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Merge pull request #138 from mapbox/sizeof
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Making a sizeof(variant) match `boost` and `std` implementation
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@artemp
artemp committed Feb 20, 2017
2 parents d2588a8 + c839c66 commit 3d807d3
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Showing 2 changed files with 38 additions and 29 deletions.
64 changes: 37 additions & 27 deletions include/mapbox/variant.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,7 @@
#include <typeinfo>
#include <utility>
#include <functional>
#include <limits>

#include <mapbox/recursive_wrapper.hpp>
#include <mapbox/variant_visitor.hpp>
Expand Down Expand Up @@ -84,25 +85,35 @@ struct MAPBOX_VARIANT_DEPRECATED static_visitor
~static_visitor() {}
};

#if !defined(MAPBOX_VARIANT_MINIMIZE_SIZE)
using type_index_t = unsigned int;
#else
#if defined(MAPBOX_VARIANT_OPTIMIZE_FOR_SPEED)
using type_index_t = std::uint_fast8_t;
#else
using type_index_t = std::uint_least8_t;
#endif
#endif

namespace detail {

static constexpr std::size_t invalid_value = std::size_t(-1);
static constexpr type_index_t invalid_value = type_index_t(-1);

template <typename T, typename... Types>
struct direct_type;

template <typename T, typename First, typename... Types>
struct direct_type<T, First, Types...>
{
static constexpr std::size_t index = std::is_same<T, First>::value
static constexpr type_index_t index = std::is_same<T, First>::value
? sizeof...(Types)
: direct_type<T, Types...>::index;
};

template <typename T>
struct direct_type<T>
{
static constexpr std::size_t index = invalid_value;
static constexpr type_index_t index = invalid_value;
};

#if __cpp_lib_logical_traits >= 201510L
Expand Down Expand Up @@ -144,26 +155,26 @@ struct convertible_type;
template <typename T, typename First, typename... Types>
struct convertible_type<T, First, Types...>
{
static constexpr std::size_t index = std::is_convertible<T, First>::value
static constexpr type_index_t index = std::is_convertible<T, First>::value
? disjunction<std::is_convertible<T, Types>...>::value ? invalid_value : sizeof...(Types)
: convertible_type<T, Types...>::index;
};

template <typename T>
struct convertible_type<T>
{
static constexpr std::size_t index = invalid_value;
static constexpr type_index_t index = invalid_value;
};

template <typename T, typename... Types>
struct value_traits
{
using value_type = typename std::remove_const<typename std::remove_reference<T>::type>::type;
static constexpr std::size_t direct_index = direct_type<value_type, Types...>::index;
static constexpr type_index_t direct_index = direct_type<value_type, Types...>::index;
static constexpr bool is_direct = direct_index != invalid_value;
static constexpr std::size_t index = is_direct ? direct_index : convertible_type<value_type, Types...>::index;
static constexpr type_index_t index = is_direct ? direct_index : convertible_type<value_type, Types...>::index;
static constexpr bool is_valid = index != invalid_value;
static constexpr std::size_t tindex = is_valid ? sizeof...(Types)-index : 0;
static constexpr type_index_t tindex = is_valid ? sizeof...(Types)-index : 0;
using target_type = typename std::tuple_element<tindex, std::tuple<void, Types...>>::type;
};

Expand Down Expand Up @@ -197,19 +208,19 @@ struct result_of_binary_visit<F, V, typename enable_if_type<typename F::result_t
using type = typename F::result_type;
};

template <std::size_t arg1, std::size_t... others>
template <type_index_t arg1, type_index_t... others>
struct static_max;

template <std::size_t arg>
template <type_index_t arg>
struct static_max<arg>
{
static const std::size_t value = arg;
static const type_index_t value = arg;
};

template <std::size_t arg1, std::size_t arg2, std::size_t... others>
template <type_index_t arg1, type_index_t arg2, type_index_t... others>
struct static_max<arg1, arg2, others...>
{
static const std::size_t value = arg1 >= arg2 ? static_max<arg1, others...>::value : static_max<arg2, others...>::value;
static const type_index_t value = arg1 >= arg2 ? static_max<arg1, others...>::value : static_max<arg2, others...>::value;
};

template <typename... Types>
Expand All @@ -218,7 +229,7 @@ struct variant_helper;
template <typename T, typename... Types>
struct variant_helper<T, Types...>
{
VARIANT_INLINE static void destroy(const std::size_t type_index, void* data)
VARIANT_INLINE static void destroy(const type_index_t type_index, void* data)
{
if (type_index == sizeof...(Types))
{
Expand All @@ -230,7 +241,7 @@ struct variant_helper<T, Types...>
}
}

