如何在检测习惯用法中要求确切的功能签名？ [英] How to require an exact function signature in the detection idiom?

问题描述

让我们假设我有一个 T 类型，并且我想检测它是否有一个下标运算符，可以用其他类型的 Index 。以下示例可以正常工作：

Let's suppose I have a type T and I want to detect whether it has a subscript operator which I can call with with another type Index. The following example works just fine:

#include <type_traits>
#include <vector>

template < typename T, typename Index >
using subscript_t = decltype(std::declval<T>()[std::declval<Index>()]);

int main()
{
    using a = subscript_t< std::vector<int>, size_t >;
    using b = subscript_t< std::vector<int>, int    >;
}

但是，我希望检测该功能当且仅当函数签名完全匹配时。在上面的示例中，我希望语句 subscript_t< std :: vector< int> ;, int> ;; 抛出类似 no可行的重载运算符[] 之类的错误，因为下标的签名 std :: vector 的运算符是

However, I want the function to be detected if and only if the function signature matches exactly. In the example above I would like the statement subscript_t< std::vector<int>, int >; to throw an error like no viable overloaded operator[], because the signature of the subscript operator for std::vector is

std::vector<T, std::allocator<T>>::operator[](size_type pos);

其中GCC中的 size_type 是无符号长。如何避免发生从 int 到 size_t 的隐式转换？

where size_type in GCC is unsigned long. How can I avoid the implicit conversion from int to size_t to take place?

推荐答案

使用 被检测到 ，您可以这样做：

With is_detected, you may do:

template <typename T, typename Ret, typename Index>
using subscript_t = std::integral_constant<Ret (T::*) (Index), & T::operator[]>;


template <typename T, typename Ret, typename Index>
using has_subscript = is_detected<subscript_t, T, Ret, Index>;

static_assert(has_subscript<std::vector<int>, int&, std::size_t>::value, "!");
static_assert(!has_subscript<std::vector<int>, int&, int>::value, "!");

演示

我在SO文档中这样写：

I wrote this in SO Documentation:

概括类型特征创建：基于SFINAE
有实验特征 detected_or ， detected_t ， is_detected 。

To generalize type_trait creation:based on SFINAE there are experimental traits detected_or, detected_t, is_detected.

使用模板参数 typename默认值，模板< typename ...> Op 和 typename ... Args ：

• 被检测到： std :: true_type 或 std :: false_type 取决于 Op< Args ...>


• detected_t ： Op< Args ...> 或 nonesuch 的别名，具体取决于 Op< Args ...> 。


• detected_or ：具有 value_t 被检测到 ，类型 $ c> Op< Args ...> 或默认，具体取决于 Op< Args ...> ;

• is_detected: alias of std::true_type or std::false_type depending of the validity of Op<Args...>
• detected_t: alias of Op<Args...> or nonesuch depending of validity of Op<Args...>.
• detected_or: alias of a struct with value_t which is is_detected, and type which is Op<Args...> or Default depending of validity of Op<Args...>

可以使用 std :: void_t 如下：

namespace detail {
    template <class Default, class AlwaysVoid,
              template<class...> class Op, class... Args>
    struct detector
    {
        using value_t = std::false_type;
        using type = Default;
    };

    template <class Default, template<class...> class Op, class... Args>
    struct detector<Default, std::void_t<Op<Args...>>, Op, Args...>
    {
        using value_t = std::true_type;
        using type = Op<Args...>;
    };

} // namespace detail

// special type to indicate detection failure
struct nonesuch {
    nonesuch() = delete;
    ~nonesuch() = delete;
    nonesuch(nonesuch const&) = delete;
    void operator=(nonesuch const&) = delete;
};

template <template<class...> class Op, class... Args>
using is_detected =
    typename detail::detector<nonesuch, void, Op, Args...>::value_t;

template <template<class...> class Op, class... Args>
using detected_t = typename detail::detector<nonesuch, void, Op, Args...>::type;

template <class Default, template<class...> class Op, class... Args>
using detected_or = detail::detector<Default, void, Op, Args...>;

检测方法是否存在的特征可以简单地实现： / p>

Traits to detect presence of method can then be simply implemented:

template <typename T, typename ...Ts>
using foo_type = decltype(std::declval<T>().foo(std::declval<Ts>()...));

struct C1 {};

struct C2 {
    int foo(char) const;
};

template <typename T>
using has_foo_char = is_detected<foo_type, T, char>;

static_assert(!has_foo_char<C1>::value, "Unexpected");
static_assert(has_foo_char<C2>::value, "Unexpected");

static_assert(std::is_same<int, detected_t<foo_type, C2, char>>::value,
              "Unexpected");

static_assert(std::is_same<void, // Default
                           detected_or<void, foo_type, C1, char>>::value,
              "Unexpected");
static_assert(std::is_same<int, detected_or<void, foo_type, C2, char>>::value,
              "Unexpected");

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