Traits.h

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/*
 * Copyright 2011-present Facebook, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

// @author: Andrei Alexandrescu

#pragma once

#include <functional>
#include <limits>
#include <memory>
#include <type_traits>

#include <folly/Portability.h>

// libc++ doesn't provide this header, nor does msvc
#if __has_include(<bits/c++config.h>)
// This file appears in two locations: inside fbcode and in the
// libstdc++ source code (when embedding fbstring as std::string).
// To aid in this schizophrenic use, two macros are defined in
// c++config.h:
//   _LIBSTDCXX_FBSTRING - Set inside libstdc++.  This is useful to
//      gate use inside fbcode v. libstdc++
#include <bits/c++config.h>
#endif

#define FOLLY_CREATE_HAS_MEMBER_TYPE_TRAITS(classname, type_name)              \
  template <typename TTheClass_>                                               \
  struct classname##__folly_traits_impl__ {                                    \
    template <typename UTheClass_>                                             \
    static constexpr bool test(typename UTheClass_::type_name*) {              \
      return true;                                                             \
    }                                                                          \
    template <typename>                                                        \
    static constexpr bool test(...) {                                          \
      return false;                                                            \
    }                                                                          \
  };                                                                           \
  template <typename TTheClass_>                                               \
  using classname = typename std::conditional<                                 \
      classname##__folly_traits_impl__<TTheClass_>::template test<TTheClass_>( \
          nullptr),                                                            \
      std::true_type,                                                          \
      std::false_type>::type

#define FOLLY_CREATE_HAS_MEMBER_FN_TRAITS_IMPL(classname, func_name, cv_qual) \
  template <typename TTheClass_, typename RTheReturn_, typename... TTheArgs_> \
  struct classname##__folly_traits_impl__<                                    \
      TTheClass_,                                                             \
      RTheReturn_(TTheArgs_...) cv_qual> {                                    \
    template <                                                                \
        typename UTheClass_,                                                  \
        RTheReturn_ (UTheClass_::*)(TTheArgs_...) cv_qual>                    \
    struct sfinae {};                                                         \
    template <typename UTheClass_>                                            \
    static std::true_type test(sfinae<UTheClass_, &UTheClass_::func_name>*);  \
    template <typename>                                                       \
    static std::false_type test(...);                                         \
  }

/*
 * The FOLLY_CREATE_HAS_MEMBER_FN_TRAITS is used to create traits
 * classes that check for the existence of a member function with
 * a given name and signature. It currently does not support
 * checking for inherited members.
 *
 * Such classes receive two template parameters: the class to be checked
 * and the signature of the member function. A static boolean field
 * named `value` (which is also constexpr) tells whether such member
 * function exists.
 *
 * Each traits class created is bound only to the member name, not to
 * its signature nor to the type of the class containing it.
 *
 * Say you need to know if a given class has a member function named
 * `test` with the following signature:
 *
 *    int test() const;
 *
 * You'd need this macro to create a traits class to check for a member
 * named `test`, and then use this traits class to check for the signature:
 *
 * namespace {
 *
 * FOLLY_CREATE_HAS_MEMBER_FN_TRAITS(has_test_traits, test);
 *
 * } // unnamed-namespace
 *
 * void some_func() {
 *   cout << "Does class Foo have a member int test() const? "
 *     << boolalpha << has_test_traits<Foo, int() const>::value;
 * }
 *
 * You can use the same traits class to test for a completely different
 * signature, on a completely different class, as long as the member name
 * is the same:
 *
 * void some_func() {
 *   cout << "Does class Foo have a member int test()? "
 *     << boolalpha << has_test_traits<Foo, int()>::value;
 *   cout << "Does class Foo have a member int test() const? "
 *     << boolalpha << has_test_traits<Foo, int() const>::value;
 *   cout << "Does class Bar have a member double test(const string&, long)? "
 *     << boolalpha << has_test_traits<Bar, double(const string&, long)>::value;
 * }
 *
 * @author: Marcelo Juchem <marcelo@fb.com>
 */
#define FOLLY_CREATE_HAS_MEMBER_FN_TRAITS(classname, func_name)               \
  template <typename, typename>                                               \
  struct classname##__folly_traits_impl__;                                    \
  FOLLY_CREATE_HAS_MEMBER_FN_TRAITS_IMPL(classname, func_name, );             \
  FOLLY_CREATE_HAS_MEMBER_FN_TRAITS_IMPL(classname, func_name, const);        \
  FOLLY_CREATE_HAS_MEMBER_FN_TRAITS_IMPL(                                     \
      classname, func_name, /* nolint */ volatile);                           \
  FOLLY_CREATE_HAS_MEMBER_FN_TRAITS_IMPL(                                     \
      classname, func_name, /* nolint */ volatile const);                     \
  template <typename TTheClass_, typename TTheSignature_>                     \
  using classname =                                                           \
      decltype(classname##__folly_traits_impl__<TTheClass_, TTheSignature_>:: \
                   template test<TTheClass_>(nullptr))

