Change array<T, 0>::begin() to return nullptr and make it constexpr.

Author: pdimov

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 ) ); }
-        const_iterator  begin() const BOOST_NOEXCEPT { return const_iterator( reinterpret_cast< const T * >( this ) ); }
-        const_iterator cbegin() const BOOST_NOEXCEPT { return const_iterator( reinterpret_cast< const T * >( this ) ); }
+        BOOST_CXX14_CONSTEXPR iterator  begin()       BOOST_NOEXCEPT { return data(); }
+        BOOST_CONSTEXPR const_iterator  begin() const BOOST_NOEXCEPT { return data(); }
+        BOOST_CONSTEXPR const_iterator cbegin() const BOOST_NOEXCEPT { return data(); }
 
-        iterator        end()       BOOST_NOEXCEPT { return  begin(); }
-        const_iterator  end() const BOOST_NOEXCEPT { return  begin(); }
-        const_iterator cend() const BOOST_NOEXCEPT { return cbegin(); }
+        BOOST_CXX14_CONSTEXPR iterator  end()       BOOST_NOEXCEPT { return  begin(); }
+        BOOST_CONSTEXPR const_iterator  end() const BOOST_NOEXCEPT { return  begin(); }
+        BOOST_CONSTEXPR const_iterator cend() const BOOST_NOEXCEPT { return cbegin(); }
 
         // reverse iterator support
         typedef std::reverse_iterator<iterator> reverse_iterator;

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!