constexpr array support C++1z (P0031)

[lefticus]

Summary

• array constexpr-ized
• associated iterator functions constexpr-ized
• unit tests added for array
• unit tests added for iterator

Unit tests for std::array required non-const constexpr context for tests, so constexpr helper functions were added in unit tests, with static_assert calls on resulting values.

Ref: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0031r0.html

[mclow.lists]

All look fine to me - but needs tests.

[lefticus]

• Updated title and summary to be more accurate.
• Implemented tests for constexpr std::array access.
• Added required constexpr iterator support.
• Implemented tests for constexpr iterator functions

[EricWF]

For the most part this LGTM. Thanks for putting in all this work. I've noted some places where the test coverage could be improved.

include/array

156

This is untested.

160

This is untested.

165

Untested.

169

Untested.

202

Untested.

include/iterator

527

Untested.

536

Untested.

565

Untested.

1132

This is a C++03 only branch, but I guess it's more consistent to apply the macro.

test/std/containers/sequences/array/indexing.pass.cpp

41 ↗

(On Diff #65143)

Missing conditional directive terminator.

84 ↗

(On Diff #65143)

C doesn't name a type.

test/std/containers/sequences/array/iterators.pass.cpp

112 ↗

(On Diff #65143)

Include test_macros.h and use TEST_STD_VER please.

[lefticus]

Fixed syntax errors in test files.

[lefticus]

I believe I messed up the context for the files, will attempt to resubmit the patch.

[lefticus]

Correct context from last patch

[lefticus]

I believe all or most of the "untested" comments are correctly tested in the updated test files via static_asserts with contexpr. I've commented on just the first one to make sure I understand correctly.

include/array

156

This should be tested in begin.pass.cpp:28 correct?

[mclow.lists]

This diff no longer applies cleanly, and I've added some formatting concerns. However, the content looks fine to me.

include/array

176

Nit: I would put the _LIBCPP_CONSTEXPR_AFTER_CXX14 on the same line as _LIBCPP_INLINE_VISIBILITY to keep the lines from getting too long.

191

And break this line after the macros for the same reason as L#176

include/iterator

526

Here, I would smoosh them all together on the same line: inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14

[lefticus]

Address formatting concerns and out-of-date-ness of patch relative to trunk/master

[mclow.lists]

Other than the iterator.primitives/iterator.operations/prev.pass.cpp test, I think this is good to go.

test/std/iterators/iterator.primitives/iterator.operations/prev.pass.cpp

47 ↗

(On Diff #69195)

Won't this fail on C++11 (i.e, doesn't it need a #if TEST_STD_VER > 14)?

[lefticus]

Add C++1z check around constexpr tests, based on mclow's feedback

[EricWF]

@lefticus Are you still interested in working on this? If so could you please split the changes for array into a separate patch? Marshall seems to have implemented some of the changes in <iterator> and that's causing merge conflicts.

[lefticus]

@EricWF I have too many other things going on right now to be able to look at it for at least the next few months.

[EricWF]

Looks like @mclow.lists just committed another version of this paper, so this patch is no longer needed.

Resigning as reviewer.

include/array

struct array
     typedef std::reverse_iterator<iterator> reverse_iterator;
     typedef std::reverse_iterator<const_iterator> const_reverse_iterator;
 
     // No explicit construct/copy/destroy for aggregate type
     void fill(const T& u);
     void swap(array& a) noexcept(is_nothrow_swappable_v<T>);
 
     // iterators:
-    iterator begin() noexcept;
-    const_iterator begin() const noexcept;
-    iterator end() noexcept;
-    const_iterator end() const noexcept;
-
-    reverse_iterator rbegin() noexcept;
-    const_reverse_iterator rbegin() const noexcept;
-    reverse_iterator rend() noexcept;
-    const_reverse_iterator rend() const noexcept;
-
-    const_iterator cbegin() const noexcept;
-    const_iterator cend() const noexcept;
-    const_reverse_iterator crbegin() const noexcept;
-    const_reverse_iterator crend() const noexcept;
+    iterator begin() noexcept; // constexpr in C++17
+    const_iterator begin() const noexcept; // constexpr in C++17
+    iterator end() noexcept; // constexpr in C++17
+    const_iterator end() const noexcept; // constexpr in C++17
+
+    reverse_iterator rbegin() noexcept; // constexpr in C++17
+    const_reverse_iterator rbegin() const noexcept; // constexpr in C++17
+    reverse_iterator rend() noexcept; // constexpr in C++17
+    const_reverse_iterator rend() const noexcept; // constexpr in C++17
+
+    const_iterator cbegin() const noexcept; // constexpr in C++17
+    const_iterator cend() const noexcept; // constexpr in C++17
+    const_reverse_iterator crbegin() const noexcept; // constexpr in C++17
+    const_reverse_iterator crend() const noexcept; // constexpr in C++17
 
