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Merged
jbcoe merged 3 commits into from
Jun 11, 2026
Merged

Fix typos #92

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1ebe71f
Edits after discussion with Tony van Eerd
jbcoe Jun 11, 2026
a60979f
Fix typos
jbcoe Jun 11, 2026
768b780
Merge remote-tracking branch 'origin/main' into fix-typos
jbcoe Jun 11, 2026
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70 changes: 43 additions & 27 deletions DRAFT.md
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Original file line number Diff line number Diff line change
Expand Up @@ -47,6 +47,7 @@ and code injection and focuses solely on the design of the class templates

- Support zero-cost conversion from a compatible `protocol` or `protocol_view` to a narrower target interface (subtype substitution).
- Add `any` to the standard library types equivalence table.
- Add `any` to the standard library types equivalence table.

### Changes in revision R1

Expand Down Expand Up @@ -89,10 +90,10 @@ and leading to inconsistent semantics across libraries.

This situation can be understood in terms of the broader polymorphism design space:

| | Static | Dynamic |
|-------------------|:------------:|:--------------:|
| Nominal typing | Templates | Virtual |
| Structural typing | Concepts | --- |
| | Static | Dynamic |
| ----------------- | :-------: | :-----: |
| Nominal typing | Templates | Virtual |
| Structural typing | Concepts | --- |

The absence of a language-supported mechanism for dynamic structural typing explains
the proliferation of type-erasure-based abstractions. Each such abstraction can be
Expand All @@ -103,10 +104,10 @@ Protocols unify and generalise existing type-erasure patterns, providing a consi
non-intrusive mechanism for expressing dynamic structural polymorphism, while also
providing consistent support for allocators:

| | Static | Dynamic |
|-------------------|:------------:|:--------------:|
| Nominal typing | Templates | Virtual |
| Structural typing | Concepts | Protocol |
| | Static | Dynamic |
| ----------------- | :-------: | :------: |
| Nominal typing | Templates | Virtual |
| Structural typing | Concepts | Protocol |

## Design

Expand Down Expand Up @@ -159,6 +160,7 @@ move assignment (and allocator-extended equivalents) are generated unconditional
```c++
template <typename Allocator>
class protocol<I, Allocator=std::allocator<void>> {
public:

// Default constructor.
explicit constexpr protocol(); // conditionally-generated
Expand Down Expand Up @@ -245,6 +247,8 @@ class protocol<I, Allocator=std::allocator<void>> {
```c++
template <>
class protocol_view<I> {
public:

// Constructor from any mutable conforming object.
template <typename T>
constexpr protocol_view(T& obj) noexcept;
Expand Down Expand Up @@ -290,6 +294,8 @@ class protocol_view<I> {
```c++
template <>
class protocol_view<const I> {
public:

// Constructor from any const conforming object.
template <typename T>
constexpr protocol_view(const T& obj) noexcept;
Expand Down Expand Up @@ -368,13 +374,20 @@ existing set of function-objects.
Consider the structural types below:

```c++
struct Any {};
struct Any {
// All special member functions are defaulted.
};
```

```c++
struct MoveOnlyAny {
// Deleted copy constructor and copy assignment.
MoveOnlyAny(const MoveOnlyAny&) = delete;
MoveOnlyAny& operator=(const MoveOnlyAny&) = delete;

// Defaulted move constructor and move assignment.
MoveOnlyAny(MoveOnlyAny&&) = default;
MoveOnlyAny& operator=(MoveOnlyAny&&) = default;
};
```

Expand Down Expand Up @@ -437,18 +450,18 @@ There is currently no function-type in the standard library that can represent a
overload set. The table below is illustrative of how flexible `protocol` and
`protocol_view` are:

| Standard library type | Protocol equivalent |
| :------------------------------------------ | :----------------------------------------------- |
| `any` | `protocol<Any>` |
| ??? |`protocol<MoveOnlyAny>` |
| `copyable_function<R(Args...) const>` | `protocol<Function<R, Args...>>` |
| `move_only_function<R(Args...) const>` | `protocol<MoveOnlyFunction<R, Args...>>` |
| `function_ref<R(Args...) const>` | `protocol_view<Function<R, Args...>>` |
| `copyable_function<R(Args...)>` | `protocol<MutatingFunction<R, Args...>>` |
| `move_only_function<R(Args...)>` | `protocol<MoveOnlyMutatingFunction<R, Args...>>` |
| `function_ref<R(Args...)>` | `protocol_view<MutatingFunction<R, Args...>>` |
| ??? | `protocol<OverloadedFunction>` |
| ??? | `protocol_view<OverloadedFunction>` |
| Standard library type | Protocol equivalent |
| :------------------------------------- | :----------------------------------------------- |
| `any` | `protocol<Any>` |
| ??? | `protocol<MoveOnlyAny>` |
| `copyable_function<R(Args...) const>` | `protocol<Function<R, Args...>>` |
| `move_only_function<R(Args...) const>` | `protocol<MoveOnlyFunction<R, Args...>>` |
| `function_ref<R(Args...) const>` | `protocol_view<Function<R, Args...>>` |
| `copyable_function<R(Args...)>` | `protocol<MutatingFunction<R, Args...>>` |
| `move_only_function<R(Args...)>` | `protocol<MoveOnlyMutatingFunction<R, Args...>>` |
| `function_ref<R(Args...)>` | `protocol_view<MutatingFunction<R, Args...>>` |
| ??? | `protocol<OverloadedFunction>` |
| ??? | `protocol_view<OverloadedFunction>` |

### Comparison with proxy

Expand Down Expand Up @@ -498,12 +511,12 @@ object is determined by the choice of pointer, not by `proxy`.

The table below summarises the main design choices side by side.

| Aspect | `protocol` | `proxy` |
| :--- | :--- | :--- |
| Interface definition | C++ struct | `facade_builder` + dispatch objects (explicit) |
| Interaction syntax | `p.draw()` | `p->draw()` |
| Layout constraints | Implementation defined | Encoded in the Facade type |
| Ownership model | Explicit | Erased |
| Aspect | `protocol` | `proxy` |
| :------------------- | :--------------------- | :--------------------------------------------- |
| Interface definition | C++ struct | `facade_builder` + dispatch objects (explicit) |
| Interaction syntax | `p.draw()` | `p->draw()` |
| Layout constraints | Implementation defined | Encoded in the Facade type |
| Ownership model | Explicit | Erased |

### Design Alternatives

Expand Down Expand Up @@ -566,6 +579,9 @@ properties required by this proposal.
The authors would like to thank Billy Baker, Tony van Eerd and the BSI C++
Panel for suggestions and useful discussion.

The authors would like to thank Billy Baker, Tony van Eerd and the BSI C++
Panel for suggestions and useful discussion.

## References

[PEP 544] _Protocols: Structural subtyping (static duck typing)_.
Expand Down
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