namespace folly {

#if __cpp_lib_bool_constant || _MSC_VER

using std::bool_constant;

#else

//  mimic: std::bool_constant, C++17
template <bool B>
using bool_constant = std::integral_constant<bool, B>;

#endif

template <std::size_t I>
using index_constant = std::integral_constant<std::size_t, I>;

/***
 *  _t
 *
 *  Instead of:
 *
 *    using decayed = typename std::decay<T>::type;
 *
 *  With the C++14 standard trait aliases, we could use:
 *
 *    using decayed = std::decay_t<T>;
 *
 *  Without them, we could use:
 *
 *    using decayed = _t<std::decay<T>>;
 *
 *  Also useful for any other library with template types having dependent
 *  member types named `type`, like the standard trait types.
 */
template <typename T>
using _t = typename T::type;

template <typename T>
struct remove_cvref {
  using type =
      typename std::remove_cv<typename std::remove_reference<T>::type>::type;
};
template <typename T>
using remove_cvref_t = typename remove_cvref<T>::type;

namespace detail {
template <typename Src>
struct like_ {
  template <typename Dst>
  using apply = Dst;
};
template <typename Src>
struct like_<Src const> {
  template <typename Dst>
  using apply = Dst const;
};
template <typename Src>
struct like_<Src volatile> {
  template <typename Dst>
  using apply = Dst volatile;
};
template <typename Src>
struct like_<Src const volatile> {
  template <typename Dst>
  using apply = Dst const volatile;
};
template <typename Src>
struct like_<Src&> {
  template <typename Dst>
  using apply = typename like_<Src>::template apply<Dst>&;
};
template <typename Src>
struct like_<Src&&> {
  template <typename Dst>
  using apply = typename like_<Src>::template apply<Dst>&&;
};
} // namespace detail

//  mimic: like_t, p0847r0
template <typename Src, typename Dst>
using like_t = typename detail::like_<Src>::template apply<remove_cvref_t<Dst>>;

//  mimic: like, p0847r0
template <typename Src, typename Dst>
struct like {
  using type = like_t<Src, Dst>;
};

namespace traits_detail {
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 = type_t<void, Ts...>;

// Older versions of libstdc++ do not provide std::is_trivially_copyable
#if defined(__clang__) && !defined(_LIBCPP_VERSION)
template <class T>
struct is_trivially_copyable : bool_constant<__is_trivially_copyable(T)> {};
#elif defined(__GNUC__) && !defined(__clang__) && __GNUC__ < 5
template <class T>
struct is_trivially_copyable : std::is_trivial<T> {};
#else
template <class T>
using is_trivially_copyable = std::is_trivially_copyable<T>;
#endif

/*
 * IsZeroInitializable describes the property that default construction is the
 * same as memset(dst, 0, sizeof(T)).
 */

namespace traits_detail {

#define FOLLY_HAS_TRUE_XXX(name)                                             \
  FOLLY_CREATE_HAS_MEMBER_TYPE_TRAITS(has_##name, name);                     \
  template <class T>                                                         \
  struct name##_is_true : std::is_same<typename T::name, std::true_type> {}; \
  template <class T>                                                         \
  struct has_true_##name : std::conditional<                                 \
                               has_##name<T>::value,                         \
                               name##_is_true<T>,                            \
                               std::false_type>::type {}

