Remove local hash_value overload; boost::hash supports array-like types natively.

doc/array/changes.adoc

@@ -14,3 +14,7 @@ http://www.boost.org/LICENSE_1_0.txt
 
 * Converted documentation to AsciiDoc (Christian Mazakas).
 * Added `noexcept` and `constexpr` as appropriate.
+* Marked obsolete functions as deprecated.
+* Removed obsolete compiler workarounds.
+* Changed `array<T, 0>::begin()`, `cbegin()`, `end()`, `cend()` to return `nullptr`, enabling `constexpr`.
+  This matches the behavior of `std::array`.

include/boost/array.hpp

@@ -207,13 +207,13 @@ namespace boost {
         typedef std::ptrdiff_t difference_type;
 
         // iterator support
-        iterator        begin()       BOOST_NOEXCEPT { return       iterator( reinterpret_cast<       T * >( this ) ); }
+        BOOST_CXX14_CONSTEXPR iterator  begin()       BOOST_NOEXCEPT { return data(); }
-        const_iterator  begin() const BOOST_NOEXCEPT { return const_iterator( reinterpret_cast< const T * >( this ) ); }
+        BOOST_CONSTEXPR const_iterator  begin() const BOOST_NOEXCEPT { return data(); }
-        const_iterator cbegin() const BOOST_NOEXCEPT { return const_iterator( reinterpret_cast< const T * >( this ) ); }
+        BOOST_CONSTEXPR const_iterator cbegin() const BOOST_NOEXCEPT { return data(); }
 
-        iterator        end()       BOOST_NOEXCEPT { return  begin(); }
+        BOOST_CXX14_CONSTEXPR iterator  end()       BOOST_NOEXCEPT { return  begin(); }
-        const_iterator  end() const BOOST_NOEXCEPT { return  begin(); }
+        BOOST_CONSTEXPR const_iterator  end() const BOOST_NOEXCEPT { return  begin(); }
-        const_iterator cend() const BOOST_NOEXCEPT { return cbegin(); }
+        BOOST_CONSTEXPR const_iterator cend() const BOOST_NOEXCEPT { return cbegin(); }
 
         // reverse iterator support
         typedef std::reverse_iterator<iterator> reverse_iterator;
@@ -303,19 +303,10 @@ namespace boost {
         BOOST_CXX14_CONSTEXPR void fill (const T& ) {}
 
         // check range (may be private because it is static)
-        static reference failed_rangecheck () {
+        static reference failed_rangecheck ()
-                std::out_of_range e("attempt to access element of an empty array");
+        {
-                boost::throw_exception(e);
+            boost::throw_exception( std::out_of_range( "attempt to access element of an empty array" ) );
-#if defined(BOOST_NO_EXCEPTIONS) || (!defined(BOOST_MSVC) && !defined(__PATHSCALE__))
+        }
-                //
-                // We need to return something here to keep
-                // some compilers happy: however we will never
-                // actually get here....
-                //
-                static T placeholder;
-                return placeholder;
-#endif
-            }
     };
 
     // comparisons
@@ -404,14 +395,6 @@ namespace boost {
         return arg.elems;
     }
 
-    template <class It> std::size_t hash_range(It, It);
-
-    template<class T, std::size_t N>
-    std::size_t hash_value(const array<T,N>& arr)
-    {
-        return boost::hash_range(arr.begin(), arr.end());
-    }
-
    template <size_t Idx, typename T, size_t N>
    BOOST_CXX14_CONSTEXPR T &get(boost::array<T,N> &arr) BOOST_NOEXCEPT {
        BOOST_STATIC_ASSERT_MSG ( Idx < N, "boost::get<>(boost::array &) index out of range" );

test/Jamfile.v2

@@ -59,6 +59,9 @@ compile array_get_test_cx.cpp ;
 
 # C++14 constexpr
 
+compile array_assign_test_cx.cpp ;
+compile array_access_test_cx2.cpp ;
+compile array_fill_test_cx.cpp ;
 compile array_eq_test_cx.cpp ;
 compile array_lt_test_cx.cpp ;
 

test/array0.cpp

@@ -44,7 +44,12 @@ void    RunTests()
     BOOST_TEST ( const_test_case.begin() == const_test_case.end());
     BOOST_TEST ( const_test_case.cbegin() == const_test_case.cend());
 
-    BOOST_TEST ( test_case.begin() != const_test_case.begin() );
+    // BOOST_TEST ( test_case.begin() != const_test_case.begin() );
+    //
+    // TR1 specified that begin() must return a unique value for zero-sized
+    // arrays. However, this makes constexpr unimplementable, and all standard
+    // libraries have converged on using nullptr instead (see LWG issue 2157.)
+
     if( test_case.data() == const_test_case.data() ) {
     //  Value of data is unspecified in TR1, so no requirement this test pass or fail
     //  However, it must compile!

