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

dave-hill · Anna Gringauze · commit c02ddae4bcff · 2018-11-05T15:39:41.000-08:00
* Span std::array c'tor uses arr.data() instead of &amp;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
diff --git a/include/gsl/span b/include/gsl/span
@@ -394,17 +394,27 @@ public:
         : storage_(KnownNotNull{std::addressof(arr[0])}, details::extent_type<N>())
     {}
 
-    template <std::size_t N, class ArrayElementType = std::remove_const_t<element_type>>
-    // GSL_SUPPRESS(bounds.4) // NO-FORMAT: attribute // TODO: parser bug
-    constexpr span(std::array<ArrayElementType, N>& arr) noexcept
-        : storage_(arr.data(), details::extent_type<N>())
-    {}
+    template <std::size_t N, class = std::enable_if_t<(N > 0)>>
+    constexpr span(std::array<std::remove_const_t<element_type>, N>& arr) noexcept
+        : storage_(KnownNotNull{arr.data()}, details::extent_type<N>())
+    {
+    }
 
-    template <std::size_t N>
-    // GSL_SUPPRESS(bounds.4) // NO-FORMAT: attribute // TODO: parser bug
+    constexpr span(std::array<std::remove_const_t<element_type>, 0>&) noexcept
+        : 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
-        : 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
     // on Container to be a contiguous sequence container.
diff --git a/tests/span_tests.cpp b/tests/span_tests.cpp
@@ -452,7 +452,13 @@ TEST_CASE("from_std_array_constructor")
         CHECK((cs.size() == narrow_cast<ptrdiff_t>(arr.size()) && cs.data() == arr.data()));
     }
 
-    std::array<AddressOverloaded, 4> ao_arr{};  
+    {
+        std::array<int, 0> empty_arr{};
+        span<int> s{empty_arr};
+        CHECK((s.size() == 0 && s.empty()));
+    }
+
+    std::array<AddressOverloaded, 4> ao_arr{};
 
     {
         span<AddressOverloaded, 4> fs{ao_arr};

Original file line number	Diff line number	Diff line change
`@@ -452,7 +452,13 @@ TEST_CASE("from_std_array_constructor")`
`452`	`452`	`CHECK((cs.size() == narrow_cast<ptrdiff_t>(arr.size()) && cs.data() == arr.data()));`
`453`	`453`	`}`
`454`	`454`
`455`		`- std::array<AddressOverloaded, 4> ao_arr{};`
	`455`	`+ {`
	`456`	`+ std::array<int, 0> empty_arr{};`
	`457`	`+ span<int> s{empty_arr};`
	`458`	`+ CHECK((s.size() == 0 && s.empty()));`
	`459`	`+ }`
	`460`	`+`
	`461`	`+ std::array<AddressOverloaded, 4> ao_arr{};`
`456`	`462`
`457`	`463`	`{`
`458`	`464`	`span<AddressOverloaded, 4> fs{ao_arr};`