FOLLY_HAS_TRUE_XXX(IsRelocatable);
FOLLY_HAS_TRUE_XXX(IsZeroInitializable);

#undef FOLLY_HAS_TRUE_XXX

} // namespace traits_detail

struct Ignore {
  Ignore() = default;
  template <class T>
  constexpr /* implicit */ Ignore(const T&) {}
  template <class T>
  const Ignore& operator=(T const&) const {
    return *this;
  }
};

template <class...>
using Ignored = Ignore;

namespace traits_detail_IsEqualityComparable {
Ignore operator==(Ignore, Ignore);

template <class T, class U = T>
struct IsEqualityComparable
    : std::is_convertible<
          decltype(std::declval<T>() == std::declval<U>()),
          bool> {};
} // namespace traits_detail_IsEqualityComparable

/* using override */ using traits_detail_IsEqualityComparable::
    IsEqualityComparable;

namespace traits_detail_IsLessThanComparable {
Ignore operator<(Ignore, Ignore);

template <class T, class U = T>
struct IsLessThanComparable
    : std::is_convertible<
          decltype(std::declval<T>() < std::declval<U>()),
          bool> {};
} // namespace traits_detail_IsLessThanComparable

/* using override */ using traits_detail_IsLessThanComparable::
    IsLessThanComparable;

namespace traits_detail_IsNothrowSwappable {
#if defined(__cpp_lib_is_swappable) || (_CPPLIB_VER && _HAS_CXX17)
// MSVC 2015+ already implements the C++17 P0185R1 proposal which
// adds std::is_nothrow_swappable, so use it instead if C++17 mode
// is enabled.
template <typename T>
using IsNothrowSwappable = std::is_nothrow_swappable<T>;
#elif _CPPLIB_VER
// MSVC 2015+ defines the base even if C++17 is disabled, and
// MSVC 2015 has issues with our fallback implementation due to
// over-eager evaluation of noexcept.
template <typename T>
using IsNothrowSwappable = std::_Is_nothrow_swappable<T>;
#else
/* using override */ using std::swap;

template <class T>
struct IsNothrowSwappable
    : bool_constant<std::is_nothrow_move_constructible<T>::value&& noexcept(
          swap(std::declval<T&>(), std::declval<T&>()))> {};
#endif
} // namespace traits_detail_IsNothrowSwappable

/* using override */ using traits_detail_IsNothrowSwappable::IsNothrowSwappable;

template <class T>
struct IsRelocatable : std::conditional<
                           traits_detail::has_IsRelocatable<T>::value,
                           traits_detail::has_true_IsRelocatable<T>,
                           // TODO add this line (and some tests for it) when we
                           // upgrade to gcc 4.7
                           // std::is_trivially_move_constructible<T>::value ||
                           is_trivially_copyable<T>>::type {};

template <class T>
struct IsZeroInitializable
    : std::conditional<
          traits_detail::has_IsZeroInitializable<T>::value,
          traits_detail::has_true_IsZeroInitializable<T>,
          bool_constant<!std::is_class<T>::value>>::type {};

template <typename...>
struct Conjunction : std::true_type {};
template <typename T>
struct Conjunction<T> : T {};
template <typename T, typename... TList>
struct Conjunction<T, TList...>
    : std::conditional<T::value, Conjunction<TList...>, T>::type {};

template <typename...>
struct Disjunction : std::false_type {};
template <typename T>
struct Disjunction<T> : T {};
template <typename T, typename... TList>
struct Disjunction<T, TList...>
    : std::conditional<T::value, T, Disjunction<TList...>>::type {};

template <typename T>
struct Negation : bool_constant<!T::value> {};

template <bool... Bs>
struct Bools {
  using valid_type = bool;
  static constexpr std::size_t size() {
    return sizeof...(Bs);
  }
};

