为什么模板实例化在这里永远存在?

钥匙

在下面的代码中,我要替换

template <typename T, typename... Args>
auto check (rank<1,T>, Args... args) const
        -> std::enable_if_t<!has_argument_type<T, Args...>(), decltype(check(rank<2, Ts...>{}, args...))> {
    return check(rank<2, Ts...>{}, args...);  // Since rank<1,T> derives immediately from rank<2, Ts...>.
}

template <typename T, typename... Args>
auto check (rank<2,T>, Args... args) const
        -> std::enable_if_t<!has_argument_type<T, Args...>(), decltype(check(rank<3, Ts...>{}, args...))> {
    return check(rank<3, Ts...>{}, args...);  // Since rank<2,T> derives immediately from rank<3, Ts...>.
}
// etc... until rank<9>.

与简单

template <std::size_t N, typename T, typename... Args>
auto check (rank<N,T>, Args... args) const
        -> std::enable_if_t<!has_argument_type<T, Args...>(), decltype(check(rank<N+1, Ts...>{}, args...))> {
    return check(rank<N+1, Ts...>{}, args...);  // Since rank<N,T> derives immediately from rank<N+1, Ts...>.
}

template <typename T, typename... Args>
auto check (rank<10, T>, Args... args) const { std::cout << "Nothing found.\n"; }

但是,当我这样做时,尽管具有终止check(rank<10, T>, Args... args)功能,但模板实例化仍会持续进行这是使用上面的长版本的完整代码。抱歉,没有将问题最小化,因为我认为我无法将其最小化并显示问题所在。跳转到main()会向您显示我要执行的简单任务,但是我想使用序列排名和重载解析来解决它。

#include <iostream>
#include <type_traits>
#include <tuple>

template <typename T, typename... Args>
constexpr auto has_argument_type_impl(int)
    -> decltype(std::is_same<typename T::argument_type, std::tuple<Args...>>{});  // Checking both that T::argument_type exists and that it is the same as std::tuple<Args...>.

template <typename T, typename... Args>
constexpr auto has_argument_type_impl(long) -> std::false_type;

template <typename T, typename... Args>
constexpr bool has_argument_type() { return decltype(has_argument_type_impl<T, Args...>(0))::value; }

template <typename T, std::size_t N, typename... Args>
constexpr auto has_argument_type_n_impl(int)
    -> decltype(std::is_same<typename T::template argument_type<N>, std::tuple<Args...>>{});  // Checking both that T::argument_type<N> exists and that it is the same as std::tuple<Args...>.

template <typename T, std::size_t N, typename... Args>
constexpr auto has_argument_type_n_impl(long) -> std::false_type;

template <typename T, std::size_t N, typename... Args>
constexpr bool has_argument_type_n() { return decltype(has_argument_type_n_impl<T, N, Args...>(0))::value; }

template <typename... Ts>
class Factory {
    template <std::size_t, typename...> struct rank;

    template <std::size_t N, typename First, typename... Rest>
    struct rank<N, First, Rest...> : rank<N, Rest...> {};

    template <std::size_t N, typename T> struct rank<N,T> : rank<N+1, Ts...> {};

    template <typename T> struct rank<10, T> {};  // Need to end the instantiations somewhere.
public:
    template <typename... Args>
    decltype(auto) create (Args... args) const {
        return check(rank<0, Ts...>{}, args...);
    }
private:
    template <typename T, typename... Rest, typename... Args>
    auto check (rank<0, T, Rest...>, Args... args) const
            -> std::enable_if_t<has_argument_type<T, Args...>(), decltype(T(args...))> {
        return T(args...);
    }

    template <typename T, typename... Rest, typename... Args>
    auto check (rank<0, T, Rest...>, Args... args) const
            -> std::enable_if_t<!has_argument_type<T, Args...>(), decltype(check(rank<0, Rest...>{}, args...))> {
        return check(rank<0, Rest...>{}, args...);
    }

    template <typename T, typename... Args>
    auto check (rank<0,T>, Args... args) const
            -> std::enable_if_t<!has_argument_type<T, Args...>(), decltype(check(rank<1, Ts...>{}, args...))> {
        return check(rank<1, Ts...>{}, args...);  // Since rank<0,T> derives immediately from rank<1, Ts...>.
    }

    template <std::size_t N, typename T, typename... Rest, typename... Args>
    auto check (rank<N, T, Rest...>, Args... args) const
            -> std::enable_if_t<has_argument_type_n<T, N-1, Args...>(), decltype(T(args...))> {
        return T(args...);
    }

    template <std::size_t N, typename T, typename... Rest, typename... Args>
    auto check (rank<N, T, Rest...>, Args... args) const
            -> std::enable_if_t<!has_argument_type_n<T, N-1, Args...>(), decltype(check(rank<N, Rest...>{}, args...))> {
        return check(rank<N, Rest...>{}, args...);
    }

