variant.hpp
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// This file is part of OpenCV project.
// It is subject to the license terms in the LICENSE file found in the top-level directory
// of this distribution and at http://opencv.org/license.html.
//
// Copyright (C) 2018 Intel Corporation
#ifndef OPENCV_GAPI_UTIL_VARIANT_HPP
#define OPENCV_GAPI_UTIL_VARIANT_HPP
#include <array>
#include <type_traits>
#include <opencv2/gapi/util/throw.hpp>
#include <opencv2/gapi/util/util.hpp> // max_of_t
// A poor man's `variant` implementation, incompletely modeled against C++17 spec.
namespace cv
{
namespace util
{
namespace detail
{
template<std::size_t I, typename Target, typename First, typename... Remaining>
struct type_list_index_helper
{
static const constexpr bool is_same = std::is_same<Target, First>::value;
static const constexpr std::size_t value =
std::conditional<is_same, std::integral_constant<std::size_t, I>, type_list_index_helper<I + 1, Target, Remaining...>>::type::value;
};
template<std::size_t I, typename Target, typename First>
struct type_list_index_helper<I, Target, First>
{
static_assert(std::is_same<Target, First>::value, "Type not found");
static const constexpr std::size_t value = I;
};
template<class T, class U, class V> using are_different =
std::enable_if<!std::is_same<typename std::decay<T>::type,
typename std::decay<U>::type>::value,
V>;
}
template<typename Target, typename... Types>
struct type_list_index
{
static const constexpr std::size_t value = detail::type_list_index_helper<0, Target, Types...>::value;
};
class bad_variant_access: public std::exception
{
public:
virtual const char *what() const noexcept override
{
return "Bad variant access";
}
};
// Interface ///////////////////////////////////////////////////////////////
struct monostate {};
inline bool operator==(const util::monostate&, const util::monostate&)
{
return true;
}
template<typename... Ts> // FIXME: no references, arrays, and void
class variant
{
// FIXME: Replace with std::aligned_union after gcc4.8 support is dropped
static constexpr const std::size_t S = cv::detail::max_of_t<sizeof(Ts)...>::value;
static constexpr const std::size_t A = cv::detail::max_of_t<alignof(Ts)...>::value;
using Memory = typename std::aligned_storage<S, A>::type[1];
template<typename T> struct cctr_h {
static void help(Memory memory, const Memory from) {
new (memory) T(*reinterpret_cast<const T*>(from));
}
};
template<typename T> struct vctr_h {
static void help(Memory memory, const void* pval) {
new (memory) T(*reinterpret_cast<const T*>(pval));
}
};
template<typename T> struct mctr_h {
static void help(Memory memory, void *pval) {
new (memory) T(std::move(*reinterpret_cast<T*>(pval)));
}
};
template<typename T> struct copy_h {
static void help(Memory to, const Memory from) {
*reinterpret_cast<T*>(to) = *reinterpret_cast<const T*>(from);
}
};
template<typename T> struct move_h {
static void help(Memory to, const Memory from) {
*reinterpret_cast<T*>(to) = std::move(*reinterpret_cast<const T*>(from));
}
};
template<typename T> struct swap_h {
static void help(Memory to, Memory from) {
std::swap(*reinterpret_cast<T*>(to), *reinterpret_cast<T*>(from));
}
};
template<typename T> struct dtor_h {