// Lighter-weight than Conjunction, but evaluates all sub-conditions eagerly.
template <class... Ts>
struct StrictConjunction
    : 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

#define FOLLY_ASSUME_RELOCATABLE(...) \
  struct IsRelocatable<__VA_ARGS__> : std::true_type {}

// Use this macro ONLY at global level (no namespace)
#define FOLLY_ASSUME_FBVECTOR_COMPATIBLE(...) \
  namespace folly {                           \
  template <>                                 \
  FOLLY_ASSUME_RELOCATABLE(__VA_ARGS__);      \
  }
// Use this macro ONLY at global level (no namespace)
#define FOLLY_ASSUME_FBVECTOR_COMPATIBLE_1(...) \
  namespace folly {                             \
  template <class T1>                           \
  FOLLY_ASSUME_RELOCATABLE(__VA_ARGS__<T1>);    \
  }
// Use this macro ONLY at global level (no namespace)
#define FOLLY_ASSUME_FBVECTOR_COMPATIBLE_2(...)  \
  namespace folly {                              \
  template <class T1, class T2>                  \
  FOLLY_ASSUME_RELOCATABLE(__VA_ARGS__<T1, T2>); \
  }
// Use this macro ONLY at global level (no namespace)
#define FOLLY_ASSUME_FBVECTOR_COMPATIBLE_3(...)      \
  namespace folly {                                  \
  template <class T1, class T2, class T3>            \
  FOLLY_ASSUME_RELOCATABLE(__VA_ARGS__<T1, T2, T3>); \
  }
// Use this macro ONLY at global level (no namespace)
#define FOLLY_ASSUME_FBVECTOR_COMPATIBLE_4(...)          \
  namespace folly {                                      \
  template <class T1, class T2, class T3, class T4>      \
  FOLLY_ASSUME_RELOCATABLE(__VA_ARGS__<T1, T2, T3, T4>); \
  }

FOLLY_NAMESPACE_STD_BEGIN

template <class T, class U>
struct pair;
#ifndef _GLIBCXX_USE_FB
FOLLY_GLIBCXX_NAMESPACE_CXX11_BEGIN
template <class T, class R, class A>
class basic_string;
FOLLY_GLIBCXX_NAMESPACE_CXX11_END
#else
template <class T, class R, class A, class S>
class basic_string;
#endif
template <class T, class A>
class vector;
template <class T, class A>
class deque;
template <class T, class C, class A>
class set;
template <class K, class V, class C, class A>
class map;
template <class T>
class shared_ptr;

FOLLY_NAMESPACE_STD_END

namespace folly {

// STL commonly-used types
template <class T, class U>
struct IsRelocatable<std::pair<T, U>>
    : bool_constant<IsRelocatable<T>::value && IsRelocatable<U>::value> {};

// Is T one of T1, T2, ..., Tn?
template <typename T, typename... Ts>
using IsOneOf = StrictDisjunction<std::is_same<T, Ts>...>;

/*
 * Complementary type traits for integral comparisons.
 *
 * For instance, `if(x < 0)` yields an error in clang for unsigned types
 *  when -Werror is used due to -Wtautological-compare
 *
 *
 * @author: Marcelo Juchem <marcelo@fb.com>
 */

namespace detail {

template <typename T, bool>
struct is_negative_impl {
  constexpr static bool check(T x) {
    return x < 0;
  }
};

template <typename T>
struct is_negative_impl<T, false> {
  constexpr static bool check(T) {
    return false;
  }
};

