
92abstract type, cannot be directly instantiated but can still be used.
93
94One of the limitations of this design is that it does not support double
95dispatching, which concretely means traits cannot have routines with more than
96one virtual parameter. The program must have a single table to look up the
97function on. Using trait references with traits with more than one parameter
98is also restricted, initially forbidden, see extension.
99
100Extension: Multi-parameter Virtual Traits:
101
102This implementation can be extended to traits with multiple parameters if
103one is called out as being the virtual trait. For example :
104
105trait iterator(otype T, dtype Item) {
106        Maybe(Item) next(virtual T *);
107}
108
109iterator(int) generators[10];
110
111Which creates a collection of iterators that produce integers, regardless of
112how those iterators are implemented. This may require a note that this trait
113is virtual on T and not Item, but noting it on the functions may be enough.
114
115
116Strict Virtual Inheritance:
117
118One powerful feature relaxed virtual does not support is the idea of down
119casting. Once something has been converted into a trait reference there is
120very little we can do to recover and of the type information, only the trait's
121required function implementations are kept.
122
123To allow down casting strict virtual requires that all traits and structures
124involved be organized into a tree. Each trait or struct must have a unique
125position on this tree (no multiple inheritance).
126
127This is declared as follows :
128
129trait error(otype T) virtual {
130        const char * msg(T *);
131}
132
133trait io_error(otype T) virtual error {
134        FILE * src(T *);
135}
136
137struct eof_error virtual io_error {
138        FILE * fd;
139};
140
141So the trait error is the head of a new tree and io_error is a child of it.
142
143Also the parent trait is implicitly part of the assertions of the children,
144so all children implement the same operations as the parent. By the unique
145path down the tree, we can also uniquely order them so that a prefix of a
146child's vtable has the same format as its parent's.
147
148This gives us an important extra feature, runtime checking of the parent-child
149relationship with a C++ dynamic_cast like operation. Allowing checked
150conversions from trait references to more particular references, which works
151if the underlying type is, or is a child of, the new trait type.
152
153Extension: Multiple Parents
154
155Although each trait/struct must have a unique position on each tree, it could
156have positions on multiple trees. All this requires is the ability to give
157multiple parents, as here :
158
159trait region(otype T) virtual drawable, collider;
160
161The restriction being, the parents must come from different trees. This
162object (and all of its children) can be cast to either tree. This is handled
163by generating a separate vtable for each tree the structure is in.
164
165Extension: Multi-parameter Strict Virtual
166
167If a trait has multiple parameters then one must be called out to be the one
168we generate separate vtables for, as in :
169
170trait example(otype T, otype U) virtual(T) ...
171
172This can generate a separate vtable for each U for which all the T+U
173implementations are provided. These are then separate nodes in the tree (or
174the root of different trees) as if each was created individually. Providing a
175single unique instance of these nodes would be the most difficult aspect of
176this extension, possibly intractable, though with sufficient hoisting and
177link-once duplication it may be possible.
178
179Example:
180
181trait argument(otype T) virtual {
182        char short_name(virtual T *);
183        bool is_set(virtual T *);
184};
185
186trait value_argument(otype T, otype U) virtual(T) argument {
187        U get_value(virtual T *);
188};
189
190Extension: Structural Inheritance
191
192Currently traits must be the internal nodes and structs the leaf nodes.
193Structs could be made internal nodes as well, in which case the child structs
194would likely structurally inherit the fields of their parents.
195
196
197Storing the Virtual Lookup Table (vtable):
198
199We have so far been silent on how the vtable is created, stored and accessed.
200
201Creation happens at compile time. Function pointers are found by using the
202same best match rules as elsewhere (additional rules for defaults from the
203parent may or may not be required). For strict virtual this must happen at the
204global scope and forbidding static functions, to ensure that a single unique
205vtable is created. Similarly, there may have to be stricter matching rules
206for the functions that go into the vtable, possibly requiring an exact match.
207Relaxed virtual could relax both restrictions, if we allow different vtable
208at different conversion (struct to trait reference) sites. If it is allowed
209local functions being bound to a vtable could cause issues when they go out
210of scope, however this should follow the lifetime rules most C programs
211already follow implicitly.
212
213Most vtables should be stored statically, the only exception being some of
214the relaxed vtables that could have local function pointers. These may be able
215to be stack allocated. All vtables should be immutable and require no manual
216cleanup.
217
218Access has two main options:
219
220The first is through the use of fat pointers, or a tuple of pointers. When the
221object is converted to a trait reference, the pointers to its vtables are
222stored along side it.
223
224This allows for compatibility with existing structures (such as those imported
225from C) and is the default storage method unless a different one is given.
226
227The other is by inlining the vtable pointer as "intrusive vtables". This adds
228a field to the structure to the vtable. The trait reference then has a single
229pointer to this field, the vtable includes an offset to find the beginning of
230the structure again.
231
232This is used if you specify a vtable field in the structure. If given in the
233trait the vtable pointer in the trait reference can then become a single
234pointer to the vtable field and use that to recover the original object
235pointer as well as retrieve all operations.
236
237trait drawable(otype T) {
238        vtable drawable;
239};
240
241struct line {
242        vtable drawable;
243        vec2 start;
244        vec2 end;
245};
246
247This inline code allows trait references to be converted to plain pointers
248(although they still must be called specially). The vtable field may just be
249an opaque block of memory or it may allow user access to the vtable. If so
250then there should be some way to retrieve the type of the vtable, which will be
251autogenerated and often unique.
252
253
254Keyword Usage:
255
256It may be desirable to add fewer new keywords than discussed in this proposal.
257It is possible that "virtual" could replace both "vtable" above with
258unambiguous contextual meaning. However, for purposes of clarity in the design
259discussion it is beneficial to keep the keywords for separate concepts distinct.
260
261
262Trait References and Operations:
263
264sizeof(drawable) will return the size of the trait object itself. However :
265
266line a_line;
267drawable widget = a_line;
268sizeof(widget);
269
270Will instead return the sizeof the underlying object, although the trait must
271require that its implementation is sized for there to be a meaningful value
272to return. You may also get the size of the trait reference with
273
274sizeof(&widget);
275
276Calling free on a trait reference will free the memory for the object. It will
277leave the vtables alone, as those are (always?) statically allocated.
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