Why Do Spirit Parsers Assigned to `auto` Variables Cause Issues, and How Can I Fix Them?

Managing Parsed Expressions in Spirit

In this query, we explore the challenges encountered when assigning Spirit parsers to auto variables. While a parser might function seamlessly when used directly with qi::parse(), issues arise when the parser is assigned to an auto variable and reused.

The heart of this behavior lies in the way Spirit parsers are implemented. Proto expression templates, which form the foundation of Spirit, maintain references to temporary variables. When a parser isassigned to an auto variable, the underlying Proto expressions also establish references to the temporary parsers.

To address this issue, several options are available:

• qi::copy(): This method, introduced in Boost V2's trunk, allows copying of parsers, ensuring distinct copies and breaking the reference chain.
• boost::proto::deep_copy: This function offers deep copying capabilities, which can also address the issue of parser references.
• BOOST_SPIRIT_AUTO: This macro, introduced in a previous Stack Overflow response, can be employed as a convenient shorthand for copying parsers.

For example:

namespace qi = boost::spirit::qi;

int main()
{
    auto bracketed_z = qi::copy( '[' >> +qi::char_('z') >> ']' ); // Uses qi::copy()

    string line = "[z]";
    auto p = line.cbegin();
    printf("%d", qi::parse(p, line.cend(), bracketed_z)); // Now works with auto variable

    // Alternative using BOOST_SPIRIT_AUTO
    BOOST_SPIRIT_AUTO(bracketed_z, '[' >> +qi::char_('z') >> ']');
}

These approaches resolve the issue by breaking the reference chain between the parser and the temporary variables, allowing auto variables to be used effectively with Spirit parsers.

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