VARIANT_INLINE static void move(const std::size_t old_type_index, void* old_value, void* new_value)
VARIANT_INLINE static void move(const type_index_t old_type_index, void* old_value, void* new_value)
{
if (old_type_index == sizeof...(Types))
{
Expand All @@ -242,7 +253,7 @@ struct variant_helper<T, Types...>
}
}

VARIANT_INLINE static void copy(const std::size_t old_type_index, const void* old_value, void* new_value)
VARIANT_INLINE static void copy(const type_index_t old_type_index, const void* old_value, void* new_value)
{
if (old_type_index == sizeof...(Types))
{
Expand All @@ -258,9 +269,9 @@ struct variant_helper<T, Types...>
template <>
struct variant_helper<>
{
VARIANT_INLINE static void destroy(const std::size_t, void*) {}
VARIANT_INLINE static void move(const std::size_t, void*, void*) {}
VARIANT_INLINE static void copy(const std::size_t, const void*, void*) {}
VARIANT_INLINE static void destroy(const type_index_t, void*) {}
VARIANT_INLINE static void move(const type_index_t, void*, void*) {}
VARIANT_INLINE static void copy(const type_index_t, const void*, void*) {}
};

template <typename T>
Expand Down Expand Up @@ -558,16 +569,15 @@ struct hasher

} // namespace detail

struct no_init
{
};
struct no_init {};

template <typename... Types>
class variant
{
static_assert(sizeof...(Types) > 0, "Template parameter type list of variant can not be empty");
static_assert(sizeof...(Types) > 0, "Template parameter type list of variant can not be empty.");
static_assert(!detail::disjunction<std::is_reference<Types>...>::value, "Variant can not hold reference types. Maybe use std::reference_wrapper?");

static_assert(!detail::disjunction<std::is_array<Types>...>::value, "Variant can not hold array types.");
static_assert(sizeof...(Types) < std::numeric_limits<type_index_t>::max(), "Internal index type must be able to accommodate all alternatives.");
private:
static const std::size_t data_size = detail::static_max<sizeof(Types)...>::value;
static const std::size_t data_align = detail::static_max<alignof(Types)...>::value;
Expand All @@ -579,14 +589,14 @@ class variant
using data_type = typename std::aligned_storage<data_size, data_align>::type;
using helper_type = detail::variant_helper<Types...>;

std::size_t type_index;
type_index_t type_index;
data_type data;

public:
VARIANT_INLINE variant() noexcept(std::is_nothrow_default_constructible<first_type>::value)
: type_index(sizeof...(Types)-1)
{
static_assert(std::is_default_constructible<first_type>::value, "First type in variant must be default constructible to allow default construction of variant");
static_assert(std::is_default_constructible<first_type>::value, "First type in variant must be default constructible to allow default construction of variant.");
new (&data) first_type();
}

Expand Down Expand Up @@ -838,7 +848,7 @@ class variant

// This function is deprecated because it returns an internal index field.
// Use which() instead.
MAPBOX_VARIANT_DEPRECATED VARIANT_INLINE std::size_t get_type_index() const
MAPBOX_VARIANT_DEPRECATED VARIANT_INLINE type_index_t get_type_index() const
{
return type_index;
}
Expand Down
3 changes: 1 addition & 2 deletions test/t/sizeof.cpp
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Original file line number Diff line number Diff line change
@@ -1,4 +1,3 @@

#include <algorithm>
#include <cstddef>
#include <cstdint>
Expand All @@ -15,7 +14,7 @@ struct some_struct
std::string c;
};

using variant_internal_index_type = size_t;
using variant_internal_index_type = mapbox::util::type_index_t;

TEST_CASE("size of variants")
{
Expand Down

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