3

I'm trying to create a C++11 implementation of Don Clugston's Member Function Pointers and the Fastest Possible C++ Delegates, and make it work as a drop-in std::function replacement.

This is what I got so far.

I construct lambda FastDelegates like this:

// FastFunc is my name for FastDelegate
template<typename LambdaType> FastFunc(LambdaType lambdaExpression)
{
    this->m_Closure.bindmemfunc(&lambdaExpression, &LambdaType::operator());
}

Now, some tests:

FastFunc<void()> test = []{ std::cout << "hello" << std::endl; };
test();
// Correctly prints "hello"

bool b{false};
FastFunc<void()> test2 = [&b]{ std::cout << b << std::endl; };
test2();
// Crash!

As you can see, when the lambda is "trivial" (no captures), copying it by value and taking its address works. But when the lambda stores some kind of state (captures), I cannot just copy it by value into the FastFunc.

I tried getting the lambda by reference, but I cannot do that when it's a temporary like in the example.

I have to somehow store the lambda inside the FastFunc, but I don't want to use std::shared_ptr because it's slow (I tried a different fastdelegate implementation that used it, and its performance was comparable to std::function).

How can I make my implementation of Don Clugston's fastest possible C++ delegates work with lambdas that capture state, preserving the amazing performance of fastdelegates?

4

4 回答 4

8

您已经很好地诊断了情况：您需要存储状态。

由于 lambda 是一个临时对象，因此实际上允许您从它（通常）移动，如果可能的话，应该优先于副本（因为移动比复制更通用）。

现在，您需要做的就是为其保留一些存储空间，如果这需要动态分配，您可能确实会降低性能。另一方面，一个物体需要有一个固定的足迹，所以 ?

一种可能的解决方案是提供可配置（但有限）的存储容量：

static size_t const Size = 32;
static size_t const Alignment = alignof(std::max_align_t);

typedef std::aligned_storage<Size, Alignment>::type Storage;
Storage storage;

现在，您可以（根据需要使用 reinterpret_cast）将您的 lambda 存储在其中，storage前提是它的大小适合（可以使用 检测static_assert）。

终于设法获得了一个工作示例（必须从头开始，因为上帝是那个快速的委托代码冗长！！），您可以在此处看到它的实际效果（代码如下）。

我只触及了表面，特别是因为它缺少复制和移动操作符。要正确执行此操作，需要按照与其他两个操作相同的模式将这些操作添加到处理程序中。

代码：

#include <cstddef>

#include <iostream>
#include <memory>
#include <type_traits>

template <typename, size_t> class FastFunc;

template <typename R, typename... Args, size_t Size>
class FastFunc<R(Args...), Size> {
public:
    template <typename F>
    FastFunc(F f): handler(&Get<F>()) {
        new (&storage) F(std::move(f));
    }

    ~FastFunc() {
        handler->destroy(&storage);
    }

    R operator()(Args&&... args) {
      return handler->apply(&storage, std::forward<Args>(args)...);
    }

private:
    using Storage = typename std::aligned_storage<Size, alignof(max_align_t)>::type;

    struct Handler {
        R (*apply)(void*, Args&&...);
        void (*destroy)(void*);
    }; // struct Handler

    template <typename F>
    static R Apply(void* f, Args&&... args) {
        (*reinterpret_cast<F*>(f))(std::forward<Args>(args)...);
    }

    template <typename F>
    static void Destroy(void* f) {
        reinterpret_cast<F*>(f)->~F();
    }

    template <typename F>
    Handler const& Get() {
        static Handler const H = { &Apply<F>, &Destroy<F> };
        return H;
    } // Get

    Handler const* handler;
    Storage storage;
}; // class FastFunc

int main() {
    FastFunc<void(), 32> stateless = []() { std::cout << "stateless\n"; };
    stateless();

    bool b = true;
    FastFunc<void(), 32> stateful = [&b]() { std::cout << "stateful: " << b << "\n"; };
    stateful();

    b = false;
    stateful();

    return 0;
}
于 2013-09-05T11:20:28.100 回答
1

你不能。

事情就是这样。Fastdelegates 仅适用于极少数非常具体的情况。这就是让它更快的原因。您不会因为实现std::function.

