c++ - 如何获取指向 Factory::method(variadics ...) 的指针以进行延迟调用？

Question

我从这个网站找到了下面的代码，但我无法适应我的目的:

我有用于创建带有可变参数的对象的工厂方法，并且有函数“apply”用于从工厂调用方法并解压到之前保存到 std::tuple 中的参数...

感谢您的帮助!

#include <iostream>
#include <utility>
#include <iostream>
#include <functional>
#include <utility>
#include <tuple>

class A {
public:
    A(int, int){}
};

class B : public A {
public:
    B(int x, int y):A(x, y){}
};

template <class Base, class Derived>
class BaseFactory {
public:
    template <class ...Args>
    static Base* create(Args&&... args){
        return new Derived(std::forward<Args>(args)...);
    }
};


// ------------- UTILITY---------------
template<int...> struct index_tuple{};

template<int I, typename IndexTuple, typename... Types>
struct make_indexes_impl;

template<int I, int... Indexes, typename T, typename ... Types>
struct make_indexes_impl<I, index_tuple<Indexes...>, T, Types...> {
    typedef typename make_indexes_impl<I + 1, index_tuple<Indexes..., I>, Types...>::type type;
};

template<int I, int... Indexes>
struct make_indexes_impl<I, index_tuple<Indexes...> > {
    typedef index_tuple<Indexes...> type;
};

template<typename ... Types>
struct make_indexes : make_indexes_impl<0, index_tuple<>, Types...> {};


// ----------UNPACK TUPLE AND APPLY TO FUNCTION ---------

template<class Ret, class... Args, int... Indexes >
Ret apply_helper( Ret (*pf)(Args...), index_tuple< Indexes... >, std::tuple<Args...>&& tup){
    return pf( std::forward<Args>( std::get<Indexes>(tup))... );
}

template<class Ret, class ... Args>
Ret apply(Ret (*pf)(Args...), const std::tuple<Args...>&  tup){
    return apply_helper(pf, typename make_indexes<Args...>::type(), std::tuple<Args...>(tup));
}

template<class Ret, class ... Args>
Ret apply(Ret (*pf)(Args...), std::tuple<Args...>&&  tup){
    return apply_helper(pf, typename make_indexes<Args...>::type(), std::forward<std::tuple<Args...>>(tup));
}

int main(){
    // I got error: apply(<unresolved overloaded function type>, std::tuple<int, int>)
    A* = apply(&BaseFactory<A, B>::create<int,int>, std::make_tuple(5, 25));
    return 0;
}

...是的，我确实读取了编译器的输出，但我如何才能获得代码的工作版本？

Answer 1

apply 的函数类型不正确。它应该是 Args&&...(就像您在 BaseFactory 中那样)而不是 Args...:

template<class Ret, class... Args, int... Indexes >
Ret apply_helper( Ret (*pf)(Args&&...), index_tuple< Indexes... >, std::tuple<Args...>&& tup){
    //                          ^^
    return pf( std::forward<Args>( std::get<Indexes>(tup))... );
}

template<class Ret, class ... Args>
Ret apply(Ret (*pf)(Args&&...), const std::tuple<Args...>&  tup){
    //                  ^^
    return apply_helper(pf, typename make_indexes<Args...>::type(), std::tuple<Args...>(tup));
}

template<class Ret, class ... Args>
Ret apply(Ret (*pf)(Args&&...), std::tuple<Args...>&&  tup){
    //                  ^^
    return apply_helper(pf, typename make_indexes<Args...>::type(), std::forward<std::tuple<Args...>>(tup));
}

int main(){
    A* a = apply(&BaseFactory<A, B>::create<int,int>, std::make_tuple(5, 25));
    return 0;
}