// folly::to integral specializations can end up generating code
// inside what are really static ifs (not executed because of the templated
// types) that violate -Wsign-compare and/or -Wbool-compare so suppress them
// in order to not prevent all calling code from using it.
FOLLY_PUSH_WARNING
FOLLY_GNU_DISABLE_WARNING("-Wsign-compare")
#if __GNUC_PREREQ(5, 0)
FOLLY_GNU_DISABLE_WARNING("-Wbool-compare")
#endif
FOLLY_MSVC_DISABLE_WARNING(4388) // sign-compare
FOLLY_MSVC_DISABLE_WARNING(4804) // bool-compare

template <typename RHS, RHS rhs, typename LHS>
bool less_than_impl(LHS const lhs) {
  // clang-format off
  return
      rhs > std::numeric_limits<LHS>::max() ? true :
      rhs <= std::numeric_limits<LHS>::min() ? false :
      lhs < rhs;
  // clang-format on
}

template <typename RHS, RHS rhs, typename LHS>
bool greater_than_impl(LHS const lhs) {
  // clang-format off
  return
      rhs > std::numeric_limits<LHS>::max() ? false :
      rhs < std::numeric_limits<LHS>::min() ? true :
      lhs > rhs;
  // clang-format on
}

FOLLY_POP_WARNING

} // namespace detail

// same as `x < 0`
template <typename T>
constexpr bool is_negative(T x) {
  return folly::detail::is_negative_impl<T, std::is_signed<T>::value>::check(x);
}

// same as `x <= 0`
template <typename T>
constexpr bool is_non_positive(T x) {
  return !x || folly::is_negative(x);
}

// same as `x > 0`
template <typename T>
constexpr bool is_positive(T x) {
  return !is_non_positive(x);
}

// same as `x >= 0`
template <typename T>
constexpr bool is_non_negative(T x) {
  return !x || is_positive(x);
}

template <typename RHS, RHS rhs, typename LHS>
bool less_than(LHS const lhs) {
  return detail::
      less_than_impl<RHS, rhs, typename std::remove_reference<LHS>::type>(lhs);
}

template <typename RHS, RHS rhs, typename LHS>
bool greater_than(LHS const lhs) {
  return detail::
      greater_than_impl<RHS, rhs, typename std::remove_reference<LHS>::type>(
          lhs);
}
} // namespace folly

// Assume nothing when compiling with MSVC.
#ifndef _MSC_VER
// gcc-5.0 changed string's implementation in libstdc++ to be non-relocatable
#if !_GLIBCXX_USE_CXX11_ABI
FOLLY_ASSUME_FBVECTOR_COMPATIBLE_3(std::basic_string)
#endif
FOLLY_ASSUME_FBVECTOR_COMPATIBLE_2(std::vector)
FOLLY_ASSUME_FBVECTOR_COMPATIBLE_2(std::deque)
FOLLY_ASSUME_FBVECTOR_COMPATIBLE_2(std::unique_ptr)
FOLLY_ASSUME_FBVECTOR_COMPATIBLE_1(std::shared_ptr)
FOLLY_ASSUME_FBVECTOR_COMPATIBLE_1(std::function)
#endif

/* Some combinations of compilers and C++ libraries make __int128 and
 * unsigned __int128 available but do not correctly define their standard type
 * traits.
 *
 * If FOLLY_SUPPLY_MISSING_INT128_TRAITS is defined, we define these traits
 * here.
 *
 * @author: Phil Willoughby <philwill@fb.com>
 */
#if FOLLY_SUPPLY_MISSING_INT128_TRAITS
FOLLY_NAMESPACE_STD_BEGIN
template <>
struct is_arithmetic<__int128> : ::std::true_type {};
template <>
struct is_arithmetic<unsigned __int128> : ::std::true_type {};
template <>
struct is_integral<__int128> : ::std::true_type {};
template <>
struct is_integral<unsigned __int128> : ::std::true_type {};
template <>
struct make_unsigned<__int128> {
  typedef unsigned __int128 type;
};
template <>
struct make_signed<__int128> {
  typedef __int128 type;
};
template <>
struct make_unsigned<unsigned __int128> {
  typedef unsigned __int128 type;
};
template <>
struct make_signed<unsigned __int128> {
  typedef __int128 type;
};
template <>
struct is_signed<__int128> : ::std::true_type {};
template <>
struct is_unsigned<unsigned __int128> : ::std::true_type {};
FOLLY_NAMESPACE_STD_END
#endif // FOLLY_SUPPLY_MISSING_INT128_TRAITS