于 2013-09-05T12:42:55.863 回答
1

我已经使用艰苦的工作和其他几个线程，例如：获取 lambda 参数类型

这里是：

namespace details{

    template<class FPtr> struct function_traits;
    template<class RT, class CT                                                                                        >struct function_traits<RT (CT::*)(                                      )     >{ typedef RT Result;                                                                                                                                                                  typedef RT (CT::*Signature)(                                      );};
    template<class RT, class CT                                                                                        >struct function_traits<RT (CT::*)(                                      )const>{ typedef RT Result;                                                                                                                                                                  typedef RT (CT::*Signature)(                                      );};
    template<class RT                                                                                                  >struct function_traits<RT        (                                      )     >{ typedef RT Result;                                                                                                                                                                  typedef RT       Signature (                                      );};
    template<class RT, class CT, class P1T                                                                             >struct function_traits<RT (CT::*)(P1T                                   )     >{ typedef RT Result;  typedef P1T Param1;                                                                                                                                             typedef RT (CT::*Signature)(P1T                                   );};
    template<class RT, class CT, class P1T                                                                             >struct function_traits<RT (CT::*)(P1T                                   )const>{ typedef RT Result;  typedef P1T Param1;                                                                                                                                             typedef RT (CT::*Signature)(P1T                                   );};
    template<class RT          , class P1T                                                                             >struct function_traits<RT        (P1T                                   )     >{ typedef RT Result;  typedef P1T Param1;                                                                                                                                             typedef RT       Signature (P1T                                   );};
    template<class RT, class CT, class P1T, class P2T                                                                  >struct function_traits<RT (CT::*)(P1T, P2T                              )     >{ typedef RT Result;  typedef P1T Param1; typedef P2T Param2;                                                                                                                         typedef RT (CT::*Signature)(P1T, P2T                              );};
    template<class RT, class CT, class P1T, class P2T                                                                  >struct function_traits<RT (CT::*)(P1T, P2T                              )const>{ typedef RT Result;  typedef P1T Param1; typedef P2T Param2;                                                                                                                         typedef RT (CT::*Signature)(P1T, P2T                              );};
    template<class RT          , class P1T, class P2T                                                                  >struct function_traits<RT        (P1T, P2T                              )     >{ typedef RT Result;  typedef P1T Param1; typedef P2T Param2;                                                                                                                         typedef RT       Signature (P1T, P2T                              );};
