Impossibly fast delegate in C++11

Question

This legendary C++ delegate article can be easily converted into C++11, without the need for fancy preprocessor magic in the original. I'd like to know if I got all the necessary C++11 nuances right. Suggestions?

#pragma once
#ifndef DELEGATE_HPP
# define DELEGATE_HPP

#include <cassert>

#include <memory>

#include <new>

#include <type_traits>

#include <utility>

template <typename T> class delegate;

template<class R, class ...A>
class delegate<R (A...)>
{
  using stub_ptr_type = R (*)(void*, A&&...);

  delegate(void* const o, stub_ptr_type const m) noexcept :
    object_ptr_(o),
    stub_ptr_(m)
  {
  }

public:
  delegate() = default;

  delegate(delegate const&) = default;

  delegate(delegate&&) = default;

  delegate(::std::nullptr_t const) noexcept : delegate() { }

  template <class C, typename =
    typename ::std::enable_if< ::std::is_class<C>{}>::type>
  explicit delegate(C const* const o) noexcept :
    object_ptr_(const_cast<C*>(o))
  {
  }

  template <class C, typename =
    typename ::std::enable_if< ::std::is_class<C>{}>::type>
  explicit delegate(C const& o) noexcept :
    object_ptr_(const_cast<C*>(&o))
  {
  }

  template <class C>
  delegate(C* const object_ptr, R (C::* const method_ptr)(A...))
  {
    *this = from(object_ptr, method_ptr);
  }

  template <class C>
  delegate(C* const object_ptr, R (C::* const method_ptr)(A...) const)
  {
    *this = from(object_ptr, method_ptr);
  }

  template <class C>
  delegate(C& object, R (C::* const method_ptr)(A...))
  {
    *this = from(object, method_ptr);
  }

  template <class C>
  delegate(C const& object, R (C::* const method_ptr)(A...) const)
  {
    *this = from(object, method_ptr);
  }

  template <
    typename T,
    typename = typename ::std::enable_if<
      !::std::is_same<delegate, typename ::std::decay<T>::type>{}
    >::type
  >
  delegate(T&& f) :
    store_(operator new(sizeof(typename ::std::decay<T>::type)),
      functor_deleter<typename ::std::decay<T>::type>),
    store_size_(sizeof(typename ::std::decay<T>::type))
  {
    using functor_type = typename ::std::decay<T>::type;

    new (store_.get()) functor_type(::std::forward<T>(f));

    object_ptr_ = store_.get();

    stub_ptr_ = functor_stub<functor_type>;

    deleter_ = deleter_stub<functor_type>;
  }

  delegate& operator=(delegate const&) = default;

  delegate& operator=(delegate&&) = default;

  template <class C>
  delegate& operator=(R (C::* const rhs)(A...))
  {
    return *this = from(static_cast<C*>(object_ptr_), rhs);
  }

  template <class C>
  delegate& operator=(R (C::* const rhs)(A...) const)
  {
    return *this = from(static_cast<C const*>(object_ptr_), rhs);
  }

  template <
    typename T,
    typename = typename ::std::enable_if<
      !::std::is_same<delegate, typename ::std::decay<T>::type>{}
    >::type
  >
  delegate& operator=(T&& f)
  {
    using functor_type = typename ::std::decay<T>::type;

    if ((sizeof(functor_type) > store_size_) || !store_.unique())
    {
      store_.reset(operator new(sizeof(functor_type)),
        functor_deleter<functor_type>);

      store_size_ = sizeof(functor_type);
    }
    else
    {
      deleter_(store_.get());
    }

    new (store_.get()) functor_type(::std::forward<T>(f));

    object_ptr_ = store_.get();

    stub_ptr_ = functor_stub<functor_type>;

    deleter_ = deleter_stub<functor_type>;

    return *this;
  }

  template <R (* const function_ptr)(A...)>
  static delegate from() noexcept
  {
    return { nullptr, function_stub<function_ptr> };
  }

  template <class C, R (C::* const method_ptr)(A...)>
  static delegate from(C* const object_ptr) noexcept
  {
    return { object_ptr, method_stub<C, method_ptr> };
  }

  template <class C, R (C::* const method_ptr)(A...) const>
  static delegate from(C const* const object_ptr) noexcept
  {
    return { const_cast<C*>(object_ptr), const_method_stub<C, method_ptr> };
  }

  template <class C, R (C::* const method_ptr)(A...)>
  static delegate from(C& object) noexcept
  {
    return { &object, method_stub<C, method_ptr> };
  }

  template <class C, R (C::* const method_ptr)(A...) const>
  static delegate from(C const& object) noexcept
  {
    return { const_cast<C*>(&object), const_method_stub<C, method_ptr> };
  }