//  I want to use the following instead of what's below it.
//  template <std::size_t N, typename T, typename... Args>
//  auto check (rank<N,T>, Args... args) const
//          -> std::enable_if_t<!has_argument_type_n<T, N-1, Args...>(), decltype(check(rank<N+1, Ts...>{}, args...))> {
//      return check(rank<N+1, Ts...>{}, args...);  // Since rank<N,T> derives immediately from rank<N+1, Ts...>.
//  }
//
//  template <typename T, typename... Args>
//  auto check (rank<10, T>, Args... args) const { std::cout << "Nothing found.\n"; }

    template <typename T, typename... Args>
    auto check (rank<1,T>, Args... args) const
            -> std::enable_if_t<!has_argument_type_n<T, 0, Args...>(), decltype(check(rank<2, Ts...>{}, args...))> {
        return check(rank<2, Ts...>{}, args...);  // Since rank<1,T> derives immediately from rank<2, Ts...>.
    }

    template <typename T, typename... Args>
    auto check (rank<2,T>, Args... args) const
            -> std::enable_if_t<!has_argument_type_n<T, 1, Args...>(), decltype(check(rank<3, Ts...>{}, args...))> {
        return check(rank<3, Ts...>{}, args...);  // Since rank<2,T> derives immediately from rank<3, Ts...>.
    }
    // etc... until rank<9>.
};

// Testing
struct Object {
    template <std::size_t, typename = void> struct ArgumentType;
    template <typename T> struct ArgumentType<0,T> { using type = std::tuple<int, bool, char, double>; };
    template <typename T> struct ArgumentType<1,T> { using type = std::tuple<bool, char, double>; };
    template <std::size_t N> using argument_type = typename ArgumentType<N>::type;

    Object (int, bool, char, double) { print(); }
    Object (bool, char, double) { print(); }
    void print() const { std::cout << "Object\n"; }
};

struct Thing {
    template <std::size_t, typename = void> struct ArgumentType;
    template <typename T> struct ArgumentType<0,T> { using type = std::tuple<int, int, char>; };
    template <typename T> struct ArgumentType<1,T> { using type = std::tuple<int, char>; };
    template <typename T> struct ArgumentType<2,T> { using type = std::tuple<char>; };
    template <std::size_t N> using argument_type = typename ArgumentType<N>::type;

    Thing (int, int, char) { print(); }
    Thing (int, char) { print(); }
    Thing (char) { print(); }
    void print() const { std::cout << "Thing\n"; }
};

struct Blob {
    using argument_type = std::tuple<int, double>;

    Blob (int, double) { print(); }
    void print() const { std::cout << "Blob\n"; }
};

struct Widget {
    using argument_type = std::tuple<int>;
    Widget (double, double, int, double) { print(); }
    Widget (int) { print(); }
    void print() const { std::cout << "Widget\n"; }
};

int main() {
    Factory<Blob, Object, Thing, Widget>().create(4,3.5);  // Blob
    Factory<Object, Blob, Widget, Thing>().create(2);  // Widget
    Factory<Object, Thing, Blob, Widget>().create(5);  // Widget
    Factory<Blob, Object, Thing, Widget>().create(4,true,'a',7.5);  // Object
    Factory<Blob, Thing, Object, Widget>().create(true,'a',7.5);  // Object
    Factory<Blob, Object, Thing, Widget>().create('a');  // Thing
}

我知道还有其他方法可以做到这一点,但是我试图更好地理解序列排名,并且想知道为什么我不能使用注释掉的部分。如何避免需要使rank<9>当前使该代码正常工作的重复代码(甚至更高等级)?谢谢你的耐心。

注意:实际上,我绝对不能像现在那样手动输入代码的重复部分。因为rank<N, Ts...>用于check重载的最高N值将在编译时确定为最高N值,使得argument_type<N>成员类型存在于所有NTs...因此,我必须使用我注释掉的通用部分,而rank<10,T>我正在使用的将必须用该特定N值替换10。因此,这不仅仅是方便的问题。我必须解决此问题才能继续开发该程序。