    template<class RT, class CT, class P1T, class P2T, class P3T                                                       >struct function_traits<RT (CT::*)(P1T, P2T, P3T                         )     >{ typedef RT Result;  typedef P1T Param1; typedef P2T Param2; typedef P3T Param3;                                                                                                     typedef RT (CT::*Signature)(P1T, P2T, P3T                         );};
    template<class RT, class CT, class P1T, class P2T, class P3T                                                       >struct function_traits<RT (CT::*)(P1T, P2T, P3T                         )const>{ typedef RT Result;  typedef P1T Param1; typedef P2T Param2; typedef P3T Param3;                                                                                                     typedef RT (CT::*Signature)(P1T, P2T, P3T                         );};
    template<class RT          , class P1T, class P2T, class P3T                                                       >struct function_traits<RT        (P1T, P2T, P3T                         )     >{ typedef RT Result;  typedef P1T Param1; typedef P2T Param2; typedef P3T Param3;                                                                                                     typedef RT       Signature (P1T, P2T, P3T                         );};
    template<class RT, class CT, class P1T, class P2T, class P3T, class P4T                                            >struct function_traits<RT (CT::*)(P1T, P2T, P3T, P4T                    )     >{ typedef RT Result;  typedef P1T Param1; typedef P2T Param2; typedef P3T Param3; typedef P4T Param4;                                                                                 typedef RT (CT::*Signature)(P1T, P2T, P3T, P4T                    );};
    template<class RT, class CT, class P1T, class P2T, class P3T, class P4T                                            >struct function_traits<RT (CT::*)(P1T, P2T, P3T, P4T                    )const>{ typedef RT Result;  typedef P1T Param1; typedef P2T Param2; typedef P3T Param3; typedef P4T Param4;                                                                                 typedef RT (CT::*Signature)(P1T, P2T, P3T, P4T                    );};
    template<class RT          , class P1T, class P2T, class P3T, class P4T                                            >struct function_traits<RT        (P1T, P2T, P3T, P4T                    )     >{ typedef RT Result;  typedef P1T Param1; typedef P2T Param2; typedef P3T Param3; typedef P4T Param4;                                                                                 typedef RT       Signature (P1T, P2T, P3T, P4T                    );};
    template<class RT, class CT, class P1T, class P2T, class P3T, class P4T, class P5T                                 >struct function_traits<RT (CT::*)(P1T, P2T, P3T, P4T, P5T               )     >{ typedef RT Result;  typedef P1T Param1; typedef P2T Param2; typedef P3T Param3; typedef P4T Param4; typedef P5T Param5;                                                             typedef RT (CT::*Signature)(P1T, P2T, P3T, P4T, P5T               );};
    template<class RT, class CT, class P1T, class P2T, class P3T, class P4T, class P5T                                 >struct function_traits<RT (CT::*)(P1T, P2T, P3T, P4T, P5T               )const>{ typedef RT Result;  typedef P1T Param1; typedef P2T Param2; typedef P3T Param3; typedef P4T Param4; typedef P5T Param5;                                                             typedef RT (CT::*Signature)(P1T, P2T, P3T, P4T, P5T               );};
    template<class RT          , class P1T, class P2T, class P3T, class P4T, class P5T                                 >struct function_traits<RT        (P1T, P2T, P3T, P4T, P5T               )     >{ typedef RT Result;  typedef P1T Param1; typedef P2T Param2; typedef P3T Param3; typedef P4T Param4; typedef P5T Param5;                                                             typedef RT       Signature (P1T, P2T, P3T, P4T, P5T               );};
