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add specialization for zero-sized array #227

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4a09d18
add specialization for zero-sized array
gberg617 Aug 31, 2026
9191e94
split zero-sized test to three tests to make it more understandable
gberg617 Aug 31, 2026
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128 changes: 128 additions & 0 deletions include/camp/array.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -192,6 +192,134 @@ struct array {
value_type elements[N];
};

template <class T>
struct array<T, 0> {
using value_type = T;
using size_type = std::size_t;
using difference_type = std::ptrdiff_t;
using reference = value_type&;
using const_reference = const value_type&;
using pointer = value_type*;
using const_pointer = const value_type*;
using iterator = pointer;
using const_iterator = const_pointer;

constexpr reference at(size_type)
{
throw std::out_of_range{"camp::array::at detected out of range access"};
}

constexpr const_reference at(size_type) const
{
throw std::out_of_range{"camp::array::at detected out of range access"};
}

CAMP_HOST_DEVICE constexpr reference operator[](size_type) noexcept
{
return *data();
}

CAMP_HOST_DEVICE constexpr const_reference operator[](
size_type) const noexcept
{
return *data();
}

reference front() = delete;
const_reference front() const = delete;
reference back() = delete;
const_reference back() const = delete;

CAMP_HOST_DEVICE constexpr pointer data() noexcept { return nullptr; }

CAMP_HOST_DEVICE constexpr const_pointer data() const noexcept
{
return nullptr;
}

CAMP_HOST_DEVICE constexpr iterator begin() noexcept
{
return iterator(data());
}

CAMP_HOST_DEVICE constexpr const_iterator begin() const noexcept
{
return const_iterator(data());
}

CAMP_HOST_DEVICE constexpr const_iterator cbegin() const noexcept
{
return const_iterator(data());
}

CAMP_HOST_DEVICE constexpr iterator end() noexcept
{
return iterator(data());
}

CAMP_HOST_DEVICE constexpr const_iterator end() const noexcept
{
return const_iterator(data());
}

CAMP_HOST_DEVICE constexpr const_iterator cend() const noexcept
{
return const_iterator(data());
}

CAMP_HOST_DEVICE constexpr bool empty() const noexcept { return true; }

CAMP_HOST_DEVICE constexpr size_type size() const noexcept { return 0; }

CAMP_HOST_DEVICE constexpr size_type max_size() const noexcept { return 0; }

CAMP_HOST_DEVICE constexpr void fill(const T&) {}

CAMP_HOST_DEVICE constexpr void swap(array&) noexcept {}
};

template <class T>
CAMP_HOST_DEVICE inline constexpr bool operator==(const array<T, 0>&,
const array<T, 0>&)
{
return true;
}

template <class T>
CAMP_HOST_DEVICE inline constexpr bool operator!=(const array<T, 0>&,
const array<T, 0>&)
{
return false;
}

template <class T>
CAMP_HOST_DEVICE inline constexpr bool operator<(const array<T, 0>&,

@gberg617 gberg617 Aug 31, 2026

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Note: C++20 replaces most of these comparison operators with the three-way comparison <=> operator

const array<T, 0>&)
{
return false;
}

template <class T>
CAMP_HOST_DEVICE inline constexpr bool operator<=(const array<T, 0>&,
const array<T, 0>&)
{
return true;
}

template <class T>
CAMP_HOST_DEVICE inline constexpr bool operator>(const array<T, 0>&,
const array<T, 0>&)
{
return false;
}

template <class T>
CAMP_HOST_DEVICE inline constexpr bool operator>=(const array<T, 0>&,
const array<T, 0>&)
{
return true;
}

template <class T, std::size_t N>
CAMP_HOST_DEVICE inline constexpr bool operator==(const array<T, N>& lhs,
const array<T, N>& rhs)
Expand Down
51 changes: 51 additions & 0 deletions test/array.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -50,6 +50,48 @@ CAMP_TEST_BEGIN(array, copy_assignment)
}
CAMP_TEST_END(array, copy_assignment)

CAMP_TEST_BEGIN(array, zero_sized_capacity)
{
camp::array<int, 0> a{};
const camp::array<int, 0>& c = a;

return a.empty() && c.empty() && a.size() == 0 && c.size() == 0
&& a.max_size() == 0 && c.max_size() == 0;
}
CAMP_TEST_END(array, zero_sized_capacity)

CAMP_TEST_BEGIN(array, zero_sized_data)
{
camp::array<int, 0> a{};
const camp::array<int, 0>& c = a;

return a.data() == nullptr && c.data() == nullptr && a.begin() == a.end()
&& c.begin() == c.end() && a.cbegin() == a.cend()
&& c.cbegin() == c.cend();
}
CAMP_TEST_END(array, zero_sized_data)

CAMP_TEST_BEGIN(array, zero_sized_modifiers)
{
camp::array<int, 0> a{};
camp::array<int, 0> b = {};

a.fill(3);
camp::swap(a, b);

return a.empty() && b.empty() && a.size() == 0 && b.size() == 0;
}
CAMP_TEST_END(array, zero_sized_modifiers)

CAMP_TEST_BEGIN(array, zero_sized_comparisons)
{
camp::array<int, 0> a{};
camp::array<int, 0> b = {};

return a == b && !(a != b) && !(a < b) && a <= b && !(a > b) && a >= b;
}
CAMP_TEST_END(array, zero_sized_comparisons)

// Not portable as currently implemented
TEST(host_array, at)
{
Expand Down Expand Up @@ -77,6 +119,15 @@ TEST(host_array, at)
EXPECT_TRUE(exception);
}

TEST(host_array, zero_sized_at)
{
camp::array<int, 0> a{};
const camp::array<int, 0>& b = a;

EXPECT_THROW(a.at(0), std::out_of_range);
EXPECT_THROW(b.at(0), std::out_of_range);
}

CAMP_TEST_BEGIN(array, subscript)
{
camp::array<int, 2> a = {1, 8};
Expand Down
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