  template <typename T>
  static delegate from(T&& f)
  {
    return ::std::forward<T>(f);
  }

  static delegate from(R (* const function_ptr)(A...))
  {
    return function_ptr;
  }

  template <class C>
  using member_pair =
    ::std::pair<C* const, R (C::* const)(A...)>;

  template <class C>
  using const_member_pair =
    ::std::pair<C const* const, R (C::* const)(A...) const>;

  template <class C>
  static delegate from(C* const object_ptr,
    R (C::* const method_ptr)(A...))
  {
    return member_pair<C>(object_ptr, method_ptr);
  }

  template <class C>
  static delegate from(C const* const object_ptr,
    R (C::* const method_ptr)(A...) const)
  {
    return const_member_pair<C>(object_ptr, method_ptr);
  }

  template <class C>
  static delegate from(C& object, R (C::* const method_ptr)(A...))
  {
    return member_pair<C>(&object, method_ptr);
  }

  template <class C>
  static delegate from(C const& object,
    R (C::* const method_ptr)(A...) const)
  {
    return const_member_pair<C>(&object, method_ptr);
  }

  void reset() { stub_ptr_ = nullptr; store_.reset(); }

  void reset_stub() noexcept { stub_ptr_ = nullptr; }

  void swap(delegate& other) noexcept { ::std::swap(*this, other); }

  bool operator==(delegate const& rhs) const noexcept
  {
    return (object_ptr_ == rhs.object_ptr_) && (stub_ptr_ == rhs.stub_ptr_);
  }

  bool operator!=(delegate const& rhs) const noexcept
  {
    return !operator==(rhs);
  }

  bool operator<(delegate const& rhs) const noexcept
  {
    return (object_ptr_ < rhs.object_ptr_) ||
      ((object_ptr_ == rhs.object_ptr_) && (stub_ptr_ < rhs.stub_ptr_));
  }

  bool operator==(::std::nullptr_t const) const noexcept
  {
    return !stub_ptr_;
  }

  bool operator!=(::std::nullptr_t const) const noexcept
  {
    return stub_ptr_;
  }

  explicit operator bool() const noexcept { return stub_ptr_; }

  R operator()(A... args) const
  {
//  assert(stub_ptr);
    return stub_ptr_(object_ptr_, ::std::forward<A>(args)...);
  }

private:
  friend struct ::std::hash<delegate>;

  using deleter_type = void (*)(void*);

  void* object_ptr_;
  stub_ptr_type stub_ptr_{};

  deleter_type deleter_;

  ::std::shared_ptr<void> store_;
  ::std::size_t store_size_;

  template <class T>
  static void functor_deleter(void* const p)
  {
    static_cast<T*>(p)->~T();

    operator delete(p);
  }

  template <class T>
  static void deleter_stub(void* const p)
  {
    static_cast<T*>(p)->~T();
  }

  template <R (*function_ptr)(A...)>
  static R function_stub(void* const, A&&... args)
  {
    return function_ptr(::std::forward<A>(args)...);
  }

  template <class C, R (C::*method_ptr)(A...)>
  static R method_stub(void* const object_ptr, A&&... args)
  {
    return (static_cast<C*>(object_ptr)->*method_ptr)(
      ::std::forward<A>(args)...);
  }

  template <class C, R (C::*method_ptr)(A...) const>
  static R const_method_stub(void* const object_ptr, A&&... args)
  {
    return (static_cast<C const*>(object_ptr)->*method_ptr)(
      ::std::forward<A>(args)...);
  }

  template <typename>
  struct is_member_pair : std::false_type { };

  template <class C>
  struct is_member_pair< ::std::pair<C* const,
    R (C::* const)(A...)> > : std::true_type
  {
  };

  template <typename>
  struct is_const_member_pair : std::false_type { };

  template <class C>
  struct is_const_member_pair< ::std::pair<C const* const,
    R (C::* const)(A...) const> > : std::true_type
  {
  };

  template <typename T>
  static typename ::std::enable_if<
    !(is_member_pair<T>{} ||
    is_const_member_pair<T>{}),
    R
  >::type
  functor_stub(void* const object_ptr, A&&... args)
  {
    return (*static_cast<T*>(object_ptr))(::std::forward<A>(args)...);
  }

  template <typename T>
  static typename ::std::enable_if<
    is_member_pair<T>{} ||
    is_const_member_pair<T>{},
    R
  >::type
  functor_stub(void* const object_ptr, A&&... args)
  {
    return (static_cast<T*>(object_ptr)->first->*
      static_cast<T*>(object_ptr)->second)(::std::forward<A>(args)...);
  }
};

namespace std
{
  template <typename R, typename ...A>
  struct hash<::delegate<R (A...)> >
  {
    size_t operator()(::delegate<R (A...)> const& d) const noexcept
    {
      auto const seed(hash<void*>()(d.object_ptr_));

      return hash<typename ::delegate<R (A...)>::stub_ptr_type>()(
        d.stub_ptr_) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
    }
  };
}