    template<class RT, class CT, class P1T, class P2T, class P3T, class P4T, class P5T, class P6T                      >struct function_traits<RT (CT::*)(P1T, P2T, P3T, P4T, P5T, P6T          )     >{ typedef RT Result;  typedef P1T Param1; typedef P2T Param2; typedef P3T Param3; typedef P4T Param4; typedef P5T Param5; typedef P6T Param6;                                         typedef RT (CT::*Signature)(P1T, P2T, P3T, P4T, P5T, P6T          );};
    template<class RT, class CT, class P1T, class P2T, class P3T, class P4T, class P5T, class P6T                      >struct function_traits<RT (CT::*)(P1T, P2T, P3T, P4T, P5T, P6T          )const>{ typedef RT Result;  typedef P1T Param1; typedef P2T Param2; typedef P3T Param3; typedef P4T Param4; typedef P5T Param5; typedef P6T Param6;                                         typedef RT (CT::*Signature)(P1T, P2T, P3T, P4T, P5T, P6T          );};
    template<class RT          , class P1T, class P2T, class P3T, class P4T, class P5T, class P6T                      >struct function_traits<RT        (P1T, P2T, P3T, P4T, P5T, P6T          )     >{ typedef RT Result;  typedef P1T Param1; typedef P2T Param2; typedef P3T Param3; typedef P4T Param4; typedef P5T Param5; typedef P6T Param6;                                         typedef RT       Signature (P1T, P2T, P3T, P4T, P5T, P6T          );};
    template<class RT, class CT, class P1T, class P2T, class P3T, class P4T, class P5T, class P6T, class P7T           >struct function_traits<RT (CT::*)(P1T, P2T, P3T, P4T, P5T, P6T, P7T     )     >{ typedef RT Result;  typedef P1T Param1; typedef P2T Param2; typedef P3T Param3; typedef P4T Param4; typedef P5T Param5; typedef P6T Param6; typedef P7T Param7;                     typedef RT (CT::*Signature)(P1T, P2T, P3T, P4T, P5T, P6T, P7T     );};
    template<class RT, class CT, class P1T, class P2T, class P3T, class P4T, class P5T, class P6T, class P7T           >struct function_traits<RT (CT::*)(P1T, P2T, P3T, P4T, P5T, P6T, P7T     )const>{ typedef RT Result;  typedef P1T Param1; typedef P2T Param2; typedef P3T Param3; typedef P4T Param4; typedef P5T Param5; typedef P6T Param6; typedef P7T Param7;                     typedef RT (CT::*Signature)(P1T, P2T, P3T, P4T, P5T, P6T, P7T     );};
    template<class RT          , class P1T, class P2T, class P3T, class P4T, class P5T, class P6T, class P7T           >struct function_traits<RT        (P1T, P2T, P3T, P4T, P5T, P6T, P7T     )     >{ typedef RT Result;  typedef P1T Param1; typedef P2T Param2; typedef P3T Param3; typedef P4T Param4; typedef P5T Param5; typedef P6T Param6; typedef P7T Param7;                     typedef RT       Signature (P1T, P2T, P3T, P4T, P5T, P6T, P7T     );};
    template<class RT, class CT, class P1T, class P2T, class P3T, class P4T, class P5T, class P6T, class P7T, class P8T>struct function_traits<RT (CT::*)(P1T, P2T, P3T, P4T, P5T, P6T, P7T, P8T)     >{ typedef RT Result;  typedef P1T Param1; typedef P2T Param2; typedef P3T Param3; typedef P4T Param4; typedef P5T Param5; typedef P6T Param6; typedef P7T Param7; typedef P8T Param8; typedef RT (CT::*Signature)(P1T, P2T, P3T, P4T, P5T, P6T, P7T, P8T);};
    template<class RT, class CT, class P1T, class P2T, class P3T, class P4T, class P5T, class P6T, class P7T, class P8T>struct function_traits<RT (CT::*)(P1T, P2T, P3T, P4T, P5T, P6T, P7T, P8T)const>{ typedef RT Result;  typedef P1T Param1; typedef P2T Param2; typedef P3T Param3; typedef P4T Param4; typedef P5T Param5; typedef P6T Param6; typedef P7T Param7; typedef P8T Param8; typedef RT (CT::*Signature)(P1T, P2T, P3T, P4T, P5T, P6T, P7T, P8T);};
    template<class RT          , class P1T, class P2T, class P3T, class P4T, class P5T, class P6T, class P7T, class P8T>struct function_traits<RT        (P1T, P2T, P3T, P4T, P5T, P6T, P7T, P8T)     >{ typedef RT Result;  typedef P1T Param1; typedef P2T Param2; typedef P3T Param3; typedef P4T Param4; typedef P5T Param5; typedef P6T Param6; typedef P7T Param7; typedef P8T Param8; typedef RT       Signature (P1T, P2T, P3T, P4T, P5T, P6T, P7T, P8T);};

    template<class T>
    typename function_traits<T>::Signature* bar_helper(T);

    template<class F>
    class FuncTraitsOf{
    public:
        typedef decltype(bar_helper(&F::operator())) fptr;
        typedef typename std::remove_pointer<fptr>::type Signature;     //Signature =   bool __cdecl(int,float)
        typedef typename function_traits< Signature > R;                //R         =   struct function_traits<bool __cdecl(int,float)>
    };

    template< class FuncTraits>class FDSel;
    template<class RT, class CT                                                                                        > struct FDSel< function_traits< RT (CT::*)(                                      )      > >{ typedef fastdelegate::FastDelegate0<                                       RT> R; };
    template<class RT, class CT                                                                                        > struct FDSel< function_traits< RT (CT::*)(                                      )const > >{ typedef fastdelegate::FastDelegate0<                                       RT> R; };
    template<class RT                                                                                                  > struct FDSel< function_traits< RT        (                                      )      > >{ typedef fastdelegate::FastDelegate0<                                       RT> R; };
    template<class RT, class CT, class P1T                                                                             > struct FDSel< function_traits< RT (CT::*)(P1T                                   )      > >{ typedef fastdelegate::FastDelegate1<P1T                                   ,RT> R; };
    template<class RT, class CT, class P1T                                                                             > struct FDSel< function_traits< RT (CT::*)(P1T                                   )const > >{ typedef fastdelegate::FastDelegate1<P1T                                   ,RT> R; };
    template<class RT          , class P1T                                                                             > struct FDSel< function_traits< RT        (P1T                                   )      > >{ typedef fastdelegate::FastDelegate1<P1T                                   ,RT> R; };
    template<class RT, class CT, class P1T, class P2T                                                                  > struct FDSel< function_traits< RT (CT::*)(P1T, P2T                              )      > >{ typedef fastdelegate::FastDelegate2<P1T, P2T                              ,RT> R; };
    template<class RT, class CT, class P1T, class P2T                                                                  > struct FDSel< function_traits< RT (CT::*)(P1T, P2T                              )const > >{ typedef fastdelegate::FastDelegate2<P1T, P2T                              ,RT> R; };
    template<class RT          , class P1T, class P2T                                                                  > struct FDSel< function_traits< RT        (P1T, P2T                              )      > >{ typedef fastdelegate::FastDelegate2<P1T, P2T                              ,RT> R; };
    template<class RT, class CT, class P1T, class P2T, class P3T                                                       > struct FDSel< function_traits< RT (CT::*)(P1T, P2T, P3T                         )      > >{ typedef fastdelegate::FastDelegate3<P1T, P2T, P3T                         ,RT> R; };
    template<class RT, class CT, class P1T, class P2T, class P3T                                                       > struct FDSel< function_traits< RT (CT::*)(P1T, P2T, P3T                         )const > >{ typedef fastdelegate::FastDelegate3<P1T, P2T, P3T                         ,RT> R; };
    template<class RT          , class P1T, class P2T, class P3T                                                       > struct FDSel< function_traits< RT        (P1T, P2T, P3T                         )      > >{ typedef fastdelegate::FastDelegate3<P1T, P2T, P3T                         ,RT> R; };
    template<class RT, class CT, class P1T, class P2T, class P3T, class P4T                                            > struct FDSel< function_traits< RT (CT::*)(P1T, P2T, P3T, P4T                    )      > >{ typedef fastdelegate::FastDelegate4<P1T, P2T, P3T, P4T                    ,RT> R; };
    template<class RT, class CT, class P1T, class P2T, class P3T, class P4T                                            > struct FDSel< function_traits< RT (CT::*)(P1T, P2T, P3T, P4T                    )const > >{ typedef fastdelegate::FastDelegate4<P1T, P2T, P3T, P4T                    ,RT> R; };