static void help(Memory memory) {
(void) memory; // MSCV warning
reinterpret_cast<T*>(memory)->~T();
}
};
template<typename T> struct equal_h {
static bool help(const Memory lhs, const Memory rhs) {
const T& t_lhs = *reinterpret_cast<const T*>(lhs);
const T& t_rhs = *reinterpret_cast<const T*>(rhs);
return t_lhs == t_rhs;
}
};
typedef void (*CCtr) (Memory, const Memory); // Copy c-tor (variant)
typedef void (*VCtr) (Memory, const void*); // Copy c-tor (value)
typedef void (*MCtr) (Memory, void*); // Generic move c-tor
typedef void (*Copy) (Memory, const Memory); // Copy assignment
typedef void (*Move) (Memory, const Memory); // Move assignment
typedef void (*Swap) (Memory, Memory); // Swap
typedef void (*Dtor) (Memory); // Destructor
typedef bool (*Equal)(const Memory, const Memory); // Equality test (external)
static constexpr std::array<CCtr, sizeof...(Ts)> cctrs(){ return {{(&cctr_h<Ts>::help)...}};}
static constexpr std::array<VCtr, sizeof...(Ts)> vctrs(){ return {{(&vctr_h<Ts>::help)...}};}
static constexpr std::array<MCtr, sizeof...(Ts)> mctrs(){ return {{(&mctr_h<Ts>::help)...}};}
static constexpr std::array<Copy, sizeof...(Ts)> cpyrs(){ return {{(©_h<Ts>::help)...}};}
static constexpr std::array<Move, sizeof...(Ts)> mvers(){ return {{(&move_h<Ts>::help)...}};}
static constexpr std::array<Swap, sizeof...(Ts)> swprs(){ return {{(&swap_h<Ts>::help)...}};}
static constexpr std::array<Dtor, sizeof...(Ts)> dtors(){ return {{(&dtor_h<Ts>::help)...}};}
std::size_t m_index = 0;
protected:
template<typename T, typename... Us> friend T& get(variant<Us...> &v);
template<typename T, typename... Us> friend const T& get(const variant<Us...> &v);
template<typename... Us> friend bool operator==(const variant<Us...> &lhs,
const variant<Us...> &rhs);
Memory memory;
public:
// Constructors
variant() noexcept;
variant(const variant& other);
variant(variant&& other) noexcept;
template<typename T> explicit variant(const T& t);
// are_different is a SFINAE trick to avoid variant(T &&t) with T=variant
// for some reason, this version is called instead of variant(variant&& o) when
// variant is used in STL containers (examples: vector assignment)
template<typename T> explicit variant(T&& t, typename detail::are_different<variant, T, int>::type = 0);
// template<class T, class... Args> explicit variant(Args&&... args);
// FIXME: other constructors
// Destructor
~variant();
// Assignment
variant& operator=(const variant& rhs);
variant& operator=(variant &&rhs) noexcept;
// SFINAE trick to avoid operator=(T&&) with T=variant<>, see comment above
template<class T>
typename detail::are_different<variant, T, variant&>
::type operator=(T&& t) noexcept;
// Observers
std::size_t index() const noexcept;
// FIXME: valueless_by_exception()
// Modifiers
// FIXME: emplace()
void swap(variant &rhs) noexcept;
// Non-C++17x!
template<typename T> static constexpr std::size_t index_of();
};
// FIMXE: visit
template<typename T, typename... Types>
T& get(util::variant<Types...> &v);
template<typename T, typename... Types>
const T& get(const util::variant<Types...> &v);
template<typename T, typename... Types>
bool holds_alternative(const util::variant<Types...> &v) noexcept;
// FIXME: T&&, const TT&& versions.