     // capacity:
     constexpr size_type size() const noexcept;
     constexpr size_type max_size() const noexcept;
     constexpr bool empty() const noexcept;
 
     // element access:
-    reference operator[](size_type n);
+    reference operator[](size_type n); // constexpr in C++17
     const_reference operator[](size_type n) const; // constexpr in C++14
     const_reference at(size_type n) const; // constexpr in C++14
-    reference at(size_type n);
+    reference at(size_type n); // constexpr in C++17
 
-    reference front();
+    reference front(); // constexpr in C++17
     const_reference front() const; // constexpr in C++14
-    reference back();
+    reference back(); // constexpr in C++17
     const_reference back() const; // constexpr in C++14
 
-    T* data() noexcept;
-    const T* data() const noexcept;
+    T* data() noexcept; // constexpr in C++17
+    const T* data() const noexcept; // constexpr in C++17
 };
 
 template <class T, size_t N>
   bool operator==(const array<T,N>& x, const array<T,N>& y);
 template <class T, size_t N>
   bool operator!=(const array<T,N>& x, const array<T,N>& y);
 template <class T, size_t N>
   bool operator<(const array<T,N>& x, const array<T,N>& y);
struct _LIBCPP_TYPE_VIS_ONLY array
     void __swap_dispatch(std::true_type, array&) {}
 
     _LIBCPP_INLINE_VISIBILITY
     void __swap_dispatch(std::false_type, array& __a)
         { _VSTD::swap_ranges(__elems_, __elems_ + _Size, __a.__elems_);}
 
     // iterators:
     _LIBCPP_INLINE_VISIBILITY
-    iterator begin() _NOEXCEPT {return iterator(__elems_);}
+    _LIBCPP_CONSTEXPR_AFTER_CXX14 iterator begin() _NOEXCEPT {return iterator(__elems_);}
     _LIBCPP_INLINE_VISIBILITY
-    const_iterator begin() const _NOEXCEPT {return const_iterator(__elems_);}
+    _LIBCPP_CONSTEXPR_AFTER_CXX14 const_iterator begin() const _NOEXCEPT {return const_iterator(__elems_);}
     _LIBCPP_INLINE_VISIBILITY
-    iterator end() _NOEXCEPT {return iterator(__elems_ + _Size);}
+    _LIBCPP_CONSTEXPR_AFTER_CXX14 iterator end() _NOEXCEPT {return iterator(__elems_ + _Size);}
     _LIBCPP_INLINE_VISIBILITY
-    const_iterator end() const _NOEXCEPT {return const_iterator(__elems_ + _Size);}
+    _LIBCPP_CONSTEXPR_AFTER_CXX14 const_iterator end() const _NOEXCEPT {return const_iterator(__elems_ + _Size);}
 
     _LIBCPP_INLINE_VISIBILITY
-    reverse_iterator rbegin() _NOEXCEPT {return reverse_iterator(end());}
+    _LIBCPP_CONSTEXPR_AFTER_CXX14 reverse_iterator rbegin() _NOEXCEPT {return reverse_iterator(end());}
     _LIBCPP_INLINE_VISIBILITY
-    const_reverse_iterator rbegin() const _NOEXCEPT {return const_reverse_iterator(end());}
+    _LIBCPP_CONSTEXPR_AFTER_CXX14 const_reverse_iterator rbegin() const _NOEXCEPT {return const_reverse_iterator(end());}
     _LIBCPP_INLINE_VISIBILITY
-    reverse_iterator rend() _NOEXCEPT {return reverse_iterator(begin());}
+    _LIBCPP_CONSTEXPR_AFTER_CXX14 reverse_iterator rend() _NOEXCEPT {return reverse_iterator(begin());}
     _LIBCPP_INLINE_VISIBILITY
-    const_reverse_iterator rend() const _NOEXCEPT {return const_reverse_iterator(begin());}
+    _LIBCPP_CONSTEXPR_AFTER_CXX14 const_reverse_iterator rend() const _NOEXCEPT {return const_reverse_iterator(begin());}
 
