假设我有一个 T 类型，我想检测它是否有一个下标运算符，我可以用另一个类型 Index 调用它.以下示例工作正常:
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，我希望当且仅当函数签名完全匹配时才能检测到该函数.在上面的例子中，我想要语句 subscript_t<std::vector<int>, int >; 抛出类似no可靠重载运算符[]的错误，因为std::vector的下标运算符的签名 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 size_type 在 GCC 中是 unsigned long.如何避免从 int 到 size_t 的隐式转换发生?
where size_type in GCC is unsigned long. How can I avoid the implicit conversion from int to size_t to take place?
With is_detected，你可以这样做:
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 文档中写了这个:
I wrote this in SO Documentation:
概括type_trait的创建: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.
带模板参数typename Default，template 和 typename ... Args:
is_detected:std::true_type 或 std::false_type 的别名，取决于 Opdetected_t:Op 或 nonesuch 的别名，取决于 Op.detected_or:value_t 是 is_detected 和 type 是 的结构的别名Op 或 Default 取决于 Opis_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 为 SFINAE 实现如下:
which can be implemented using std::void_t for SFINAE as following:
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...>;
然后可以简单地实现检测方法存在的特征:
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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