#pragma once
-#include <memory>
+#include <functional>
#include <limits>
+#include <memory>
#include <type_traits>
-#include <functional>
#include <folly/Portability.h>
using _t = typename T::type;
/**
+ * type_t
+ *
+ * A type alias for the first template type argument. `type_t` is useful for
+ * controlling class-template and function-template partial specialization.
+ *
+ * Example:
+ *
+ * template <typename Value>
+ * class Container {
+ * public:
+ * template <typename... Args>
+ * Container(
+ * type_t<in_place_t, decltype(Value(std::declval<Args>()...))>,
+ * Args&&...);
+ * };
+ *
* void_t
*
* A type alias for `void`. `void_t` is useful for controling class-template
- * partial specialization.
+ * and function-template partial specialization.
*
* Example:
*
* struct has_value_type<T, folly::void_t<typename T::value_type>>
* : std::true_type {};
*/
-#if defined(__cpp_lib_void_t) || defined(_MSC_VER)
-
-/* using override */ using std::void_t;
-#else // defined(__cpp_lib_void_t) || defined(_MSC_VER)
+/**
+ * There is a bug in libstdc++, libc++, and MSVC's STL that causes it to
+ * ignore unused template parameter arguments in template aliases and does not
+ * cause substitution failures. This defect has been recorded here:
+ * http://open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#1558.
+ *
+ * This causes the implementation of std::void_t to be buggy, as it is likely
+ * defined as something like the following:
+ *
+ * template <typename...>
+ * using void_t = void;
+ *
+ * This causes the compiler to ignore all the template arguments and does not
+ * help when one wants to cause substitution failures. Rather declarations
+ * which have void_t in orthogonal specializations are treated as the same.
+ * For example, assuming the possible `T` types are only allowed to have
+ * either the alias `one` or `two` and never both or none:
+ *
+ * template <typename T,
+ * typename std::void_t<std::decay_t<T>::one>* = nullptr>
+ * void foo(T&&) {}
+ * template <typename T,
+ * typename std::void_t<std::decay_t<T>::two>* = nullptr>
+ * void foo(T&&) {}
+ *
+ * The second foo() will be a redefinition because it conflicts with the first
+ * one; void_t does not cause substitution failures - the template types are
+ * just ignored.
+ */
namespace traits_detail {
-template <class...>
-struct void_t_ {
- using type = void;
+template <class T, class...>
+struct type_t_ {
+ using type = T;
};
} // namespace traits_detail
+template <class T, class... Ts>
+using type_t = typename traits_detail::type_t_<T, Ts...>::type;
template <class... Ts>
-using void_t = _t<traits_detail::void_t_<Ts...>>;
-
-#endif // defined(__cpp_lib_void_t) || defined(_MSC_VER)
+using void_t = type_t<void, Ts...>;
/**
* IsRelocatable<T>::value describes the ability of moving around
template <class T>
using is_trivially_copyable = std::is_trivially_copyable<T>;
#endif
-}
+} // namespace traits_detail
struct Ignore {
+ Ignore() = default;
template <class T>
- /* implicit */ Ignore(const T&) {}
+ constexpr /* implicit */ Ignore(const T&) {}
template <class T>
const Ignore& operator=(T const&) const { return *this; }
};
decltype(std::declval<T>() == std::declval<U>()),
bool
> {};
-}
+} // namespace traits_detail_IsEqualityComparable
/* using override */ using traits_detail_IsEqualityComparable::
IsEqualityComparable;
decltype(std::declval<T>() < std::declval<U>()),
bool
> {};
-}
+} // namespace traits_detail_IsLessThanComparable
/* using override */ using traits_detail_IsLessThanComparable::
IsLessThanComparable;
noexcept(swap(std::declval<T&>(), std::declval<T&>()))
> {};
#endif
-}
+} // namespace traits_detail_IsNothrowSwappable
/* using override */ using traits_detail_IsNothrowSwappable::IsNothrowSwappable;
// Lighter-weight than Conjunction, but evaluates all sub-conditions eagerly.
