Add operator&=( result&, unary-returning-value )

include/boost/system/result.hpp

@@ -1118,6 +1118,25 @@ U operator&( result<T, E>&& r, F&& f )
     }
 }
 
+// operator&=
+
+// result &= unary-returning-value
+
+template<class T, class E, class F,
+    class U = decltype( std::declval<F>()( std::declval<T>() ) ),
+    class En1 = typename std::enable_if<!detail::is_result<U>::value>::type,
+    class En2 = typename std::enable_if<detail::is_value_convertible_to<U, T>::value>::type
+>
+result<T, E>& operator&=( result<T, E>& r, F&& f )
+{
+    if( r )
+    {
+        r = std::forward<F>( f )( *std::move( r ) );
+    }
+
+    return r;
+}
+
 } // namespace system
 } // namespace boost
 

test/CMakeLists.txt

@@ -174,3 +174,4 @@ boost_test(TYPE run SOURCES result_or_fn0v.cpp)
 boost_test(TYPE run SOURCES result_or_fn0r.cpp)
 boost_test(TYPE run SOURCES result_and_fn1v.cpp)
 boost_test(TYPE run SOURCES result_and_fn1r.cpp)
+boost_test(TYPE run SOURCES result_and_eq_fn1v.cpp)

test/Jamfile.v2

@@ -204,3 +204,4 @@ run result_or_fn0v.cpp : : : $(CPP11) ;
 run result_or_fn0r.cpp : : : $(CPP11) ;
 run result_and_fn1v.cpp : : : $(CPP11) ;
 run result_and_fn1r.cpp : : : $(CPP11) ;
+run result_and_eq_fn1v.cpp : : : $(CPP11) ;

test/result_and_eq_fn1v.cpp

@@ -0,0 +1,114 @@
+// 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&& r )
+    {
+        if( &r != this )
+        {
+            v_ = r.v_;
+            r.v_ = 0;
+        }
+
+        return *this;
+    }
+};
+
+struct E
+{
+};
+
+int f( int x )
+{
+    return x * 2 + 1;
+}
+
+X g( Y y )
+{
+    return X{ y.v_ * 2 + 1 };
+}
+
+int& h( int& )
+{
+    static int x = 2;
+    return x;
+}
+
+int main()
+{
+    {
+        result<int> r( 1 );
+
+        r &= f;
+
+        BOOST_TEST( r.has_value() ) && BOOST_TEST_EQ( *r, 3 );
+    }
+
+    {
+        result<int, E> r( in_place_error );
+
+        r &= f;
+
+        BOOST_TEST( r.has_error() );
+    }
+
+    {
+        result<Y> r( in_place_value, 1 );
+
+        r &= g;
+
+        BOOST_TEST( r.has_value() ) && BOOST_TEST_EQ( r->v_, 3 );
+    }
+
+    {
+        result<Y, E> r( in_place_error );
+
+        r &= g;
+
+        BOOST_TEST( r.has_error() );
+    }
+
+    {
+        int x1 = 1;
+        result<int&> r( x1 );
+
+        r &= h;
+
+        BOOST_TEST( r.has_value() ) && BOOST_TEST_EQ( &*r, &h( x1 ) );
+    }
+
+    {
+        result<int&, E> r( in_place_error );
+
+        r &= h;
+
+        BOOST_TEST( r.has_error() );
+    }
+
+    return boost::report_errors();
+}