Add operator|=(result&, fn0v)

include/boost/system/result.hpp

@@ -1084,6 +1084,22 @@ result<T, E>& operator|=( result<T, E>& r, U&& u )
     return r;
 }
 
+// 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
+>
+result<T, E>& operator|=( result<T, E>& r, F&& f )
+{
+    if( !r )
+    {
+        r = std::forward<F>( f )();
+    }
+
+    return r;
+}
+
 // operator&
 
 // result & unary-returning-value

test/CMakeLists.txt

@@ -175,3 +175,4 @@ boost_test(TYPE run SOURCES result_and_eq_fn1v.cpp)
 boost_test(TYPE run SOURCES result_and_eq_fn1r.cpp)
 boost_test(TYPE run SOURCES result_in_place_use.cpp)
 boost_test(TYPE run SOURCES result_or_eq_value.cpp)
+boost_test(TYPE run SOURCES result_or_eq_fn0v.cpp)

test/Jamfile.v2

@@ -205,3 +205,4 @@ run result_and_eq_fn1v.cpp ;
 run result_and_eq_fn1r.cpp ;
 run result_in_place_use.cpp ;
 run result_or_eq_value.cpp ;
+run result_or_eq_fn0v.cpp ;

test/result_or_eq_fn0v.cpp

@@ -0,0 +1,115 @@
+// Copyright 2017, 2021-2024 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()
+{
+    return 2;
+}
+
+X g()
+{
+    return { 2 };
+}
+
+int& h()
+{
+    static int x = 2;
+    return x;
+}
+
+int main()
+{
+    {
+        result<int> r( 1 );
+
+        r |= f;
+
+        BOOST_TEST( r.has_value() ) && BOOST_TEST_EQ( *r, 1 );
+    }
+
+    {
+        result<int, E> r( in_place_error );
+
+        r |= f;
+
+        BOOST_TEST( r.has_value() ) && BOOST_TEST_EQ( *r, 2 );
+    }
+
+    {
+        result<Y> r( in_place_value, 1 );
+
+        r |= g;
+
+        BOOST_TEST( r.has_value() ) && BOOST_TEST_EQ( r->v_, 1 );
+    }
+
+    {
+        result<Y, E> r( in_place_error );
+
+        r |= g;
+
+        BOOST_TEST( r.has_value() ) && BOOST_TEST_EQ( r->v_, 2 );
+    }
+
+    {
+        int x1 = 1;
+
+        result<int&> r( x1 );
+
+        r |= h;
+
+        BOOST_TEST( r.has_value() ) && BOOST_TEST_EQ( &*r, &x1 );
+    }
+
+    {
+        result<int&, E> r( in_place_error );
+
+        r |= h;
+
+        BOOST_TEST( r.has_value() ) && BOOST_TEST_EQ( &*r, &h() );
+    }
+
+    return boost::report_errors();
+}