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+---
+marp: true
+theme: default
+paginate: true
+size: 16:9
+#footer: https://github.com/jbcoe/cc-protocol
+---
+
+# Structural Subtyping for C++
+
+## [P4148R0](https://wg21.link/P4148R0)
+
+### 11 May 2026
+
+---
+
+# Introduction
+
+We propose adding two new class templates to the C++ Standard Library to support structural sub-typing at runtime:
+
+```c++
+template <typename T, typename Allocator = std::allocator<T>>
+class protocol;
+```
+
+```c++
+template <typename T>
+class protocol_view;
+```
+
+---
+
+# Polymorphism
+
+polymorphism (from the Greek for "many forms") is the ability of different objects to respond to the same function call in their own specific ways.
+
+C++ achieves polymorphism through mechanisms: Compile-time and Runtime.
+
+---
+
+# Compile-time Polymorphism
+
+Overloaded functions or function templates allow the compiler to detemrine which function to call during the build process:
+
+```c++
+double func(const A& a);
+double func(const B& b);
+double func(const C& c);
+```
+
+```c++
+<template typename T>
+double func(const T& t);
+```
+
+---
+
+# Runtime Polymorphism
+
+Inheritance-based polymorphism enables runtime dispatch by using base-class interfaces to invoke overridden virtual functions across an extensible, open set of derived types.
+
+```c++
+struct Shape {
+    virtual size_t sides() const = 0;
+    virtual ~Shape() = default;
+};
+
+class Triangle : public Shape {
+public:
+    size_t sides() const override { return 3; }
+};
+
+class Square : public Shape {
+public:
+    size_t sides() const override { return 4; }
+};
+```
+
+---
+
+# Closed set Runtime Polymorphism
+
+In C++ we can use `std::variant` and `std::visit` to implement closed-set polymorphism: runtime dispatch over a predefined set of types.
+
+```c++
+template<class... Ts> struct overload : Ts... { using Ts::operator()...; };
+
+...
+
+std::variant<int, std::string> myData = "Polymorphic String";
+
+std::visit(overload {
+    [](int val) {
+        std::cout << "Processing int: " << val * 2 << "\n";
+    },
+    [](const std::string& val) {
+        std::cout << "Processing string: " << val << " (length: " << val.size() << ")\n";
+    }
+}, myData);
+```
+
+---
+
+# Type erasure
+
+Type erasure enables runtime polymorphism for an open set of unrelated types.
+
+```c++
+class AnyDrawable {
+    struct Interface {
+        virtual ~Interface() = default;
+        virtual void draw() = 0;
+    };
+
+    template<typename T> struct Model : Interface {
+        T d; Model(T t) : d(t) {}
+        void draw() override { d.draw(); }
+    };
+
+    Interface* i_;
+
+    void draw() { i_->draw(); }
+};
+```
+
+---
+
+# `polymorphic (C++26)`
+
+C++ 26 introduces `polymorphic<T, A>`, a value-type which owns an object of a class derived from `T`.
+
+As it is an owning type, `polymorphic` takes a second template argument for the allocator.  If no allocator is specified, the default allocator will be used.
+
+`polymorphic` is a value-type: only const qualified methods of the owned object can be accessed from a const-access path; copies have their own distinct, non-sliced, copies of the owned object.
+
+`polymorphic` provides `operator->` and `operator*` to access the owned object.
+
+---
+
+# `protocol (Proposed addition)`
+
+The class template `protocol<T, A>` owns an object whose interface structurally subtypes the interface of the first template argument `T`.
+
+As it is an owning type, `protocol` takes a second template argument for the allocator.  If no allocator is specified, the default allocator will be used.
+
+`protocol` is a value-type: only const qualified methods of the owned object can be accessed from a const-access path; copies have their own distinct, non-sliced, copies of the owned object.
+
+`protocol<T>` has all of the member functions of `T` and forwards them to the owned object.
+
+---
+
+# `protocol_view (Proposed addition)`
+
+The class template `protocol_view<T>` is a non-owning view on an object whose interface structurally subtypes the interface of the template argument `T`.
+
+`protocol_view` can be cheaply copied and passed by value.
+
+`protocol<T>` has all of the member functions of `T` and forwards them to the viewed object.
+
+`protocol_view<const T>` can be used for const-only access to an object in a similar manner to `std::span`.
+
+---
+
+# Function-like objects
+
+C++ has a plethora of function-like type-erasing class templates.
+
+- `std::function` (C++11)
+- `std::function_ref` (C++26)
+- `std::move_only_function` (C++23)
+- `std::copyable_function` (C++26)
+
+Each of these can be implemented using `protocol<T>` or `protocol_view<T>` by defining a suitable `T`.
+
+---
+
+# Overload sets
+
+`protocol` and `protocol_view` can be used to implement an overload set that can be used as a class member or passed as a function argument.
+
+```c++
+struct OverloadedFunction {
+    R1 operator()(Args1&&... args) const;
+    R2 operator()(Args2&&... args);
+    R3 operator()(Args3&&... args);
+};
+```
+
+---
+
+# Indexed containers
+
+`protocol` and `protocol_view` can be used for indexed container access.
+
+```c++
+struct IndexedContainer<T> {
+    const T& operator[](size_t i) const;
+    T& operator[](size_t i);
+    size_t size() const;
+};
+```
+
+---
+
+# Implementation
+
+A reference implementation, using an AST-based Python code generator to simulate
+post-C++26 code injection, is available at <https://github.com/jbcoe/cc-protocol>.
+
+There is an open PR to illustrate the work (currently) needed to add new types
+<https://github.com/jbcoe/cc-protocol/pull/53>
+
+---
+
+# Future work
+
+Future extensions to C++ reflection should allow `protocol` and `protocol_view` to be  implemented without the need for a custom build step.