// Implementation //////////////////////////////////////////////////////////
template<typename... Ts>
variant<Ts...>::variant() noexcept
{
typedef typename std::tuple_element<0, std::tuple<Ts...> >::type TFirst;
new (memory) TFirst();
}
template<typename... Ts>
variant<Ts...>::variant(const variant &other)
: m_index(other.m_index)
{
(cctrs()[m_index])(memory, other.memory);
}
template<typename... Ts>
variant<Ts...>::variant(variant &&other) noexcept
: m_index(other.m_index)
{
(mctrs()[m_index])(memory, other.memory);
}
template<typename... Ts>
template<class T>
variant<Ts...>::variant(const T& t)
: m_index(util::type_list_index<T, Ts...>::value)
{
(vctrs()[m_index])(memory, &t);
}
template<typename... Ts>
template<class T>
variant<Ts...>::variant(T&& t, typename detail::are_different<variant, T, int>::type)
: m_index(util::type_list_index<typename std::remove_reference<T>::type, Ts...>::value)
{
(mctrs()[m_index])(memory, &t);
}
template<typename... Ts>
variant<Ts...>::~variant()
{
(dtors()[m_index])(memory);
}
template<typename... Ts>
variant<Ts...>& variant<Ts...>::operator=(const variant<Ts...> &rhs)
{
if (m_index != rhs.m_index)
{
(dtors()[ m_index])(memory);
(cctrs()[rhs.m_index])(memory, rhs.memory);
m_index = rhs.m_index;
}
else
{
(cpyrs()[rhs.m_index])(memory, rhs.memory);
}
return *this;
}
template<typename... Ts>
variant<Ts...>& variant<Ts...>::operator=(variant<Ts...> &&rhs) noexcept
{
if (m_index != rhs.m_index)
{
(dtors()[ m_index])(memory);
(mctrs()[rhs.m_index])(memory, rhs.memory);
m_index = rhs.m_index;
}
else
{
(mvers()[rhs.m_index])(memory, rhs.memory);
}
return *this;
}
template<typename... Ts>
template<class T> typename detail::are_different<variant<Ts...>, T, variant<Ts...>&>
::type variant<Ts...>::operator=(T&& t) noexcept
{
// FIXME: No version with implicit type conversion available!
static const constexpr std::size_t t_index =
util::type_list_index<T, Ts...>::value;
if (t_index == m_index)
{
util::get<T>(*this) = std::move(t);
return *this;
}
else return (*this = variant(std::move(t)));
}
template<typename... Ts>
std::size_t util::variant<Ts...>::index() const noexcept
{
return m_index;
}
template<typename... Ts>
void variant<Ts...>::swap(variant<Ts...> &rhs) noexcept
{
if (m_index == rhs.index())
{
(swprs()[m_index](memory, rhs.memory));
}
else
{
variant<Ts...> tmp(std::move(*this));
*this = std::move(rhs);
rhs = std::move(tmp);
}
}
template<typename... Ts>
template<typename T>
constexpr std::size_t variant<Ts...>::index_of()
{
return util::type_list_index<T, Ts...>::value; // FIXME: tests!
}
template<typename T, typename... Types>
T& get(util::variant<Types...> &v)
{
const constexpr std::size_t t_index =
util::type_list_index<T, Types...>::value;
if (v.index() == t_index)
return *(T*)(&v.memory); // workaround for ICC 2019
// original code: return reinterpret_cast<T&>(v.memory);
else
throw_error(bad_variant_access());
}
template<typename T, typename... Types>
const T& get(const util::variant<Types...> &v)
{
const constexpr std::size_t t_index =
util::type_list_index<T, Types...>::value;
if (v.index() == t_index)
return *(const T*)(&v.memory); // workaround for ICC 2019
// original code: return reinterpret_cast<const T&>(v.memory);
else
throw_error(bad_variant_access());
}
template<typename T, typename... Types>
bool holds_alternative(const util::variant<Types...> &v) noexcept
{
return v.index() == util::variant<Types...>::template index_of<T>();
}
template<typename... Us> bool operator==(const variant<Us...> &lhs,
const variant<Us...> &rhs)
{
using V = variant<Us...>;
// Instantiate table only here since it requires operator== for <Us...>
// <Us...> should have operator== only if this one is used, not in general
static const std::array<typename V::Equal, sizeof...(Us)> eqs = {
{(&V::template equal_h<Us>::help)...}
};
if (lhs.index() != rhs.index())
return false;
return (eqs[lhs.index()])(lhs.memory, rhs.memory);
}
template<typename... Us> bool operator!=(const variant<Us...> &lhs,
const variant<Us...> &rhs)
{
return !(lhs == rhs);
}
} // namespace cv
} // namespace util
#endif // OPENCV_GAPI_UTIL_VARIANT_HPP