test/array_access_test_cx2.cpp

@@ -0,0 +1,45 @@
+// Copyright 2025 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/array.hpp>
+#include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
+#include <boost/config/workaround.hpp>
+#include <cstddef>
+
+#if defined(BOOST_NO_CXX14_CONSTEXPR)
+
+BOOST_PRAGMA_MESSAGE("Test skipped because BOOST_NO_CXX14_CONSTEXPR is defined")
+
+#else
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+template<class T, std::size_t N> constexpr boost::array<T, N> modified( boost::array<T, N> a1 )
+{
+    a1.front() = 1;
+    a1[ 1 ] = 2;
+    a1.at( 2 ) = 3;
+    a1.back() = 4;
+
+    return a1;
+}
+
+template<class T> void test1()
+{
+    constexpr boost::array<T, 4> a1 = {};
+    constexpr boost::array<T, 4> a2 = modified( a1 );
+
+    STATIC_ASSERT( a2[0] == 1 );
+    STATIC_ASSERT( a2[1] == 2 );
+    STATIC_ASSERT( a2[2] == 3 );
+    STATIC_ASSERT( a2[3] == 4 );
+}
+
+int main()
+{
+    test1<int>();
+}
+
+#endif

test/array_assign_test_cx.cpp

@@ -0,0 +1,54 @@
+// Copyright 2025 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/array.hpp>
+#include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
+#include <boost/config/workaround.hpp>
+#include <cstddef>
+
+#if defined(BOOST_NO_CXX14_CONSTEXPR)
+
+BOOST_PRAGMA_MESSAGE("Test skipped because BOOST_NO_CXX14_CONSTEXPR is defined")
+
+#else
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+template<class T, std::size_t N> constexpr boost::array<T, N> assigned( boost::array<T, N> const& a1 )
+{
+    boost::array<T, N> a2 = {};
+
+    a2 = a1;
+
+    return a2;
+}
+
+template<class T> void test1()
+{
+    constexpr boost::array<T, 4> a1 = {{ 1, 2, 3, 4 }};
+    constexpr boost::array<T, 4> a2 = assigned( a1 );
+
+    STATIC_ASSERT( a1[0] == a2[0] );
+    STATIC_ASSERT( a1[1] == a2[1] );
+    STATIC_ASSERT( a1[2] == a2[2] );
+    STATIC_ASSERT( a1[3] == a2[3] );
+}
+
+template<class T> void test2()
+{
+    constexpr boost::array<T, 0> a1 = {};
+    constexpr boost::array<T, 0> a2 = assigned( a1 );
+
+    (void)a1;
+    (void)a2;
+}
+
+int main()
+{
+    test1<int>();
+    test2<int>();
+}
+
+#endif

test/array_fill_test_cx.cpp

@@ -0,0 +1,51 @@
+// Copyright 2025 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/array.hpp>
+#include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
+#include <boost/config/workaround.hpp>
+#include <cstddef>
+
+#if defined(BOOST_NO_CXX14_CONSTEXPR)
+
+BOOST_PRAGMA_MESSAGE("Test skipped because BOOST_NO_CXX14_CONSTEXPR is defined")
+
+#else
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+template<class T, std::size_t N> constexpr boost::array<T, N> filled( T const& v )
+{
+    boost::array<T, N> r = {};
+
+    r.fill( v );
+
+    return r;
+}
+
+template<class T> void test1()
+{
+    constexpr boost::array<T, 4> a1 = filled<T, 4>( 7 );
+
+    STATIC_ASSERT( a1[0] == 7 );
+    STATIC_ASSERT( a1[1] == 7 );
+    STATIC_ASSERT( a1[2] == 7 );
+    STATIC_ASSERT( a1[3] == 7 );
+}
+
+template<class T> void test2()
+{
+    constexpr boost::array<T, 0> a1 = filled<T, 0>( 7 );
+
+    (void)a1;
+}
+
+int main()
+{
+    test1<int>();
+    test2<int>();
+}
+
+#endif

test/array_init_test.cpp

@@ -58,7 +58,7 @@ int main()
     test2<int, 1>();
     test2<int, 7>();
 
-    // test2<int const, 0>();
+    test2<int const, 0>();
 
 #if BOOST_WORKAROUND(BOOST_MSVC, < 1910) || BOOST_WORKAROUND(BOOST_GCC, < 50000)
 

test/array_iterator_test_cx.cpp

@@ -39,20 +39,17 @@ template<class T> void test2()
 {
     constexpr boost::array<T, 0> a = {};
 
-    // iterator access is not constexpr for boost::array<T, 0>
-    // it is for std::array<T, 0>, though, so we should change it
-
     STATIC_ASSERT( a.begin() == a.end() );
     STATIC_ASSERT( a.cbegin() == a.cend() );
 }
 
 int main()
 {
-    // test1<int, 0>();
+    test1<int, 0>();
     test1<int, 1>();
     test1<int, 7>();
 
-    // test2<int>();
+    test2<int>();
 }
 
 #endif