#endif // DELEGATE_HPP

Example use:

#include <iostream>

#include "delegate.hpp"

struct A
{
  void foo(int a)
  {
    std::cout << "method got: " << a << std::endl;
  }
};

void foo(int a)
{
  std::cout << "function got: " << a << std::endl;
}

int main(int argc, char* argv[])
{
  auto d1(delegate<void (int)>::from<foo>());

  A a;
  auto d2(delegate<void (int)>::from<A, &A::foo>(&a));
  auto d3(delegate<void (int)>{foo});
  auto d4(delegate<void (int)>(&a, &A::foo));

  d1(1);
  d2(2);
  d3(3);
  d4(4);

  int b(2);

  auto dx(delegate<void ()>(
    [b](){std::cout << "hello world: " << b << std::endl;}));

  dx();

  return 0;
}

Answer 1

It looks by and large OK. Just some nitpicks

• the default constructor could just be delegate() = default;, or otherwise initialize all members... or remove it entirely if it makes no sense.

• the copy constructor should use initialization, not assignment.

• swap should return void.

• Assignment should pass-by-value:

  delegate& operator=(delegate rhs) { rhs.swap(*this); return *this; }
  

• Invocation should use arbitrary arguments and forwarding:

  template <typename ...B>
  R operator()(B &&... b)
  {
      return (*stub_ptr)(object_ptr, std::forward<B>(b)...);
  }
  

  In fact, you should add enable_if with some variadic version of is_constructible<A, B>... to the operator so you don't create impossible overloads.

Answer 2

I love this! Ran some quick benchmarks:

int x{0}; 
for(int xx = 0; xx < 5; ++xx)
{
    startBenchmark();
    {
        std::function<int(int)> t2 = [&x](int i){ return i + x; };
        std::function<void(int)> t1 = [&x, &t2](int i){ x = t2(i); };
        for(int i = 0; i < 1000000000; ++i) t1(i);
    } lo << lt("std::func") << endBenchmark() << endl;

    startBenchmark();
    {
        delegate<int(int)> t2 = [&x](int i){ return i + x; };
        delegate<void(int)> t1 = [&x, &t2](int i){ x = t2(i); };
        for(int i = 0; i < 1000000000; ++i) t1(i);
    } lo << lt("ssvu::fastfunc") << endBenchmark() << endl;
}

Results:

[std::func]                           3278 ms
[ssvu::fastfunc]                      2147 ms
[std::func]                           3264 ms
[ssvu::fastfunc]                      2126 ms
[std::func]                           3302 ms
[ssvu::fastfunc]                      2182 ms
[std::func]                           3306 ms
[ssvu::fastfunc]                      2135 ms
...

Your delegate class is always faster by 1 seconds, even in this naive benchmark.

---

Actual review:

I tested changing all std::function<R(A...)> to your delegate<R(A...)> in all of my projects.

After making a small adjustment to the delegate code, everything works as expected!

In some of my code I used to do this:

void a(std::function<void()> mMaybe = nullptr)
{
    // do something
    if(mMaybe != nullptr) mMaybe();
}

This code broke when using delegate instead of std::function. I fixed it by adding these lines to your delegate class:

delegate(std::nullptr_t) : object_ptr_{nullptr}, stub_ptr_{nullptr} { }
bool operator==(std::nullptr_t) const noexcept      { return object_ptr_ == nullptr; }
bool operator!=(std::nullptr_t) const noexcept      { return object_ptr_ != nullptr; }

Now the above code compiles and works correctly. With this small adjustment, the delegate class is a probably faster drop-in replacement for std::function.

Answer 3

Problematic results:

[std::func]                           31855 ms
[ssvu::fastfunc]                      123848 ms

Benchmark code:

int x{0};
for(int xx = 0; xx < 5; ++xx)
{
    startBenchmark();
    {
        for(int i = 0; i < 1000000000; ++i)
        {
            std::function<int(int)> t2 = t2impl;
            std::function<void(int)> t1 = [&x, &t2](int i){ x = t2(i); };
            t1(i);
        }
    } lo << lt("std::func") << endBenchmark() << endl;
    startBenchmark();
    {
        for(int i = 0; i < 1000000000; ++i)
        {
            FastFunc<int(int)> t2 = t2impl;
            FastFunc<void(int)> t1 = [&x, &t2](int i){ x = t2(i); };
            t1(i);
        }
    } lo << lt("ssvu::fastfunc") << endBenchmark() << endl;
}

std::function performed way better on this benchmark. Any idea why?