编辑:这是一个更小的示例,显示了相同的问题:

#include <iostream>
#include <type_traits>
#include <tuple>

template <typename T>
constexpr auto has_argument_type_impl(int)
    -> decltype(typename T::argument_type{}, std::true_type{});

template <typename T>
constexpr auto has_argument_type_impl(long) -> std::false_type;

template <typename T>
constexpr bool has_argument_type() { return decltype(has_argument_type_impl<T>(0))::value; }

template <typename... Ts>
class Factory {
    template <std::size_t, typename...> struct rank;

    template <std::size_t N, typename First, typename... Rest>
    struct rank<N, First, Rest...> : rank<N, Rest...> {};

    template <std::size_t N, typename T> struct rank<N,T> : rank<N+1, Ts...> {};

    template <typename T> struct rank<10, T> {};  // Need to end the instantiations somewhere.
public:
    template <typename... Args>
    decltype(auto) create (Args... args) const {
        return check(rank<0, Ts...>{}, args...);
    }
private:
    template <std::size_t N, typename T, typename... Rest, typename... Args>
    auto check (rank<N, T, Rest...>, Args... args) const
            -> std::enable_if_t<has_argument_type<T>(), decltype(T(args...))> {
        return T(args...);
    }

    template <std::size_t N, typename T, typename... Rest, typename... Args>
    auto check (rank<N, T, Rest...>, Args... args) const
            -> std::enable_if_t<!has_argument_type<T>(), decltype(check(rank<N, Rest...>{}, args...))> {
        return check(rank<N, Rest...>{}, args...);
    }

    template <typename T, typename... Args>
    auto check (rank<0,T>, Args... args) const
            -> std::enable_if_t<!has_argument_type<T>(), decltype(check(rank<1, Ts...>{}, args...))> {
        return check(rank<1, Ts...>{}, args...);  // Since rank<0,T> derives immediately from rank<1, Ts...>.
    }

//  I want to use the following instead of what's below it.
//  template <std::size_t N, typename T, typename... Args>
//  auto check (rank<N,T>, Args... args) const
//          -> std::enable_if_t<!has_argument_type<T>(), decltype(check(rank<N+1, Ts...>{}, args...))> {
//      return check(rank<N+1, Ts...>{}, args...);  // Since rank<N,T> derives immediately from rank<N+1, Ts...>.
//  }
//
//  template <typename T, typename... Args>
//  auto check (rank<10, T>, Args... args) const { std::cout << "Nothing found.\n"; }

    template <typename T, typename... Args>
    auto check (rank<1,T>, Args... args) const
            -> std::enable_if_t<!has_argument_type<T>(), decltype(check(rank<2, Ts...>{}, args...))> {
        return check(rank<2, Ts...>{}, args...);  // Since rank<1,T> derives immediately from rank<2, Ts...>.
    }

    template <typename T, typename... Args>
    auto check (rank<2,T>, Args... args) const
            -> std::enable_if_t<!has_argument_type<T>(), decltype(check(rank<3, Ts...>{}, args...))> {
        return check(rank<3, Ts...>{}, args...);  // Since rank<2,T> derives immediately from rank<3, Ts...>.
    }
    // etc... until rank<9>.
};

// Testing
struct Object {};
struct Thing {};

struct Blob {
    using argument_type = std::tuple<int, double>;
    Blob (int, double) { std::cout << "Blob\n"; }
};

int main() {
    Factory<Object, Thing, Blob>().create(4,3.5);  // Blob
}
TC

直到过载解析过程中的很晚才开始执行部分排序。

忽略各种check过载之间的所有ping-poning ,最终您最终会遇到

template <std::size_t N, typename T, typename... Args>
auto check (rank<N,T>, Args... args) const
      -> std::enable_if_t<!has_argument_type_n<T, N, Args...>(), 
                          decltype(check(rank<N+1, Ts...>{}, args...))>;

template <typename T, typename... Args>
auto check (rank<10, T>, Args... args) const;

与一个rank<10, something I frankly don't care about>两个重载都将进行推论和替换作为替换为第一个签名的返回类型的一部分,您将实例化rank<11, Ts...>,而后者又绕过了的终止特化rank,从而产生了无限的模板实例化链。您甚至没有达到部分订购决胜局选择第二个过载的地步。

只需将第一个重载限制为即可N < 10它需要按词法在返回类型之前(以便N >= 10在编译器不尝试替代返回类型时),因此请将其放在默认模板参数中。

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