system/ulib/fbl/include/fbl/type_support.h

// Copyright 2016 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#pragma once

namespace fbl {

template <typename T, T v>
struct integral_constant {
    static constexpr T value = v;

    using value_type = T;
    using type = integral_constant<T, v>;
};

using true_type = integral_constant<bool, true>;
using false_type = integral_constant<bool, false>;

// is_void:
template <typename T>
struct is_void : false_type {};

template <>
struct is_void<void> : true_type {};

template <>
struct is_void<const void> : true_type {};

template <>
struct is_void<volatile void> : true_type {};

template <>
struct is_void<const volatile void> : true_type {};

// is_null_pointer:
template <typename T>
struct is_null_pointer : false_type {};

template <>
struct is_null_pointer<decltype(nullptr)> : true_type {};

template <>
struct is_null_pointer<const decltype(nullptr)> : true_type {};

template <>
struct is_null_pointer<volatile decltype(nullptr)> : true_type {};

template <>
struct is_null_pointer<const volatile decltype(nullptr)> : true_type {};

// is_const:

template <typename T>
struct is_const : false_type {};

template <typename T>
struct is_const<const T> : true_type {};

// is_lvalue_reference:

template <typename T>
struct is_lvalue_reference : false_type {};

template <typename T>
struct is_lvalue_reference<T&> : true_type {};

// is_rvalue_reference:

template <typename T>
struct is_rvalue_reference : false_type {};

template <typename T>
struct is_rvalue_reference<T&&> : true_type {};

// is_reference:
template <typename T>
struct is_reference : false_type {};

template <typename T>
struct is_reference<T&> : true_type {};

template <typename T>
struct is_reference<T&&> : true_type {};

// remove_reference:

template <typename T>
struct remove_reference {
    using type = T;
};

template <typename T>
struct remove_reference<T&> {
    using type = T;
};

template <typename T>
struct remove_reference<T&&> {
    using type = T;
};

// remove_pointer:

template <typename T>
struct remove_pointer {
    using type = T;
};

template <typename T>
struct remove_pointer<T*> {
    using type = T;
};

// remove_const:

template <typename T>
struct remove_const {
    typedef T type;
};

template <typename T>
struct remove_const<const T> {
    typedef T type;
};

// remove_volatile:

template <typename T>
struct remove_volatile {
    typedef T type;
};

template <typename T>
struct remove_volatile<volatile T> {
    typedef T type;
};

// remove_cv:

template <typename T>
struct remove_cv {
    typedef typename remove_volatile<typename remove_const<T>::type>::type type;
};

// remove_extent:

template <typename T>
struct remove_extent {
    using type = T;
};

template <typename T>
struct remove_extent<T[]> {
    using type = T;
};

// Avoid having to pull in a header to name size_t; just use sizeof to
// get at it.
template <typename T, decltype(sizeof(nullptr)) N>
struct remove_extent<T[N]> {
    using type = T;
};

// forward:

template <typename T>
constexpr T&& forward(typename remove_reference<T>::type& t) {
    return static_cast<T&&>(t);
}

template <typename T>
constexpr T&& forward(typename remove_reference<T>::type&& t) {
    static_assert(!is_lvalue_reference<T>::value, "bad fbl::forward call");
    return static_cast<T&&>(t);
}

// is_same:

template<class T, class U> struct is_same : false_type {};
template<class T> struct is_same<T, T> : true_type {};

// enable_if:

template<bool B, class T = void> struct enable_if { };
template<class T> struct enable_if<true, T> {
    typedef T type;
};

// conditional:

template<bool B, class T, class F>
struct conditional { typedef T type; };

template<class T, class F>
struct conditional<false, T, F> { typedef F type; };

// is_integral.  By default, T is not integral (aka, not an integer)
template <typename T>
struct is_integral : false_type {};

// Specializations.  Every basic integral type needs to be called out.
template <> struct is_integral<bool>                   : true_type {};
template <> struct is_integral<char>                   : true_type {};
template <> struct is_integral<char16_t>               : true_type {};
template <> struct is_integral<char32_t>               : true_type {};
template <> struct is_integral<wchar_t>                : true_type {};
template <> struct is_integral<signed char>            : true_type {};
template <> struct is_integral<unsigned char>          : true_type {};
template <> struct is_integral<short int>              : true_type {};
template <> struct is_integral<unsigned short int>     : true_type {};
template <> struct is_integral<int>                    : true_type {};
template <> struct is_integral<unsigned int>           : true_type {};
template <> struct is_integral<long int>               : true_type {};
template <> struct is_integral<unsigned long int>      : true_type {};
template <> struct is_integral<long long int>          : true_type {};
template <> struct is_integral<unsigned long long int> : true_type {};

// is_floating_point.  By default, T is not a floating point type.
template <typename T>
struct is_floating_point : false_type {};

// Specializations.  Every basic floating point type needs to be called out.
template <> struct is_floating_point<float>       : true_type {};
template <> struct is_floating_point<double>      : true_type {};
template <> struct is_floating_point<long double> : true_type {};

// Arithmetic data types are either floats or integers
template <typename T>
struct is_arithmetic :
    integral_constant<bool, is_integral<T>::value || is_floating_point<T>::value> { };

namespace internal {

template<typename T, bool = is_arithmetic<T>::value>
struct is_signed : integral_constant<bool, T(-1) < T(0)> {};
template<typename T>
struct is_signed<T, false> : fbl::false_type {};

template<typename T, bool = is_arithmetic<T>::value>
struct is_unsigned : integral_constant<bool, T(0) < T(-1)> {};
template<typename T>
struct is_unsigned<T, false> : fbl::false_type {};

template<typename T, bool = is_integral<T>::value>
struct is_signed_integer : integral_constant<bool, T(-1) < T(0)> {};
template<typename T>
struct is_signed_integer<T, false> : fbl::false_type {};

template<typename T, bool = is_integral<T>::value>
struct is_unsigned_integer : integral_constant<bool, T(0) < T(-1)> {};
template<typename T>
struct is_unsigned_integer<T, false> : fbl::false_type {};