template <class... Ts>
struct StrictConjunction
- : std::is_same<Bools<Ts::value..., true>, Bools<true, Ts::value...>> {};
+ : std::is_same<Bools<Ts::value...>, Bools<(Ts::value || true)...>> {};
+
+template <class... Ts>
+struct StrictDisjunction
+ : Negation<
+ std::is_same<Bools<Ts::value...>, Bools<(Ts::value && false)...>>
+ > {};
} // namespace folly
* regular type, use it like this:
*
* // Make sure you're at namespace ::folly scope
- * template<> FOLLY_ASSUME_RELOCATABLE(MyType)
+ * template <> FOLLY_ASSUME_RELOCATABLE(MyType)
*
* When using it with a template type, use it like this:
*
* // Make sure you're at namespace ::folly scope
- * template<class T1, class T2>
+ * template <class T1, class T2>
* FOLLY_ASSUME_RELOCATABLE(MyType<T1, T2>)
*/
#define FOLLY_ASSUME_RELOCATABLE(...) \
IsRelocatable<U>::value> {};
// Is T one of T1, T2, ..., Tn?
-template <class T, class... Ts>
-struct IsOneOf {
- enum { value = false };
-};
-
-template <class T, class T1, class... Ts>
-struct IsOneOf<T, T1, Ts...> {
- enum { value = std::is_same<T, T1>::value || IsOneOf<T, Ts...>::value };
-};
+template <typename T, typename... Ts>
+using IsOneOf = StrictDisjunction<std::is_same<T, Ts>...>;
/*
* Complementary type traits for integral comparisons.
FOLLY_POP_WARNING
-} // namespace detail {
+} // namespace detail
// same as `x < 0`
template <typename T>
RHS, rhs, typename std::remove_reference<LHS>::type
>(lhs);
}
-
-namespace traits_detail {
-struct InPlaceTag {};
-template <class>
-struct InPlaceTypeTag {};
-template <std::size_t>
-struct InPlaceIndexTag {};
-}
-
-/**
- * Like std::piecewise_construct, a tag type & instance used for in-place
- * construction of non-movable contained types, e.g. by Synchronized.
- * Follows the naming and design of std::in_place suggested in
- * http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0032r2.pdf
- */
-using in_place_t = traits_detail::InPlaceTag (&)(traits_detail::InPlaceTag);
-
-template <class T>
-using in_place_type_t =
- traits_detail::InPlaceTypeTag<T> (&)(traits_detail::InPlaceTypeTag<T>);
-
-template <std::size_t I>
-using in_place_index_t =
- traits_detail::InPlaceIndexTag<I> (&)(traits_detail::InPlaceIndexTag<I>);
-
-inline traits_detail::InPlaceTag in_place(traits_detail::InPlaceTag = {}) {
- return {};
-}
-
-template <class T>
-inline traits_detail::InPlaceTypeTag<T> in_place_type(
- traits_detail::InPlaceTypeTag<T> = {}) {
- return {};
-}
-
-template <std::size_t I>
-inline traits_detail::InPlaceIndexTag<I> in_place_index(
- traits_detail::InPlaceIndexTag<I> = {}) {
- return {};
-}
-
-// For backwards compatibility:
-using construct_in_place_t = in_place_t;
-
-inline traits_detail::InPlaceTag construct_in_place(
- traits_detail::InPlaceTag = {}) {
- return {};
-}
-
-/**
- * Initializer lists are a powerful compile time syntax introduced in C++11
- * but due to their often conflicting syntax they are not used by APIs for
- * construction.
- *
- * Further standard conforming compilers *strongly* favor an
- * std::initalizer_list overload for construction if one exists. The
- * following is a simple tag used to disambiguate construction with
- * initializer lists and regular uniform initialization.
- *
- * For example consider the following case
- *
- * class Something {
- * public:
- * explicit Something(int);
- * Something(std::intiializer_list<int>);
- *
- * operator int();
- * };
- *
- * ...
- * Something something{1}; // SURPRISE!!
- *
- * The last call to instantiate the Something object will go to the
- * initializer_list overload. Which may be surprising to users.
- *
- * If however this tag was used to disambiguate such construction it would be
- * easy for users to see which construction overload their code was referring
- * to. For example
- *
- * class Something {
- * public:
- * explicit Something(int);
- * Something(folly::initlist_construct_t, std::initializer_list<int>);
- *
- * operator int();
- * };
- *
- * ...
- * Something something_one{1}; // not the initializer_list overload
- * Something something_two{folly::initlist_construct, {1}}; // correct
- */
-struct initlist_construct_t {};
-constexpr initlist_construct_t initlist_construct{};
-
} // namespace folly
// Assume nothing when compiling with MSVC.