Boost.Spirit.Qi crashes when assigning rule to a sequence including itself

Question

I have the following MWE:

#include 

#include 
#include 
#include &l         


        

Answer 1

I'm not sure what you were trying to achieve, but copy() would seem to be what you're after

    parser = parser.copy() >> parser2;

See it Live on Coliru

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Background

The problem is Qi takes non-terminals by reference, so you get the parser semantics a PEG grammar would suggest.

Besides that, Proto expression trees (expression templates) do take some of their arguments by reference.

These two combined have a potential to really mess up your life, especially when construction parsers dynamically. In short, I'd argue that, outside

• using inherited attributes
• and qi::symbols (including the Nabialek trick)

constructing rules on the fly is not well supported in Spirit V2. Proto x11 / Spirit X3 may change this for the better.

See more background here:

• C++ Boost qi recursive rule construction
• Generating Spirit parser expressions from a variadic list of alternative parser expressions
• Can Boost Spirit Rules be parameterized

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Sample code

#include 
#include 
#include 

namespace spirit = boost::spirit;
namespace qi = boost::spirit::qi;
namespace phoenix = boost::phoenix;


int main() {

    std::string input("1 2");

    qi::rule parser;
    qi::rule parser2;
    qi::rule parser3;

    parser = qi::int_[
        std::cerr << phoenix::val("First int: ") << qi::_1 << std::endl
    ];

    parser2 = qi::int_[
        std::cerr << phoenix::val("Second int: ") << qi::_1 << std::endl
    ];

    try {
        // Comment out these two lines, (finished below ...)
        parser3 = parser >> parser2;
        phrase_parse(input.begin(), input.end(), parser3, qi::space);

        parser = parser.copy() >> parser2;
        phrase_parse(input.begin(), input.end(), parser, qi::space);
    }
    catch (...) {
        std::cerr << "Exception caught." << std::endl;
    }

}