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Span can be constructed from empty std::array safely (#686)

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* Span std::array c'tor uses arr.data() instead of &arr[0]
- Fixes runtime issues when constructing from an empty std::array

* Construct span with std::data if C++17 detected

* Specialize span c'tor for std::array of length 0, set storage to nullptr
This commit is contained in:
Dave Hill 2018-11-05 15:39:41 -08:00 committed by Anna Gringauze
parent 2bf9f137a6
commit c02ddae4bc
2 changed files with 26 additions and 10 deletions
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28
include/gsl/span
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@ -394,17 +394,27 @@ public:
: storage_(KnownNotNull{std::addressof(arr[0])}, details::extent_type<N>()) : storage_(KnownNotNull{std::addressof(arr[0])}, details::extent_type<N>())
{} {}
template <std::size_t N, class ArrayElementType = std::remove_const_t<element_type>> template <std::size_t N, class = std::enable_if_t<(N > 0)>>
// GSL_SUPPRESS(bounds.4) // NO-FORMAT: attribute // TODO: parser bug constexpr span(std::array<std::remove_const_t<element_type>, N>& arr) noexcept
constexpr span(std::array<ArrayElementType, N>& arr) noexcept : storage_(KnownNotNull{arr.data()}, details::extent_type<N>())
: storage_(arr.data(), details::extent_type<N>()) {
{} }
template <std::size_t N> constexpr span(std::array<std::remove_const_t<element_type>, 0>&) noexcept
// GSL_SUPPRESS(bounds.4) // NO-FORMAT: attribute // TODO: parser bug : storage_(static_cast<pointer>(nullptr), details::extent_type<0>())
{
}
template <std::size_t N, class = std::enable_if_t<(N > 0)>>
constexpr span(const std::array<std::remove_const_t<element_type>, N>& arr) noexcept constexpr span(const std::array<std::remove_const_t<element_type>, N>& arr) noexcept
: storage_(arr.data(), details::extent_type<N>()) : storage_(KnownNotNull{arr.data()}, details::extent_type<N>())
{} {
}
constexpr span(const std::array<std::remove_const_t<element_type>, 0>&) noexcept
: storage_(static_cast<pointer>(nullptr), details::extent_type<0>())
{
}
// NB: the SFINAE here uses .data() as a incomplete/imperfect proxy for the requirement // NB: the SFINAE here uses .data() as a incomplete/imperfect proxy for the requirement
// on Container to be a contiguous sequence container. // on Container to be a contiguous sequence container.

6
tests/span_tests.cpp
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@ -452,6 +452,12 @@ TEST_CASE("from_std_array_constructor")
CHECK((cs.size() == narrow_cast<ptrdiff_t>(arr.size()) && cs.data() == arr.data())); CHECK((cs.size() == narrow_cast<ptrdiff_t>(arr.size()) && cs.data() == arr.data()));
} }
{
std::array<int, 0> empty_arr{};
span<int> s{empty_arr};
CHECK((s.size() == 0 && s.empty()));
}
std::array<AddressOverloaded, 4> ao_arr{}; std::array<AddressOverloaded, 4> ao_arr{};
{ {