C++模板替换错误

Question

我真的很想自己解决这个问题，但花几个小时一次又一次地看同一个代码并不会让我开心。该错误消息看起来很简单，它指向Not2模板。它抱怨说没有Apply成员，但显然有Apply，其中两个。 Function2和Not2有一个非常相似的形式，但编译器并没有说任何关于Function2，所以Not2内部有问题。 Not2意味着采取Function2模板并取消结果。

我尽可能地删除了不相关的代码，但代码看起来相当长。如果你认为阅读所有不值得花费的时间，请跳过。

main.cpp:130:24: error: type/value mismatch at argument 1 in template parameter list for 'template<class T> constexpr const int unbox<T>' 

    using Apply = Box<!unbox<T::template Apply<U>::template Apply<V>>>; 

         ^

main.cpp:130:24: note: expected a type, got 'typename T::Apply<U>::Apply<V>' 

main.cpp:130:70: error: template argument 1 is invalid 

    using Apply = Box<!unbox<T::template Apply<U>::template Apply<V>>>; 

                    ^

main.cpp: In instantiation of 'struct Sort_<List<Box<2>, Box<1> > >': 

main.cpp:155:37: required by substitution of 'template<class T> using Sort = typename Sort_::Type [with T = List<Box<2>, Box<1> >]' 

main.cpp:159:38: required from here 

main.cpp:151:77: error: no class template named 'Apply' in 'Not2<Function2<LessThan_> >::Apply<Box<2> > {aka struct Not2<Function2<LessThan_> >::Apply_<Box<2> >}' 

    Filter<Tail<T>, Not2<LessThan>::template Apply<Head<T>>::template Apply>> Type; 

                      ^

---

template<int n> 
struct Box 
{ 
}; 

template<typename T> 
struct Unbox_; 

template<int n> 
struct Unbox_<Box<n>> 
{ 
    static constexpr int value = n; 
}; 

template<typename T> 
constexpr int unbox = Unbox_<T>::value; 

template<typename ...> 
struct List 
{ 
}; 

template<> 
struct List<> 
{ 
}; 

template<typename, typename> 
struct Cons_; 

template<typename T, typename ...Ts> 
struct Cons_<T, List<Ts...>> 
{ 
    typedef List<T, Ts...> Type; 
}; 

template<typename T, typename U> 
using Cons = typename Cons_<T, U>::Type; 

template<typename> 
struct Head_; 

template<typename T, typename ...Ts> 
struct Head_<List<T, Ts...>> 
{ 
    typedef T Type; 
}; 

template<typename T> 
using Head = typename Head_<T>::Type; 

template<typename> 
struct Tail_; 

template<typename T, typename ...Ts> 
struct Tail_<List<T, Ts...>> 
{ 
    typedef List<Ts...> Type; 
}; 

template<typename T> 
using Tail = typename Tail_<T>::Type; 

template<typename T, typename U> 
struct Merge_ 
{ 
    typedef Cons<Head<T>, typename Merge_<Tail<T>, U>::Type> Type; 
}; 

template<typename T> 
struct Merge_<List<>, T> 
{ 
    typedef T Type; 
}; 

template<typename T, typename U> 
using Merge = typename Merge_<T, U>::Type; 

template<typename, typename T, typename> 
struct If_ 
{ 
    typedef T Type; 
}; 

template<typename T, typename U> 
struct If_<Box<0>, T, U> 
{ 
    typedef U Type; 
}; 

template<typename T, typename U, typename V> 
using If = typename If_<T, U, V>::Type; 

template<typename T, template<typename> class U> 
struct Filter_ 
{ 
    typedef If<U<Head<T>>, Cons<Head<T>, typename Filter_<Tail<T>, U>::Type>, 
    typename Filter_<Tail<T>, U>::Type> Type; 
}; 

template<template<typename> class T> 
struct Filter_<List<>, T> 
{ 
    typedef List<> Type; 
}; 

template<typename T, template<typename> class U> 
using Filter = typename Filter_<T, U>::Type; 

template<template<typename, typename> class T> 
struct Function2 
{ 
    template<typename U> 
    struct Apply_ 
    { 
    template<typename V> 
    using Apply = T<U, V>; 
    }; 
    template<typename U> 
    using Apply = Apply_<U>; 
}; 

template<typename T> 
struct Not2 
{ 
    template<typename U> 
    struct Apply_ 
    { 
    template<typename V> 
    using Apply = Box<!unbox<T::template Apply<U>::template Apply<V>>>; 
    }; 
    template<typename U> 
    using Apply = Apply_<U>; 
}; 

template<typename T, typename U> 
struct LessThan_2 
{ 
    typedef Box<unbox<T> < unbox<U>> Type; 
}; 

template<typename T, typename U> 
using LessThan_ = typename LessThan_2<T, U>::Type; 

using LessThan = Function2<LessThan_>; 

template<typename T> 
struct Sort_ 
{ 
    typedef Merge<Merge<Filter<Tail<T>, LessThan::Apply<Head<T>>::template Apply>, List<Head<T>>>, 
    Filter<Tail<T>, Not2<LessThan>::template Apply<Head<T>>::template Apply>> Type; 
}; 

template<typename T> 
using Sort = typename Sort_<T>::Type; 

int main() 
{ 
    typedef Sort<List<Box<2>, Box<1>>> L; 
} 

Answer 1

我相信你只是缺少一个typename上给你错误的行（130）的unbox模板参数列表内。

此外，您似乎没有对Sort_和Merge_模板进行任何排序，因为它们只是将头以Coliru illustration为中心连接起来。我猜你的代码还没有完成。