    template<class RT          , class P1T, class P2T, class P3T, class P4T                                            > struct FDSel< function_traits< RT        (P1T, P2T, P3T, P4T                    )      > >{ typedef fastdelegate::FastDelegate4<P1T, P2T, P3T, P4T                    ,RT> R; };
    template<class RT, class CT, class P1T, class P2T, class P3T, class P4T, class P5T                                 > struct FDSel< function_traits< RT (CT::*)(P1T, P2T, P3T, P4T, P5T               )      > >{ typedef fastdelegate::FastDelegate5<P1T, P2T, P3T, P4T, P5T               ,RT> R; };
    template<class RT, class CT, class P1T, class P2T, class P3T, class P4T, class P5T                                 > struct FDSel< function_traits< RT (CT::*)(P1T, P2T, P3T, P4T, P5T               )const > >{ typedef fastdelegate::FastDelegate5<P1T, P2T, P3T, P4T, P5T               ,RT> R; };
    template<class RT          , class P1T, class P2T, class P3T, class P4T, class P5T                                 > struct FDSel< function_traits< RT        (P1T, P2T, P3T, P4T, P5T               )      > >{ typedef fastdelegate::FastDelegate5<P1T, P2T, P3T, P4T, P5T               ,RT> R; };
    template<class RT, class CT, class P1T, class P2T, class P3T, class P4T, class P5T, class P6T                      > struct FDSel< function_traits< RT (CT::*)(P1T, P2T, P3T, P4T, P5T, P6T          )      > >{ typedef fastdelegate::FastDelegate6<P1T, P2T, P3T, P4T, P5T, P6T          ,RT> R; };
    template<class RT, class CT, class P1T, class P2T, class P3T, class P4T, class P5T, class P6T                      > struct FDSel< function_traits< RT (CT::*)(P1T, P2T, P3T, P4T, P5T, P6T          )const > >{ typedef fastdelegate::FastDelegate6<P1T, P2T, P3T, P4T, P5T, P6T          ,RT> R; };
    template<class RT          , class P1T, class P2T, class P3T, class P4T, class P5T, class P6T                      > struct FDSel< function_traits< RT        (P1T, P2T, P3T, P4T, P5T, P6T          )      > >{ typedef fastdelegate::FastDelegate6<P1T, P2T, P3T, P4T, P5T, P6T          ,RT> R; };
    template<class RT, class CT, class P1T, class P2T, class P3T, class P4T, class P5T, class P6T, class P7T           > struct FDSel< function_traits< RT (CT::*)(P1T, P2T, P3T, P4T, P5T, P6T, P7T     )      > >{ typedef fastdelegate::FastDelegate7<P1T, P2T, P3T, P4T, P5T, P6T, P7T     ,RT> R; };
    template<class RT, class CT, class P1T, class P2T, class P3T, class P4T, class P5T, class P6T, class P7T           > struct FDSel< function_traits< RT (CT::*)(P1T, P2T, P3T, P4T, P5T, P6T, P7T     )const > >{ typedef fastdelegate::FastDelegate7<P1T, P2T, P3T, P4T, P5T, P6T, P7T     ,RT> R; };
    template<class RT          , class P1T, class P2T, class P3T, class P4T, class P5T, class P6T, class P7T           > struct FDSel< function_traits< RT        (P1T, P2T, P3T, P4T, P5T, P6T, P7T     )      > >{ typedef fastdelegate::FastDelegate7<P1T, P2T, P3T, P4T, P5T, P6T, P7T     ,RT> R; };
    template<class RT, class CT, class P1T, class P2T, class P3T, class P4T, class P5T, class P6T, class P7T, class P8T> struct FDSel< function_traits< RT (CT::*)(P1T, P2T, P3T, P4T, P5T, P6T, P7T, P8T)      > >{ typedef fastdelegate::FastDelegate8<P1T, P2T, P3T, P4T, P5T, P6T, P7T, P8T,RT> R; };
    template<class RT, class CT, class P1T, class P2T, class P3T, class P4T, class P5T, class P6T, class P7T, class P8T> struct FDSel< function_traits< RT (CT::*)(P1T, P2T, P3T, P4T, P5T, P6T, P7T, P8T)const > >{ typedef fastdelegate::FastDelegate8<P1T, P2T, P3T, P4T, P5T, P6T, P7T, P8T,RT> R; };
    template<class RT          , class P1T, class P2T, class P3T, class P4T, class P5T, class P6T, class P7T, class P8T> struct FDSel< function_traits< RT        (P1T, P2T, P3T, P4T, P5T, P6T, P7T, P8T)      > >{ typedef fastdelegate::FastDelegate8<P1T, P2T, P3T, P4T, P5T, P6T, P7T, P8T,RT> R; };


}


template<class F>
typename details::FDSel< typename details::FuncTraitsOf<F>::R >::R MakeDelegate(F& f){
    return fastdelegate::MakeDelegate(&f, &F::operator());
}

将其复制/粘贴到您的 FastDelegate.h 文件中。

不要像这样使用它：

home.visit(fastdelegate::MakeDelegate([&](const Room& a){ /* ... */ }));

而是这样做：

auto d = [&](const Room& a){ /* ... */ };
home.visit(fastdelegate::MakeDelegate(d));

如果我错过了什么，请告诉我。

于 2015-02-25T02:02:58.480 回答
0

“平凡”和一般 lambda 函数之间的区别在于，如果它不属于第一类（没有捕获），它就是一个函数对象。

如果您复制对象 (lambda) 并且它包含对临时对象的引用，或者对将在您的 FastDelegate 销毁之前释放的堆栈分配对象的引用，则您有一个悬空引用，因此会崩溃。

尝试通过复制而不是通过引用来捕获

于 2013-09-05T10:38:42.310 回答