     _LIBCPP_INLINE_VISIBILITY
     const_iterator cbegin() const _NOEXCEPT {return begin();}
     _LIBCPP_INLINE_VISIBILITY
     const_iterator cend() const _NOEXCEPT {return end();}
     _LIBCPP_INLINE_VISIBILITY
     const_reverse_iterator crbegin() const _NOEXCEPT {return rbegin();}
     _LIBCPP_INLINE_VISIBILITY
     const_reverse_iterator crend() const _NOEXCEPT {return rend();}
 
     // capacity:
     _LIBCPP_INLINE_VISIBILITY
     _LIBCPP_CONSTEXPR size_type size() const _NOEXCEPT {return _Size;}
     _LIBCPP_INLINE_VISIBILITY
     _LIBCPP_CONSTEXPR size_type max_size() const _NOEXCEPT {return _Size;}
     _LIBCPP_INLINE_VISIBILITY
     _LIBCPP_CONSTEXPR bool empty() const _NOEXCEPT {return _Size == 0;}
 
     // element access:
-    _LIBCPP_INLINE_VISIBILITY reference operator[](size_type __n)             {return __elems_[__n];}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14 reference operator[](size_type __n)             {return __elems_[__n];}
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11 const_reference operator[](size_type __n) const {return __elems_[__n];}
-    reference at(size_type __n);
+    _LIBCPP_CONSTEXPR_AFTER_CXX14 reference at(size_type __n);
     _LIBCPP_CONSTEXPR_AFTER_CXX11 const_reference at(size_type __n) const;
 
-    _LIBCPP_INLINE_VISIBILITY reference front()             {return __elems_[0];}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14 reference front()             {return __elems_[0];}
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11 const_reference front() const {return __elems_[0];}
-    _LIBCPP_INLINE_VISIBILITY reference back()              {return __elems_[_Size > 0 ? _Size-1 : 0];}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14 reference back()              {return __elems_[_Size > 0 ? _Size-1 : 0];}
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11 const_reference back() const  {return __elems_[_Size > 0 ? _Size-1 : 0];}
 
     _LIBCPP_INLINE_VISIBILITY
-    value_type* data() _NOEXCEPT {return __elems_;}
+    value_type* _LIBCPP_CONSTEXPR_AFTER_CXX14 data() _NOEXCEPT {return __elems_;}
     _LIBCPP_INLINE_VISIBILITY
-    const value_type* data() const _NOEXCEPT {return __elems_;}
+    const value_type* _LIBCPP_CONSTEXPR_AFTER_CXX14 data() const _NOEXCEPT {return __elems_;}
 };
 
 template <class _Tp, size_t _Size>
+_LIBCPP_CONSTEXPR_AFTER_CXX14
 typename array<_Tp, _Size>::reference
 array<_Tp, _Size>::at(size_type __n)
 {
     if (__n >= _Size)
 #ifndef _LIBCPP_NO_EXCEPTIONS
         throw out_of_range("array::at");
 #else
         assert(!"array::at out_of_range");

include/iterator

public:
     ostreambuf_iterator(streambuf_type* s) noexcept;
     ostreambuf_iterator& operator=(charT c);
     ostreambuf_iterator& operator*();
     ostreambuf_iterator& operator++();
     ostreambuf_iterator& operator++(int);
     bool failed() const noexcept;
 };
 
