Add operator|( result, nullary-returning-value )

include/boost/system/result.hpp

@@ -935,6 +935,40 @@ template<class T, class E, class U,
     }
 }
 
+// result | nullary-returning-value
+
+template<class T, class E, class F,
+    class U = decltype( std::declval<F>()() ),
+    class En = typename std::enable_if<detail::is_value_convertible_to<U, T>::value>::type
+>
+    T operator|( result<T, E> const& r, F&& f )
+{
+    if( r )
+    {
+        return *r;
+    }
+    else
+    {
+        return std::forward<F>( f )();
+    }
+}
+
+template<class T, class E, class F,
+    class U = decltype( std::declval<F>()() ),
+    class En = typename std::enable_if<detail::is_value_convertible_to<U, T>::value>::type
+>
+    T operator|( result<T, E>&& r, F&& f )
+{
+    if( r )
+    {
+        return *std::move( r );
+    }
+    else
+    {
+        return std::forward<F>( f )();
+    }
+}
+
 } // namespace system
 } // namespace boost
 

test/CMakeLists.txt

@@ -170,3 +170,4 @@ 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)
+boost_test(TYPE run SOURCES result_or_fn0v.cpp)

test/Jamfile.v2

@@ -200,3 +200,4 @@ 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) ;
+run result_or_fn0v.cpp : : : $(CPP11) ;

test/result_or_fn0v.cpp

@@ -0,0 +1,162 @@
+// 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 f()
+{
+    return 2;
+}
+
+X g()
+{
+    return { 2 };
+}
+
+int& h()
+{
+    static int x = 2;
+    return x;
+}
+
+int main()
+{
+    {
+        result<int> r( 1 );
+
+        int x = r | f;
+
+        BOOST_TEST_EQ( x, 1 );
+    }
+
+    {
+        result<int> const r( 1 );
+
+        int x = r | f;
+
+        BOOST_TEST_EQ( x, 1 );
+    }
+
+    {
+        int x = result<int>( 1 ) | f;
+
+        BOOST_TEST_EQ( x, 1 );
+    }
+
+    {
+        result<int, E> r( in_place_error );
+
+        int x = r | f;
+
+        BOOST_TEST_EQ( x, 2 );
+    }
+
+    {
+        result<int, E> const r( in_place_error );
+
+        int x = r | f;
+
+        BOOST_TEST_EQ( x, 2 );
+    }
+
+    {
+        int x = result<int, E>( in_place_error ) | f;
+
+        BOOST_TEST_EQ( x, 2 );
+    }
+
+    {
+        Y y = result<Y>( in_place_value, 1 ) | g;
+
+        BOOST_TEST_EQ( y.v_, 1 );
+    }
+
+    {
+        Y y = result<Y, E>( in_place_error ) | g;
+
+        BOOST_TEST_EQ( y.v_, 2 );
+    }
+
+    {
+        int x1 = 1;
+
+        result<int&> r( x1 );
+
+        int& x = r | h;
+
+        BOOST_TEST_EQ( &x, &x1 );
+    }
+
+    {
+        int x1 = 1;
+
+        result<int&> const r( x1 );
+
+        int& x = r | h;
+
+        BOOST_TEST_EQ( &x, &x1 );
+    }
+
+    {
+        int x1 = 1;
+
+        int& x = result<int&>( x1 ) | h;
+
+        BOOST_TEST_EQ( &x, &x1 );
+    }
+
+    {
+        result<int&, E> r( in_place_error );
+
+        int& x = r | h;
+
+        BOOST_TEST_EQ( &x, &h() );
+    }
+
+    {
+        result<int&, E> const r( in_place_error );
+
+        int& x = r | h;
+
+        BOOST_TEST_EQ( &x, &h() );
+    }
+
+    {
+        int& x = result<int&, E>( in_place_error ) | h;
+
+        BOOST_TEST_EQ( &x, &h() );
+    }
+
+    return boost::report_errors();
+}