} // namespace internal

template <typename T>
struct is_signed : internal::is_signed<T>::type {};

template <typename T>
struct is_unsigned : internal::is_unsigned<T>::type {};

template <typename T>
struct is_signed_integer : internal::is_signed_integer<T>::type {};

template <typename T>
struct is_unsigned_integer : internal::is_unsigned_integer<T>::type {};

// is_enum is a builtin
template<typename T>
struct is_enum : integral_constant<bool, __is_enum(T)> { };

// is_pod is a builtin
template<typename T>
struct is_pod : integral_constant<bool, __is_pod(T)> { };

// is_standard_layout is a builtin
template<typename T>
struct is_standard_layout : integral_constant<bool, __is_standard_layout(T)> { };

// underlying_type is a builtin
template<typename T>
struct underlying_type {
  using type = __underlying_type(T);
};

// match_cv: match_cv<SrcType, DestType>::type is DestType cv-qualified in the same way as SrcType.

// Primary template:
template <typename SrcType, typename DestType>
struct match_cv {
  using type = typename remove_cv<DestType>::type;
};

// Specializations for const/volatile/const volatile:
template <typename SrcType, typename DestType>
struct match_cv<const SrcType, DestType> {
  using type = typename remove_cv<DestType>::type const;
};
template <typename SrcType, typename DestType>
struct match_cv<volatile SrcType, DestType> {
  using type = typename remove_cv<DestType>::type volatile;
};
template <typename SrcType, typename DestType>
struct match_cv<const volatile SrcType, DestType> {
  using type = typename remove_cv<DestType>::type const volatile;
};

// is_class builtin
template <typename T>
struct is_class : public integral_constant<bool, __is_class(T)> { };

// is_union builtin
template <typename T>
struct is_union : public integral_constant<bool, __is_union(T)> { };

// is_base_of builtin
template <typename Base, typename Derived>
struct is_base_of : public integral_constant<bool, __is_base_of(Base, Derived)> { };

// has_virtual_destructor
template <typename T>
struct has_virtual_destructor : public integral_constant<bool, __has_virtual_destructor(T)> { };

// has_trivial_destructor
template <typename T>
struct has_trivial_destructor : public integral_constant<bool, __has_trivial_destructor(T)> { };

// is_pointer
namespace internal {
template <typename T>
struct is_pointer : public false_type { };

template <typename T>
struct is_pointer<T*> : public true_type { };
}  // namespace internal

template <typename T>
struct is_pointer :
    public integral_constant<bool,
                             internal::is_pointer<typename remove_cv<T>::type>::value> { };

// is_convertible_pointer
//
// Note: this is a simplified version of std::is_convertible.  Whereas
// std::is_convertible will check to see if any two types are implicitly
// convertible, is_convertible_pointer will only check to see if two pointer
// types are implicitly convertible.  Additionally, no explicit support or tests
// have been added for function pointers conversion.
template <typename From, typename To>
struct is_convertible_pointer {
private:
    static true_type  test(To);
    static false_type test(...);
    static From       make_from_type();

public:
    static constexpr bool value =
        is_pointer<From>::value &&
        is_pointer<To>::value &&
        decltype(test(make_from_type()))::value;
};

namespace internal {

template <typename...>
struct make_void {
    using type = void;
};

} // namespace internal

// Utility type for SFINAE expression evaluation, equivalent to C++17 std::void_t.
template <typename... Ts>
using void_t = typename internal::make_void<Ts...>::type;

// is_function:

// Morally, is_function could be implemented in the same style as
// e.g. is_reference: a base case of false, and specializations of
// true for every kind of function. However, listing all of those
// cases is brittle and language version dependent. For instance,
// C++17 makes noexcept specifiers as part of the type.

// Instead, this implementation uses a pair of tricky overloaded
// functions to detect every type that _isn't_ a function. This
// includes:
// - objects (classes, unions, and primitives) (including incomplete types)
// - references (including references to functions)
// - nullptr_t
// - void
// - pointers (including pointers to functions)
// - pointers to members
// - arrays of both complete and incomplete type

namespace internal {

// This leaves pointers, pointer-to-members, arrays, and functions.
template <typename T>
struct is_object_void_reference_or_null_pointer {
    static constexpr bool value =
        is_class<T>::value ||
        is_union<T>::value ||
        is_void<T>::value ||
        is_reference<T>::value ||
        is_null_pointer<T>::value;
};

struct dummy {};

struct func_tag {};

struct nonfunc_tag {};

template <typename T>
func_tag choose(T*);

template <typename T>
func_tag choose(dummy);

template <typename T>
nonfunc_tag choose(...);

template <typename T>
T& make(int);

template <typename T>
dummy make(...);

} // namespace internal

template <typename T,
          bool = internal::is_object_void_reference_or_null_pointer<T>::value>
struct is_function : public integral_constant<bool,
                                              is_same<internal::func_tag,
                                                      decltype(internal::choose<T>(internal::make<T>(0)))
                                                      >::value> {
};

template <typename T>
struct is_function<T, true> : public false_type {
};

}  // namespace fbl