Add operator|( result, value )

include/boost/system/result.hpp

@@ -885,6 +885,56 @@ public:
     }
 };
 
+// operator|
+
+namespace detail
+{
+
+template<class T, class U> struct is_value_convertible_to: std::is_convertible<T, U>
+{
+};
+
+template<class T, class U> struct is_value_convertible_to<T, U&>:
+    std::integral_constant<bool,
+        std::is_lvalue_reference<T>::value &&
+        std::is_convertible<std::remove_reference_t<T>*, U*>::value>
+{
+};
+
+} // namespace detail
+
+// result | value
+
+template<class T, class E, class U,
+    class En = typename std::enable_if<detail::is_value_convertible_to<U, T>::value>::type
+>
+    T operator|( result<T, E> const& r, U&& u )
+{
+    if( r )
+    {
+        return *r;
+    }
+    else
+    {
+        return std::forward<U>( u );
+    }
+}
+
+template<class T, class E, class U,
+    class En = typename std::enable_if<detail::is_value_convertible_to<U, T>::value>::type
+>
+    T operator|( result<T, E>&& r, U&& u )
+{
+    if( r )
+    {
+        return *std::move( r );
+    }
+    else
+    {
+        return std::forward<U>( u );
+    }
+}
+
 } // namespace system
 } // namespace boost
 

test/CMakeLists.txt

@@ -167,3 +167,6 @@ boost_test(TYPE run SOURCES result_error_move.cpp)
 boost_test(TYPE run SOURCES result_value_construct6.cpp)
 boost_test(TYPE run SOURCES result_value_construct7.cpp)
 boost_test(TYPE run SOURCES result_error_construct5.cpp)
+boost_test(TYPE run SOURCES result_or_value.cpp)
+boost_test(TYPE compile-fail SOURCES result_or_value_fail.cpp)
+boost_test(TYPE compile-fail SOURCES result_or_value_fail2.cpp)

test/Jamfile.v2

@@ -197,3 +197,6 @@ run result_error_move.cpp : : : $(CPP11) ;
 run result_value_construct6.cpp : : : $(CPP11) ;
 run result_value_construct7.cpp : : : $(CPP11) ;
 run result_error_construct5.cpp : : : $(CPP11) ;
+run result_or_value.cpp : : : $(CPP11) ;
+compile-fail result_or_value_fail.cpp : $(CPP11) ;
+compile-fail result_or_value_fail2.cpp : $(CPP11) ;

test/result_or_value.cpp

@@ -0,0 +1,167 @@
+// Copyright 2017, 2021, 2022 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/system/result.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+using namespace boost::system;
+
+struct X
+{
+    int v_;
+};
+
+struct Y
+{
+    int v_;
+
+    explicit Y( int v ): v_( v ) {}
+    Y( X x ): v_( x.v_) {}
+
+    Y( Y const& ) = delete;
+    Y& operator=( Y const& ) = delete;
+
+    Y( Y&& r ): v_( r.v_ )
+    {
+        r.v_ = 0;
+    }
+
+    Y& operator=( Y&& ) = delete;
+};
+
+struct E
+{
+};
+
+int main()
+{
+    {
+        result<int> r( 1 );
+
+        int x = r | 2;
+
+        BOOST_TEST_EQ( x, 1 );
+    }
+
+    {
+        result<int> const r( 1 );
+
+        int x = r | 2;
+
+        BOOST_TEST_EQ( x, 1 );
+    }
+
+    {
+        int x = result<int>( 1 ) | 2;
+
+        BOOST_TEST_EQ( x, 1 );
+    }
+
+    {
+        result<int, E> r( in_place_error );
+
+        int x = r | 2;
+
+        BOOST_TEST_EQ( x, 2 );
+    }
+
+    {
+        result<int, E> const r( in_place_error );
+
+        int x = r | 2;
+
+        BOOST_TEST_EQ( x, 2 );
+    }
+
+    {
+        int x = result<int, E>( in_place_error ) | 2;
+
+        BOOST_TEST_EQ( x, 2 );
+    }
+
+    {
+        Y y = result<Y>( in_place_value, 1 ) | Y{2};
+
+        BOOST_TEST_EQ( y.v_, 1 );
+    }
+
+    {
+        Y y = result<Y, E>( in_place_error ) | Y{2};
+
+        BOOST_TEST_EQ( y.v_, 2 );
+    }
+
+    {
+        Y y = result<Y>( in_place_value, 1 ) | X{2};
+
+        BOOST_TEST_EQ( y.v_, 1 );
+    }
+
+    {
+        Y y = result<Y, E>( in_place_error ) | X{2};
+
+        BOOST_TEST_EQ( y.v_, 2 );
+    }
+
+    {
+        int x1 = 1;
+        int x2 = 2;
+
+        result<int&> r( x1 );
+
+        int& x = r | x2;
+
+        BOOST_TEST_EQ( &x, &x1 );
+    }
+
+    {
+        int x1 = 1;
+        int x2 = 2;
+
+        result<int&> const r( x1 );
+
+        int& x = r | x2;
+
+        BOOST_TEST_EQ( &x, &x1 );
+    }
+
+    {
+        int x1 = 1;
+        int x2 = 2;
+
+        int& x = result<int&>( x1 ) | x2;
+
+        BOOST_TEST_EQ( &x, &x1 );
+    }
+
+    {
+        int x2 = 2;
+
+        result<int&, E> r( in_place_error );
+
+        int& x = r | x2;
+
+        BOOST_TEST_EQ( &x, &x2 );
+    }
+
+    {
+        int x2 = 2;
+
+        result<int&, E> const r( in_place_error );
+
+        int& x = r | x2;
+
+        BOOST_TEST_EQ( &x, &x2 );
+    }
+
+    {
+        int x2 = 2;
+
+        int& x = result<int&, E>( in_place_error ) | x2;
+
+        BOOST_TEST_EQ( &x, &x2 );
+    }
+
+    return boost::report_errors();
+}

test/result_or_value_fail.cpp

@@ -0,0 +1,14 @@
+// Copyright 2017, 2021, 2022 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/system/result.hpp>
+
+using namespace boost::system;
+
+int main()
+{
+    int x = 1;
+    result<int const&> r( x );
+    r | 2;
+}

test/result_or_value_fail2.cpp

@@ -0,0 +1,13 @@
+// Copyright 2017, 2021, 2022 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/system/result.hpp>
+
+using namespace boost::system;
+
+int main()
+{
+    int x = 1;
+    result<int const&>( x ) | 2;
+}