X3: Optional container parsing when it is a part of the sequence and the optional parser subject is a sequence too

[pulver]

In Boost 1.70.0, starting with example/x3/employee.cpp and applying the diff:

diff --git a/employee.cpp b/test.cpp
index 6257102..cb0fdbb 100644
--- a/employee.cpp
+++ b/test.cpp
@@ -31,7 +31,7 @@ namespace client { namespace ast
     {
         int age;
         std::string forename;
-        std::string surname;
+        boost::optional<std::string> surname;
         double salary;
     };

@@ -71,7 +71,7 @@ namespace client
             >> '{'
             >>  int_ >> ','
             >>  quoted_string >> ','
-            >>  quoted_string >> ','
+            >>  -quoted_string >> ','
             >>  double_
             >>  '}'
             ;
@@ -118,7 +118,6 @@ main()

             std::cout << "-------------------------\n";
             std::cout << "Parsing succeeded\n";
-            std::cout << "got: " << emp << std::endl;
             std::cout << "\n-------------------------\n";
         }
         else

results in compiler errors, including

/usr/include/boost/spirit/home/x3/support/traits/move_to.hpp:194:24: error: no matching function for call to ‘move_to(std::remove_reference<const char&>::type, std::__cxx11::basic_string<char>&, boost::mpl::identity<boost::spirit::x3::traits::container_attribute>::type)’

In contrast, the following diff does successfully compile:

diff --git a/employee.cpp b/test.cpp
index 6257102..79f35a0 100644
--- a/employee.cpp
+++ b/test.cpp
@@ -32,7 +32,7 @@ namespace client { namespace ast
         int age;
         std::string forename;
         std::string surname;
-        double salary;
+        boost::optional<double> salary;
     };

     using boost::fusion::operator<<;
@@ -72,7 +72,7 @@ namespace client
             >>  int_ >> ','
             >>  quoted_string >> ','
             >>  quoted_string >> ','
-            >>  double_
+            >>  -double_
             >>  '}'
             ;

@@ -118,7 +118,6 @@ main()

             std::cout << "-------------------------\n";
             std::cout << "Parsing succeeded\n";
-            std::cout << "got: " << emp << std::endl;
             std::cout << "\n-------------------------\n";
         }
         else

Summary

The optional operator unary minus (-) works as expected when parsing a single double_ into a boost::optional<double> member variable, but does not work as expected when parsing the compound parser quoted_string into a corresponding boost::optional<std::string>.

[pulver]

Issue resolved. A string is a sequence/vector of characters, so when an optional operator - is applied, it collapses to the sequence. In other words, remove the boost::optional and test to see if it is empty to see if there was a match or not.

[djowel]

Issue resolved. A string is a sequence/vector of characters, so when an optional operator - is applied, it collapses to the sequence. In other words, remove the boost::optional and test to see if it is empty to see if there was a match or not.

Yes, but I still want to investigate if this is a bug or not. The optional and use of - seems like a valid use-case, at first glance.

[Kojoley]

It is definitely a bug.

I recently observed the root issue, but have not come to a solution. The issue is about handling container attributes in sequence && tuple attribute partitioning in sequence && passing thru unary a partitioned tuple attribute. In more details: tuple partitioning creates an iterator range of single element and it is not unwrapped because it will be passed to another sequence parser, and it is something that parse_into_container_impl relies on currently.

MWE:

#include <boost/spirit/home/x3.hpp>
#include <boost/fusion/include/std_pair.hpp>

int main()
{
    using boost::spirit::x3::char_;
    using boost::spirit::x3::int_;
    char const* const it = "123:abc";
    std::pair<int, boost::optional<std::string>> v;
    parse(it, it, int_ >> -(':' >> +char_), v);
}