Impossibly fast Delegate in C++11 on MSVC++14

问题:

The Problem

This question relates to several questions about function pointers here (I will not list them), but most importantly it concerns the code posted here. The post claims the code should be standard compliant and it compiles on Apple Clang just fine.

I have not managed to compile it in VS2015, i.e. MSVC++14.

What could be the source of this error and why not with Clang?

---

First Error

First error repeats multiple times:

...\Delegate.hpp(329): error C2514: 'Delegate<R(A...)>::is_member_pair<<unnamed-symbol>>':
class has no constructors

And the same for is_member_pair_const. It relates to the following part of the code:

  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)...);
  }

---

Second Error

Multiple errors appear, but the first is usually most important:

...\Delegate.hpp(340): error C2059: syntax error: '<end Parse>'
...\Delegate.hpp(349): note: see reference to class template instantiation 'Delegate<R(A...)>' being compiled

Then follow a few syntax errors, unrecognizable template declaration/definition etc. The code in question:

  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)...);
  }

---

For Completeness: The entire Delegate.hpp:

Code Source and Base Article

#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

回答1:

As Igor commented, the following solves the problem: replace is_member_pair<T>{} with is_member_pair<T>::value everywhere and do the same for is_const_member_pair.