Add operator<=>, array_thw_test.cpp

include/boost/array.hpp

@@ -381,6 +381,23 @@ namespace boost {
         x.swap(y);
     }
 
+#if defined(__cpp_impl_three_way_comparison) && __cpp_impl_three_way_comparison >= 201907L
+
+    template<class T, std::size_t N>
+    constexpr auto operator<=> (const array<T,N>& x, const array<T,N>& y)
+        -> decltype( x.elems[ 0 ] <=> y.elems[ 0 ] )
+    {
+        for( std::size_t i = 0; i < N; ++i )
+        {
+            auto r = x.elems[ i ] <=> y.elems[ i ];
+            if( r != 0 ) return r;
+        }
+
+        return 0 <=> 0; // std::strong_ordering::equal
+    }
+
+#endif
+
     // undocumented and obsolete
     template <typename T, std::size_t N>
     BOOST_DEPRECATED( "please use `elems` instead" )

test/Jamfile.v2

@@ -45,6 +45,7 @@ run array_swap_test.cpp ;
 run array_swap_test2.cpp ;
 run array_eq_test.cpp ;
 run array_lt_test.cpp ;
+run array_thw_test.cpp ;
 run array_get_test.cpp ;
 
 # C++11 constexpr

test/array_thw_test.cpp

@@ -0,0 +1,87 @@
+// 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/core/lightweight_test.hpp>
+#include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
+#include <cstddef>
+
+#if !defined(__cpp_impl_three_way_comparison)
+
+BOOST_PRAGMA_MESSAGE( "Test skipped because __cpp_impl_three_way_comparison is not defined" )
+int main() {}
+
+#elif !( __cpp_impl_three_way_comparison >= 201907L )
+
+BOOST_PRAGMA_MESSAGE( "Test skipped because __cpp_impl_three_way_comparison is defined to " BOOST_STRINGIZE(__cpp_impl_three_way_comparison) )
+int main() {}
+
+#else
+
+template<class T, std::size_t N> void test()
+{
+    constexpr auto eq = 0 <=> 0;
+    constexpr auto lt = 0 <=> 1;
+    constexpr auto gt = 1 <=> 0;
+
+    {
+        boost::array<T, N> const a1 = {};
+        boost::array<T, N> const a2 = {};
+
+        BOOST_TEST( ( a1 <=> a2 ) == eq );
+    }
+
+    {
+        boost::array<T, N> a1;
+        boost::array<T, N> a2;
+
+        a1.fill( 1 );
+        a2.fill( 1 );
+
+        BOOST_TEST( ( a1 <=> a2 ) == eq );
+    }
+
+    for( std::size_t i = 0; i < N; ++i )
+    {
+        boost::array<T, N> a1;
+        boost::array<T, N> a2;
+
+        a1.fill( 1 );
+        a2.fill( 1 );
+
+        a1[ i ] = 0;
+
+        BOOST_TEST( ( a1 <=> a2 ) == lt );
+
+        {
+            boost::array<T, N> const a3 = a1;
+            boost::array<T, N> const a4 = a2;
+
+            BOOST_TEST( ( a3 <=> a4 ) == lt );
+        }
+
+        a1[ i ] = 2;
+
+        BOOST_TEST( ( a1 <=> a2 ) == gt );
+
+        {
+            boost::array<T, N> const a3 = a1;
+            boost::array<T, N> const a4 = a2;
+
+            BOOST_TEST( ( a3 <=> a4 ) == gt );
+        }
+    }
+}
+
+int main()
+{
+    // test<int, 0>();
+    test<int, 1>();
+    test<int, 7>();
+
+    return boost::report_errors();
+}
+
+#endif