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// Copyright 2017 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include <array>
#include <memory>
namespace v8 {
namespace base {
namespace detail {
// make_array_helper statically iteratively creates the index list 0 .. Size-1.
// A specialization for the base case (first index is 0) finally constructs the
// array.
// TODO(clemensh): Use std::index_sequence once we have C++14 support.
template <class Function, std::size_t... Indexes>
struct make_array_helper;
template <class Function, std::size_t... Indexes>
struct make_array_helper<Function, 0, Indexes...> {
constexpr static std::array<typename std::result_of<Function(size_t)>::type,
sizeof...(Indexes) + 1>
make_array(Function f) {
return {{f(0), f(Indexes)...}};
template <class Function, std::size_t FirstIndex, std::size_t... Indexes>
struct make_array_helper<Function, FirstIndex, Indexes...>
: make_array_helper<Function, FirstIndex - 1, FirstIndex, Indexes...> {};
} // namespace detail
// base::make_array: Create an array of fixed length, initialized by a function.
// The content of the array is created by calling the function with 0 .. Size-1.
// Example usage to create the array {0, 2, 4}:
// std::array<int, 3> arr = base::make_array<3>(
// [](std::size_t i) { return static_cast<int>(2 * i); });
// The resulting array will be constexpr if the passed function is constexpr.
template <std::size_t Size, class Function>
constexpr std::array<typename std::result_of<Function(size_t)>::type, Size>
make_array(Function f) {
static_assert(Size > 0, "Can only create non-empty arrays");
return detail::make_array_helper<Function, Size - 1>::make_array(f);
// base::make_unique<T>: Construct an object of type T and wrap it in a
// std::unique_ptr.
// Replacement for C++14's std::make_unique.
template <typename T, typename... Args>
std::unique_ptr<T> make_unique(Args&&... args) {
return std::unique_ptr<T>(new T(std::forward<Args>(args)...));
// implicit_cast<A>(x) triggers an implicit cast from {x} to type {A}. This is
// useful in situations where static_cast<A>(x) would do too much.
template <class A>
A implicit_cast(A x) {
return x;
// Helper to determine how to pass values: Pass scalars and arrays by value,
// others by const reference (even if it was a non-const ref before; this is
// disallowed by the style guide anyway).
// The default is to also remove array extends (int[5] -> int*), but this can be
// disabled by setting {remove_array_extend} to false.
template <typename T, bool remove_array_extend = true>
struct pass_value_or_ref {
using noref_t = typename std::remove_reference<T>::type;
using decay_t = typename std::conditional<
std::is_array<noref_t>::value && !remove_array_extend, noref_t,
typename std::decay<noref_t>::type>::type;
using type = typename std::conditional<std::is_scalar<decay_t>::value ||
decay_t, const decay_t&>::type;
template <typename T>
struct has_output_operator {
// This template is only instantiable if U provides operator<< with ostream.
// Its return type is uint8_t.
template <typename U>
static auto __check_operator(U u)
-> decltype(*(std::ostream*)nullptr << *u, uint8_t{0});
// This is a fallback implementation, returning uint16_t. If the template
// above is instantiable, is has precedence over this varargs function.
static uint16_t __check_operator(...);
using ptr_t = typename std::add_pointer<T>::type;
static constexpr bool value = sizeof(__check_operator(ptr_t{nullptr})) == 1;
namespace detail {
template <typename Func, typename T, typename... Ts>
struct fold_helper {
static_assert(sizeof...(Ts) == 0, "this is the base case");
using result_t = typename std::remove_reference<T>::type;
static constexpr T&& fold(Func func, T&& first) {
return std::forward<T>(first);
template <typename Func, typename T1, typename T2, typename... Ts>
struct fold_helper<Func, T1, T2, Ts...> {
using folded_t = typename std::result_of<Func(T1, T2)>::type;
using next_fold_helper = fold_helper<Func, folded_t&&, Ts...>;
using result_t = typename next_fold_helper::result_t;
static constexpr result_t fold(Func func, T1&& first, T2&& second,
Ts&&... more) {
return next_fold_helper::fold(
func, func(std::forward<T1>(first), std::forward<T2>(second)),
} // namespace detail
// Fold all arguments from left to right with a given function.
template <typename Func, typename... Ts>
constexpr auto fold(Func func, Ts&&... more) ->
typename detail::fold_helper<Func, Ts...>::result_t {
return detail::fold_helper<Func, Ts...>::fold(func,
} // namespace base
} // namespace v8