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Fix return type of templated span.subspan() (#625)

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* Added support for returning fixed-spize spans from subspan().

* Addressed issues from code review.

* Took simpler approach to static data member.

* Subtle fix to support MSVC 15.

* Helps to not introduce extraneous >
This commit is contained in:
Neil MacIntosh 2018-03-03 19:12:45 -08:00 committed by GitHub
parent cbd64c9ff6
commit 6a33b97a84
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2 changed files with 38 additions and 2 deletions
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28
include/gsl/span
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@ -45,8 +45,17 @@
#if _MSC_VER < 1910
#pragma push_macro("constexpr")
#define constexpr /*constexpr*/
#define GSL_USE_STATIC_CONSTEXPR_WORKAROUND
#endif // _MSC_VER < 1910
#else // _MSC_VER
// See if we have enough C++17 power to use a static constexpr data member
// without needing an out-of-line definition
#if !(defined(__cplusplus) && (__cplusplus >= 201703L))
#define GSL_USE_STATIC_CONSTEXPR_WORKAROUND
#endif // !(defined(__cplusplus) && (__cplusplus >= 201703L))
#endif // _MSC_VER
#define GSL_NOEXCEPT noexcept
@ -302,6 +311,12 @@ namespace details
private:
index_type size_;
};
template <class ElementType, std::ptrdiff_t Extent, std::ptrdiff_t Offset, std::ptrdiff_t Count>
struct calculate_subspan_type
{
using type = span<ElementType, Count != dynamic_extent ? Count : (Extent != dynamic_extent ? Extent - Offset : Extent)>;
};
} // namespace details
// [span], class template span
@ -323,7 +338,11 @@ public:
using size_type = index_type;
constexpr static const index_type extent = Extent;
#if defined(GSL_USE_STATIC_CONSTEXPR_WORKAROUND)
static constexpr const index_type extent { Extent };
#else
static constexpr index_type extent { Extent };
#endif
// [span.cons], span constructors, copy, assignment, and destructor
template <bool Dependent = false,
@ -412,7 +431,7 @@ public:
}
template <std::ptrdiff_t Offset, std::ptrdiff_t Count = dynamic_extent>
constexpr span<element_type, Count> subspan() const
constexpr auto subspan() const -> typename details::calculate_subspan_type<ElementType, Extent, Offset, Count>::type
{
Expects((Offset >= 0 && size() - Offset >= 0) &&
(Count == dynamic_extent || (Count >= 0 && Offset + Count <= size())));
@ -525,6 +544,11 @@ private:
}
};
#if defined(GSL_USE_STATIC_CONSTEXPR_WORKAROUND)
template <class ElementType, std::ptrdiff_t Extent>
constexpr const typename span<ElementType, Extent>::index_type span<ElementType, Extent>::extent;
#endif
// [span.comparison], span comparison operators
template <class ElementType, std::ptrdiff_t FirstExtent, std::ptrdiff_t SecondExtent>

12
tests/span_tests.cpp
View File

@ -776,6 +776,7 @@ TEST_CASE("subspan")
{
span<int, 5> av = arr;
CHECK((av.subspan<2, 2>().size() == 2));
CHECK(decltype(av.subspan<2, 2>())::extent == 2);
CHECK(av.subspan(2, 2).size() == 2);
CHECK(av.subspan(2, 3).size() == 3);
}
@ -783,13 +784,16 @@ TEST_CASE("subspan")
{
span<int, 5> av = arr;
CHECK((av.subspan<0, 0>().size() == 0));
CHECK(decltype(av.subspan<0,0>())::extent == 0);
CHECK(av.subspan(0, 0).size() == 0);
}
{
span<int, 5> av = arr;
CHECK((av.subspan<0, 5>().size() == 5));
CHECK(decltype(av.subspan<0, 5>())::extent == 5);
CHECK(av.subspan(0, 5).size() == 5);
CHECK_THROWS_AS(av.subspan(0, 6).size(), fail_fast);
CHECK_THROWS_AS(av.subspan(1, 5).size(), fail_fast);
}
@ -797,14 +801,22 @@ TEST_CASE("subspan")
{
span<int, 5> av = arr;
CHECK((av.subspan<4, 0>().size() == 0));
CHECK(decltype(av.subspan<4, 0>())::extent == 0);
CHECK(av.subspan(4, 0).size() == 0);
CHECK(av.subspan(5, 0).size() == 0);
CHECK_THROWS_AS(av.subspan(6, 0).size(), fail_fast);
}
{
span<int, 5> av = arr;
CHECK((av.subspan<1>().size() == 4));
CHECK(decltype(av.subspan<1>())::extent == 4);
}
{
span<int> av;
CHECK((av.subspan<0, 0>().size() == 0));
CHECK((decltype(av.subspan<0, 0>())::extent == 0));
CHECK(av.subspan(0, 0).size() == 0);
CHECK_THROWS_AS((av.subspan<1, 0>().size()), fail_fast);
}