Add cast_empty() for value casting

1 files changed, 6 insertions, 0 deletions

diff --git a/build2/variable.cxx b/build2/variable.cxx
index ce88712..9ce9fe8 100644
--- a/build2/variable.cxx
+++ b/build2/variable.cxx
@@ -533,6 +533,8 @@ namespace build2
     return s;
   }
 
+  const string& value_traits<string>::empty_instance = empty_string;
+
   const char* const value_traits<string>::type_name = "string";
 
   const value_type value_traits<string>::value_type
@@ -585,6 +587,8 @@ namespace build2
     throw_invalid_argument (n, r, "path");
   }
 
+  const path& value_traits<path>::empty_instance = empty_path;
+
   const char* const value_traits<path>::type_name = "path";
 
   const value_type value_traits<path>::value_type
@@ -630,6 +634,8 @@ namespace build2
     throw_invalid_argument (n, r, "dir_path");
   }
 
+  const dir_path& value_traits<dir_path>::empty_instance = empty_dir_path;
+
   const char* const value_traits<dir_path>::type_name = "dir_path";
 
   const value_type value_traits<dir_path>::value_type