Do you want to facilitate lifetime management and maintenance of polymorphic objects in C++?

Do you want to be able to write polymorphic code in C++ as easily as in languages with GC (like Java or C#), while still having excellent runtime performance?

Have you tried other polymorphic programming libraries in C++ but found them deficient?

If so, this library is for you. 😉

The "proxy" is a single-header, cross-platform C++ library that Microsoft uses to make runtime polymorphism easier to implement and faster. Please find the design details at https://wg21.link/p0957.

The "proxy" is a header-only C++20 library. Once you set the language level of your compiler not earlier than C++20 and get the header file (proxy.h), you are all set. You can also install the library via vcpkg, which is a C++ library manager invented by Microsoft, by searching for "proxy" (see vcpkg.info).

All the facilities of the library are defined in namespace pro. The 3 major class templates are dispatch, facade and proxy. Here is a demo showing how to use this library to implement runtime polymorphism in a different way from the traditional inheritance-based approach:

// Abstraction
struct Draw : pro::dispatch<void(std::ostream&)> {
  template <class T>
  void operator()(const T& self, std::ostream& out) { self.Draw(out); }
};
struct Area : pro::dispatch<double()> {
  template <class T>
  double operator()(const T& self) { return self.Area(); }
};
struct DrawableFacade : pro::facade<Draw, Area> {};

// Implementation
class Rectangle {
 public:
  void Draw(std::ostream& out) const
      { out << "{Rectangle: width = " << width_ << ", height = " << height_ << "}"; }
  void SetWidth(double width) { width_ = width; }
  void SetHeight(double height) { height_ = height; }
  double Area() const { return width_ * height_; }

 private:
  double width_;
  double height_;
};

// Client - Consumer
std::string PrintDrawableToString(pro::proxy<DrawableFacade> p) {
  std::stringstream result;
  result << "shape = ";
  p.invoke<Draw>(result);
  result << ", area = " << p.invoke<Area>();
  return std::move(result).str();
}

// Client - Producer
pro::proxy<DrawableFacade> CreateRectangleAsDrawable(int width, int height) {
  Rectangle rect;
  rect.SetWidth(width);
  rect.SetHeight(height);
  return pro::make_proxy<DrawableFacade>(rect);
}

Please find more details and discussions in the spec. The complete version of the "drawable" demo could be found in tests/proxy_integration_tests.cpp (also available on Compiler Explorer).

See more details in samples

1. Set up vcpkg manifest

{
  "name": "<project_name>",
  "version": "0.1.0",
  "dependencies": [
    {
      "name": "proxy"
    }
  ]
}

2. Integrate proxy in CMakeLists.txt

find_package(proxy CONFIG REQUIRED)
target_link_libraries(<target_name> PRIVATE msft_proxy)

3. Run CMake with vcpkg toolchain file

cmake <source_dir> -B <build_dir> -DCMAKE_TOOLCHAIN_FILE=<vcpkg_dir>/scripts/buildsystems/vcpkg.cmake

git clone https://github.com/microsoft/proxy.git
cd proxy
cmake -S . -B build
cmake --build ./build -j8
cd ./build
ctest -j8

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This project has adopted the Microsoft Open Source Code of Conduct. For more information see the Code of Conduct FAQ or contact [email protected] with any additional questions or comments.

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