We are thrilled to announce that Proxy 3, our latest and greatest solution for polymorphism in C++, is now feature complete! Since the library was initially open-sourced, we have heard much positive feedback and received many brilliant feature requests. Big thanks to all of you who have contributed any code or idea that made the library better!

Our Mission

“Proxy” is a modern C++ library that helps you use polymorphism (a way to use different types of objects interchangeably) without needing inheritance.

“Proxy” was created by Microsoft engineers and has been used in the Windows operating system since 2022. For many years, using inheritance was the main way to achieve polymorphism in C++. However, new programming languages like Rust offer better ways to do this. We have improved our understanding of object-oriented programming and decided to use pointers in C++ as the foundation for “Proxy”. Specifically, the “Proxy” library is designed to be:

• Portable: “Proxy” was implemented as a single-header library in standard C++20. It can be used on any platform while the compiler supports C++20. The majority of the library is freestanding, making it feasible for embedded engineering or kernel design of an operating system.
• Non-intrusive: An implementation type is no longer required to inherit from an abstract binding.
• Well-managed: “Proxy” provides a GC-like capability that manages the lifetimes of different objects efficiently without the need for an actual garbage collector.
• Fast: With typical compiler optimizations, “Proxy” produces high-quality code that is as good as or better than hand-written code. In many cases, “Proxy” performs better than traditional inheritance-based approaches, especially in managing the lifetimes of objects.
• Accessible: Learned from user feedback, accessibility has been significantly improved in “Proxy 3” with intuitive syntax, good IDE compatibility, and accurate diagnostics.
• Flexible: Not only member functions, the “abstraction” of “Proxy” allows any expression to be polymorphic, including free functions, operators, conversions, etc. Different abstractions can be freely composed on demand. Performance tuning is supported for experts to balance between extensibility and performance.

The New Look

Comparing to our previous releases, there are some major improvements in the syntax and utilities. Let’s get started with some examples!

Hello World

#include <iostream>
#include <string>

#include "proxy.h"

struct Streamable : pro::facade_builder
    ::add_convention<pro::operator_dispatch<"<<", true>, std::ostream&(std::ostream& out) const>
    ::build {};

int main() {
  std::string str = "Hello World";
  pro::proxy<Streamable> p1 = &str;
  std::cout << "p1 = " << *p1 << "\n";  // Prints: "p1 = Hello World"

  pro::proxy<Streamable> p2 = std::make_unique<int>(123);
  std::cout << "p2 = " << *p2 << "\n";  // Prints: "p2 = 123"

  pro::proxy<Streamable> p3 = pro::make_proxy<Streamable>(3.14);
  std::cout << "p3 = " << *p3 << "\n";  // Prints: "p3 = 3.14"
}

Here is a step-by-step explanation:

• #include <iostream>: For std::cout.
• #include <string>: For std::string.
• #include "proxy.h": For the “Proxy” library. Most of the facilities of the library are defined in namespace pro. If the library is consumed via vcpkg or conan, this line should be changed into #include <proxy/proxy.h>.
• struct Streamable : pro::facade_builder ... ::build {}: Defines a facade type Streamable. The term “facade” is how the “Proxy” library models runtime abstraction. Specifically,
  • pro::facade_builder: Provides capability to build a facade type at compile-time.
  • add_convention: Adds a generalized “calling convention”, defined by a “dispatch” and several “overloads”, to the build context.
  • pro::operator_dispatch<"<<", true>: Specifies a dispatch for operator << expressions where the primary operand (proxy) is on the right-hand side (specified by the second template parameter true). Note that polymorphism in the “Proxy” library is defined by expressions rather than member functions, which is different from C++ virtual functions or other OOP languages.
  • std::ostream&(std::ostream& out) const: The signature of the calling convention, similar with std::move_only_function. const specifies that the primary operand is const.
  • build: Builds the context into a facade type.
• pro::proxy<Streamable> p1 = &str: Creates a proxy object from a raw pointer of std::string. p1 behaves like a raw pointer, and does not have ownership of the underlying std::string. If the lifetime of str ends before p1, p1 becomes dangling.
• std::cout << *p1: This is how it works. It prints “Hello World” because the calling convention is defined in the facade Streamable, so it works as if by calling std::cout << str.
• pro::proxy<Streamable> p2 =std::make_unique<int>(123): Creates a std::unique_ptr<int> and converts to a proxy. Different from p1, p2 has ownership of the underlying int because it is instantiated from a value of std::unique_ptr, and will call the destructor of std::unique_ptr when p2 is destroyed, while p1 does not have ownership of the underlying int because it is instantiated from a raw pointer. p1 and p2 are of the same type pro::proxy<Streamable>, which means you can have a function that returns pro::proxy<Streamable> without exposing any information about the implementation details to its caller.
• std::cout << *p2: Prints “123” with no surprise.
• pro::proxy<Streamable> p3 = pro::make_proxy<Streamable>(3.14): Creates a proxy from a double without specifying the underlying pointer type. Specifically,
  • Similar with p2, p3 also has ownership of the underlying double value, but can effectively avoid heap allocation.
  • Since the size of the underlying type (double) is known to be small (on major 32- or 64-bit platforms), pro::make_proxy realizes the fact at compile-time and guarantees no heap allocation.
  • Library “Proxy” explicitly defines when heap allocation occurs or not to avoid users falling into performance hell, which is different from std::function and other existing polymorphic wrappers in the standard.
• std::cout << *p3: Prints “3.14” with no surprise.
• When main returns, p2 and p3 will destroy the underlying objects, while p1 does nothing because it holds a raw pointer that does not have ownership of the underlying std::string.

