Use of optional parser in spirit qi - c++

Question

I'm trying to parse either an additive expression of the form "A+C", or "A" alone. After a few tests I realized that the problem is apparently my use of the optional parser, so to exemplify:
qi::rule<string::iterator, string()> Test;
Test =
(
qi::string("A")[qi::_val= qi::_1]
>> -(
qi::string("B")[qi::_val += qi::_1]
>> qi::string("C")[qi::_val += qi::_1]
)
)
;
string s1, s2;
s1 = "AB";
bool a= qi::parse(s1.begin(), s1.end(), Test, s2);
The idea is to parse 'A' or "ABC", but if the s1 value is "AB" without 'C', the value of a is true. I believe that although I put parenthesis after the operator '-' and then I use the ">>" operator, the 'C' part is considered optional, and not the B>>C as a whole. Any ideas?

Answer 1

Container attributes are not backtracked.
That's a performance choice. You need to explicitly control propagation using e.g. qi::hold:
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#include <boost/spirit/include/qi.hpp>
namespace qi = boost::spirit::qi;
int main() {
using It = std::string::const_iterator;
qi::rule<It, std::string()> Test;
Test =
(
qi::char_('A')
>> -qi::hold [
qi::char_('B')
>> qi::char_('C')
]
)
;
for (std::string const input : { "A", "AB", "ABC" })
{
std::cout << "-------------------------\nTesting '" << input << "'\n";
It f = input.begin(), l = input.end();
std::string parsed;
bool ok = qi::parse(f, l, Test, parsed);
if (ok)
std::cout << "Parsed success: " << parsed << "\n";
else
std::cout << "Parsed failed\n";
if (f != l)
std::cout << "Remaining unparsed: '" << std::string(f,l) << "'\n";
}
}
Prints:
-------------------------
Testing 'A'
Parsed success: A
-------------------------
Testing 'AB'
Parsed success: A
Remaining unparsed: 'B'
-------------------------
Testing 'ABC'
Parsed success: ABC
Note I have made a number of simplifications.
See also:
Boost Spirit: "Semantic actions are evil"?
boost::spirit::qi duplicate parsing on the output
Understanding Boost.spirit's string parser