-template <class C> auto begin(C& c) -> decltype(c.begin());
-template <class C> auto begin(const C& c) -> decltype(c.begin());
-template <class C> auto end(C& c) -> decltype(c.end());
-template <class C> auto end(const C& c) -> decltype(c.end());
-template <class T, size_t N> T* begin(T (&array)[N]);
-template <class T, size_t N> T* end(T (&array)[N]);
-
-template <class C> auto cbegin(const C& c) -> decltype(std::begin(c));        // C++14
-template <class C> auto cend(const C& c) -> decltype(std::end(c));            // C++14
-template <class C> auto rbegin(C& c) -> decltype(c.rbegin());                 // C++14
-template <class C> auto rbegin(const C& c) -> decltype(c.rbegin());           // C++14
-template <class C> auto rend(C& c) -> decltype(c.rend());                     // C++14
-template <class C> auto rend(const C& c) -> decltype(c.rend());               // C++14
+template <class C> _LIBCPP_CONSTEXPR_AFTER_CXX14 auto begin(C& c) -> decltype(c.begin());
+template <class C> _LIBCPP_CONSTEXPR_AFTER_CXX14 auto begin(const C& c) -> decltype(c.begin());
+template <class C> _LIBCPP_CONSTEXPR_AFTER_CXX14 auto end(C& c) -> decltype(c.end());
+template <class C> _LIBCPP_CONSTEXPR_AFTER_CXX14 auto end(const C& c) -> decltype(c.end());
+template <class T, size_t N> _LIBCPP_CONSTEXPR_AFTER_CXX14 T* begin(T (&array)[N]);
+template <class T, size_t N> _LIBCPP_CONSTEXPR_AFTER_CXX14 T* end(T (&array)[N]);
+
+template <class C> _LIBCPP_CONSTEXPR_AFTER_CXX14 auto cbegin(const C& c) -> decltype(std::begin(c));        // C++14
+template <class C> _LIBCPP_CONSTEXPR_AFTER_CXX14 auto cend(const C& c) -> decltype(std::end(c));            // C++14
+template <class C> _LIBCPP_CONSTEXPR_AFTER_CXX14 auto rbegin(C& c) -> decltype(c.rbegin());                 // C++14
+template <class C> _LIBCPP_CONSTEXPR_AFTER_CXX14 auto rbegin(const C& c) -> decltype(c.rbegin());           // C++14
+template <class C> _LIBCPP_CONSTEXPR_AFTER_CXX14 auto rend(C& c) -> decltype(c.rend());                     // C++14
+template <class C> _LIBCPP_CONSTEXPR_AFTER_CXX14 auto rend(const C& c) -> decltype(c.rend());               // C++14
 template <class E> reverse_iterator<const E*> rbegin(initializer_list<E> il); // C++14
 template <class E> reverse_iterator<const E*> rend(initializer_list<E> il);   // C++14
-template <class T, size_t N> reverse_iterator<T*> rbegin(T (&array)[N]);      // C++14
-template <class T, size_t N> reverse_iterator<T*> rend(T (&array)[N]);        // C++14
-template <class C> auto crbegin(const C& c) -> decltype(std::rbegin(c));      // C++14
-template <class C> auto crend(const C& c) -> decltype(std::rend(c));          // C++14
+template <class T, size_t N> _LIBCPP_CONSTEXPR_AFTER_CXX14 reverse_iterator<T*> rbegin(T (&array)[N]);      // C++14
+template <class T, size_t N> _LIBCPP_CONSTEXPR_AFTER_CXX14 reverse_iterator<T*> rend(T (&array)[N]);        // C++14
+template <class C> _LIBCPP_CONSTEXPR_AFTER_CXX14 auto crbegin(const C& c) -> decltype(std::rbegin(c));      // C++14
+template <class C> _LIBCPP_CONSTEXPR_AFTER_CXX14 auto crend(const C& c) -> decltype(std::rend(c));          // C++14
 
 // 24.8, container access:
 template <class C> constexpr auto size(const C& c) -> decltype(c.size());         // C++17
 template <class T, size_t N> constexpr size_t size(const T (&array)[N]) noexcept; // C++17
 template <class C> constexpr auto empty(const C& c) -> decltype(c.empty());       // C++17
 template <class T, size_t N> constexpr bool empty(const T (&array)[N]) noexcept;  // C++17
 template <class E> constexpr bool empty(initializer_list<E> il) noexcept;         // C++17
 template <class C> constexpr auto data(C& c) -> decltype(c.data());               // C++17
struct _LIBCPP_TYPE_VIS_ONLY iterator
     typedef _Tp        value_type;
     typedef _Distance  difference_type;
     typedef _Pointer   pointer;
     typedef _Reference reference;
     typedef _Category  iterator_category;
 };
 
