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Document that extend base must appear early in a class definition (#4401)

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Note: some of this is from the principle of information accumulation,
and some is from [proposal
#2760](https://github.com/carbon-language/carbon-lang/blob/trunk/proposals/p2760.md#class-inheritance).

Also: change terminology from "virtual override keywords" to "virtual
modifier keywords", to be consistent with our other modifier keywords.

---------

Co-authored-by: Josh L <josh11b@users.noreply.github.com>
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josh11b
2024-10-11 19:51:15 +00:00
committed by GitHub
co-authored by Josh L
parent b0cde707f8
commit 4994e13068
1 changed files with 18 additions and 7 deletions
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@@ -45,7 +45,7 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
- [Alias](#alias)
- [Inheritance](#inheritance)
- [Virtual methods](#virtual-methods)
- [Virtual override keywords](#virtual-override-keywords)
- [Virtual modifier keywords](#virtual-modifier-keywords)
- [Subtyping](#subtyping)
- [`Self` refers to the current type](#self-refers-to-the-current-type)
- [Constructors](#constructors)
@@ -1182,9 +1182,9 @@ direct call to the method does.
[Class functions](#class-functions) may not be declared virtual.
##### Virtual override keywords
##### Virtual modifier keywords
A method is declared as virtual by using a _virtual override keyword_ in its
A method is declared as virtual by using a _virtual modifier keyword_ in its
declaration before `fn`.
```
@@ -1204,7 +1204,7 @@ _non-virtual_. This means:
- they have an implementation in the current class, and that implementation
must work for all derived classes.
There are three virtual override keywords:
There are three virtual modifier keywords:
- `virtual` - This marks a method as not present in bases of this class and
having an implementation in this class. That implementation may be
@@ -1230,6 +1230,12 @@ There are three virtual override keywords:
| `abstract` | ✅ | ❌ | ❌ | _not present_<br />`virtual`<br />`abstract`<br />`impl` | `abstract`<br />`impl`<br />_may not be<br />mentioned if<br />`D` is not final_ |
| `impl` | ✅ | ✅ | ✅ | `virtual`<br />`abstract`<br />`impl` | `abstract`<br />`impl` |
Since validating a method with a virtual modifier keyword involves looking for
methods with the same name in the base class, virtual methods must be declared
after the `extend base` declaration when present in a class definition. This
simplifies the compiler, and follows the
[information accumulation principle](/docs/project/principles/information_accumulation.md).
#### Subtyping
A pointer to a base class, like `MyBaseClass*` is actually considered to be a
@@ -1246,11 +1252,16 @@ or _vtable_. Any calls to virtual methods will perform
the method using the function pointer in the vtable, to get the overridden
implementation from the most derived class that implements the method.
This data layout is reflected in the order of declarations in a class
definition. An `extend base` declaration, when present in a class definition,
must appear before any other declarations adding data to the class instances,
such as instance variables.
Since a final class may not be extended, the compiler can bypass the vtable and
use [static dispatch](https://en.wikipedia.org/wiki/Static_dispatch). In
general, you can use a combination of an abstract base class and a final class
instead of an extensible class if you need to distinguish between exactly a type
and possibly a subtype.
instead of an extensible class if you need to distinguish between "exactly a
type" and "possibly a subtype."
```
base class Extensible { ... }
@@ -1713,7 +1724,7 @@ declaration. Access modifiers are how Carbon supports
[encapsulation](#encapsulated-types).
The [access modifier](https://en.wikipedia.org/wiki/Access_modifiers) is written
before any [virtual override keyword](#virtual-override-keywords).
before any [virtual modifier keyword](#virtual-modifier-keywords).
**Rationale:** Carbon makes members public by default for a few reasons: