c – std :: function就像委托模板类一样

template<class T>
struct FastDelegate {};

template<class R,class... Args>
struct FastDelegate<R (Args...)> 
{
    template <typename T>
    FastDelegate(T* t,R (T::*f)(Args...)) : m_t(t),m_f(f) {} 

    R operator()(Args... p)
    {
        return (m_t->*m_f)(std::forward<Args>(p)...);
    }

    T* m_t;  // How can I capture T as a type in this partial specialization?
    R  (T::*m_f)(Args...);
};

struct Test
{
 int add ( int x,int y ) { return x+y; }
};

int main ()
{
 int x = 5;
 int y = 4;

 Tester t;
 FastDelegate<int (int,int)> d (&t,&Test::calc );
 int z = d(x,y);
}

您可以将对象捕获为void *,将成员函数存储在随机成??员函数类型中,并具有恢复必要类型的函数.这种方法避免在堆上分配任何内存.有问题的步骤是从某种类型的成员函数转换到另一个成员函数.但是,根据5.2.10 [expr.reinterpret.cast]第10段,只要成员函数在使用之前被转换回其原始类型,就可以安全地使用这种方法：

[…] The result of this conversion is unspecified,except in the following cases:

• converting a prvalue of type “pointer to member function” to a different pointer to member function type and back to its original type yields the original pointer to member value.

下面是一个实现此方法的示例.但是,请注意,使用std :: function< R(Args ...)>可能更容易.使用合适的lambda作为标准库可能会首先实现类似的方法.

#include <iostream>
#include <utility>

template<class T>
struct FastDelegate {};

template<class R,class... Args>
struct FastDelegate<R (Args...)> 
{
    struct dummy {};
    template <typename T>
    FastDelegate(T* t,R (T::*f)(Args...))
        : m_t(t),m_f(reinterpret_cast<void (dummy::*)()>(f)),m_call([](void(dummy::*d)(),void* v,Args... a){
                typedef R (T::*mem)(Args...);
                T* t = static_cast<T*>(v);
                mem f = reinterpret_cast<mem>(d);
                return (t->*f)(std::forward<Args>(a)...);
            }) {
    }

    R operator()(Args... p) {
        return (this->m_call)(this->m_f,this->m_t,std::forward<Args>(p)...);
    }

    void* m_t;
    void  (dummy::*m_f)();
    R     (*m_call)(void (dummy::*)(),void*,Args...);
};

struct Tester
{
 int add ( int x,int y ) {
     std::cout << "add(" << x << "," << y << ")n";
     return x+y;
 }
};

int main ()
{
 int x = 5;
 int y = 4;

 Tester t;
 FastDelegate<int (int,&Tester::add);
 int z = d(x,y);
}