More Expressions

In addition to the operator expressions demonstrated in the previous example, the library supports almost all forms of expressions in C++ and can make them polymorphic. Specifically,

• The PRO_DEF_MEM_DISPATCH macro: Defines a dispatch type for member function call expressions.
• The PRO_DEF_FREE_DISPATCH macro: Defines a dispatch type for free function call expressions.
• The pro::operator_dispatch class template: Dispatch type for operator expressions.
• The pro::conversion_dispatch class template: Dispatch type for conversion expressions.

Note that some facilities are provided as macros, because C++ templates today do not support generating a function with an arbitrary name. Here is another example that makes member function call expressions polymorphic:

#include <iostream>
#include <sstream>

#include "proxy.h"

PRO_DEF_MEM_DISPATCH(MemDraw, Draw);
PRO_DEF_MEM_DISPATCH(MemArea, Area);

struct Drawable : pro::facade_builder
    ::add_convention<MemDraw, void(std::ostream& output)>
    ::add_convention<MemArea, double() noexcept>
    ::support_copy<pro::constraint_level::nontrivial>
    ::build {};

class Rectangle {
 public:
  Rectangle(double width, double height) : width_(width), height_(height) {}
  Rectangle(const Rectangle&) = default;

  void Draw(std::ostream& out) const {
    out << "{Rectangle: width = " << width_ << ", height = " << height_ << "}";
  }
  double Area() const noexcept { return width_ * height_; }

 private:
  double width_;
  double height_;
};

std::string PrintDrawableToString(pro::proxy<Drawable> p) {
  std::stringstream result;
  result << "entity = ";
  p->Draw(result);
  result << ", area = " << p->Area();
  return std::move(result).str();
}

int main() {
  pro::proxy<Drawable> p = pro::make_proxy<Drawable, Rectangle>(3, 5);
  std::string str = PrintDrawableToString(p);
  std::cout << str << "\n";  // Prints: "entity = {Rectangle: width = 3, height = 5}, area = 15"
}

Here is a step-by-step explanation:

• #include <iostream>: For std::cout.
• #include <sstream>: For std::stringstream.
• #include "proxy.h": For the “Proxy” library.
• PRO_DEF_MEM_DISPATCH(MemDraw, Draw): Defines a dispatch type MemDraw for expressions of calling member function Draw.
• PRO_DEF_MEM_DISPATCH(MemArea, Area): Defines a dispatch type MemArea for expressions of calling member function Area.
• struct Drawable : pro::facade_builder ... ::build {}: Defines a facade type Drawable. Specifically,
  • add_convention: Adds calling conventions to the build context.
  • support_copy<pro::constraint_level::nontrivial>: Specifies the underlying pointer type shall be copyable, which also makes the resulting proxy type copyable.
• class Rectangle: An implementation of Drawable.
• Function PrintDrawableToString: Converts a Drawable into a std::string. Note that this is a function rather than a function template, which means it can generate ABI in a larger build system.
• pro::proxy<Drawable> p = pro::make_proxy<Drawable, Rectangle>(3, 5): Creates a proxy<Drawable> object containing a Rectangle.
• std::string str = PrintDrawableToString(p): Converts p into a std::string, implicitly creates a copy of p.
• std::cout << str: Prints the string.

Other Useful Features

In addition to the features mentioned above, here is a curated list of the most popular features based on user feedback:

• Overloading: facade_builder::add_convention is more powerful than demonstrated above. It can take any number of overload types and perform standard overload resolution when invoking a proxy.
• Facade composition: facade_builder::add_facade allows flexible composition of different abstractions.
• Weak dispatch: When an object does not implement a convention, and we do not want it to trigger a hard compile error, it is allowed to define a “weak dispatch” with macro PRO_DEF_WEAK_DISPATCH from an existing dispatch type and a default implementation.
• Allocator awareness: Function template allocate_proxy is able to create a proxy from a value with any custom allocator. In C++11, std::function and std::packaged_task had constructors that accepted custom allocators for performance tuning, but these were removed in C++17 because “the semantics are unclear, and there are technical issues with storing an allocator in a type-erased context and then recovering that allocator later for any allocations needed during copy assignment”. These issues do not apply to allocate_proxy.
• Configurable constraints: facade_builder provides full support for constraints configuration, including memory layout (by restrict_layout), copyability (by support_copy), relocatability (by support_relocation), and destructibility (by support_destruction).
• Reflection: proxy supports type-based compile-time reflection for runtime queries, specifically with facade_builder::add_reflection and function template proxy_reflect.

Summary

We hope this library could empower more C++ users outside Microsoft to write polymorphic code easier. The full documentation of Proxy 3 will be published soon. Please stay tuned for more technical details. We are also actively working on several ISO C++ proposals for further standardization.