 template <class _InputIter>
 inline _LIBCPP_INLINE_VISIBILITY
 void __advance(_InputIter& __i,
              typename iterator_traits<_InputIter>::difference_type __n, input_iterator_tag)
 {
     for (; __n > 0; --__n)
         ++__i;
 }
 
 template <class _BiDirIter>
 inline _LIBCPP_INLINE_VISIBILITY
 void __advance(_BiDirIter& __i,
              typename iterator_traits<_BiDirIter>::difference_type __n, bidirectional_iterator_tag)
 {
     if (__n >= 0)
         for (; __n > 0; --__n)
             ++__i;
     else
         for (; __n < 0; ++__n)
             --__i;

 void advance(_InputIter& __i,
              typename iterator_traits<_InputIter>::difference_type __n)
 {
     __advance(__i, __n, typename iterator_traits<_InputIter>::iterator_category());
 }
 
 template <class _InputIter>
 inline _LIBCPP_INLINE_VISIBILITY
 typename iterator_traits<_InputIter>::difference_type
 __distance(_InputIter __first, _InputIter __last, input_iterator_tag)
 {
     typename iterator_traits<_InputIter>::difference_type __r(0);
     for (; __first != __last; ++__first)
         ++__r;
     return __r;
 }
 

 operator-(const move_iterator<_Iter1>& __x, const move_iterator<_Iter2>& __y)
 -> decltype(__x.base() - __y.base())
 {
     return __x.base() - __y.base();
 }
 #else
 template <class _Iter1, class _Iter2>
 inline _LIBCPP_INLINE_VISIBILITY
 typename move_iterator<_Iter1>::difference_type
 operator-(const move_iterator<_Iter1>& __x, const move_iterator<_Iter2>& __y)
 {
     return __x.base() - __y.base();
 }
 #endif
 
 template <class _Iter>
 inline _LIBCPP_INLINE_VISIBILITY

 end(_Tp (&__array)[_Np])
 {
     return __array + _Np;
 }
 
 #if !defined(_LIBCPP_HAS_NO_RVALUE_REFERENCES) && !defined(_LIBCPP_HAS_NO_TRAILING_RETURN)
 
 template <class _Cp>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
 auto
 begin(_Cp& __c) -> decltype(__c.begin())
 {
     return __c.begin();
 }
 
 template <class _Cp>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
 auto
 begin(const _Cp& __c) -> decltype(__c.begin())
 {
     return __c.begin();
 }
 
 template <class _Cp>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
 auto
 end(_Cp& __c) -> decltype(__c.end())
 {
     return __c.end();
 }
 
 template <class _Cp>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
 auto
 end(const _Cp& __c) -> decltype(__c.end())
 {
     return __c.end();
 }
 
 #if _LIBCPP_STD_VER > 11
 
 template <class _Tp, size_t _Np>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
 reverse_iterator<_Tp*> rbegin(_Tp (&__array)[_Np])
 {
     return reverse_iterator<_Tp*>(__array + _Np);
 }
 
 template <class _Tp, size_t _Np>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
 reverse_iterator<_Tp*> rend(_Tp (&__array)[_Np])
 {
     return reverse_iterator<_Tp*>(__array);
 }
 
 template <class _Ep>
 inline _LIBCPP_INLINE_VISIBILITY
 reverse_iterator<const _Ep*> rbegin(initializer_list<_Ep> __il)

 template <class _Cp>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
 auto cend(const _Cp& __c) -> decltype(end(__c))
 {
     return end(__c);
 }
 
 template <class _Cp>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
 auto rbegin(_Cp& __c) -> decltype(__c.rbegin())
 {
     return __c.rbegin();
 }
 
 template <class _Cp>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
 auto rbegin(const _Cp& __c) -> decltype(__c.rbegin())
 {
     return __c.rbegin();
 }
 
 template <class _Cp>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
 auto rend(_Cp& __c) -> decltype(__c.rend())
 {
     return __c.rend();
 }
 
 template <class _Cp>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
 auto rend(const _Cp& __c) -> decltype(__c.rend())
 {
     return __c.rend();
 }
 
 template <class _Cp>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
 auto crbegin(const _Cp& __c) -> decltype(rbegin(__c))
 {
     return rbegin(__c);
 }
 
 template <class _Cp>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
 auto crend(const _Cp& __c) -> decltype(rend(__c))
 {
     return rend(__c);
 }
 
 #endif