proxy
Proxy 3.1.0 Release
Are you looking to simplify the lifetime management and maintenance of polymorphic objects in C++?
Do you want to write polymorphic code in C++ as easily as in GC languages like Java or C#, without sacrificing performance?
Have you tried other polymorphic programming libraries in C++ but found them deficient?
If so, this library is for you.
"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:
Please refer to the Proxy's Frequently Asked Questions for more background, and refer to the specifications for more technical details.
"Proxy" is a header-only C++20 library. To use the library, make sure your compiler meets the minimum requirements and just include the header file proxy.h in your source code. Alternatively, you can install the library via vcpkg or conan, by searching for "proxy" (see vcpkg.io and conan.io).
Let's get started with the following "Hello World" example:
#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", formally defined as the ProFacade requirements, 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,
p2, p3 also has ownership of the underlying double value, but can effectively avoid heap allocation.double) is known to be small (on major 32- or 64-bit platforms), pro::make_proxy realizes the fact at compile-time, and falls back to pro::make_proxy_inplace, which guarantees no heap allocation.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.
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,
PRO_DEF_MEM_DISPATCH: Defines a dispatch type for member function call expressions.PRO_DEF_FREE_DISPATCH: Defines a dispatch type for free function call expressions.pro::operator_dispatch: Dispatch type for operator expressions.pro::conversion_dispatch: Dispatch type for conversion expressions.Note that some facilities are provided as macro, 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.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.The "Proxy" library is a self-contained solution for runtime polymorphism in C++. There are many other capabilities documented in the specifications. In addition to the features mentioned above, here is a curated list of the most popular features based on user feedback:
facade_builder::add_convention is more powerful than demonstrated above. It can take any number of overload types (formally, any type meeting the ProOverload requirements) and perform standard overload resolution when invoking a proxy.facade_builder::add_facade allows flexible composition of different abstractions.PRO_DEF_WEAK_DISPATCH from an existing dispatch type and a default implementation.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.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).proxy supports type-based compile-time reflection for runtime queries. Please refer to facade_builder::add_reflection and function template proxy_reflect for more details.| Family | Minimum version | Required flags |
|---|---|---|
| GCC | 13.1 | -std=c++20 |
| Clang | 15.0.0 | -std=c++20 |
| MSVC | 19.31 | /std:c++20 |
| NVIDIA HPC | 24.1 | -std=c++20 |
git clone https://github.com/microsoft/proxy.git
cd proxy
cmake -B build
cmake --build build -j
ctest --test-dir build -j
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