Why Do Spirit V2 Parsers Crash When Assigned to `auto` Variables?

Assigning Parsers to Auto Variables: Spirit V2's Hidden Weakness

Spirit parsers in Spirit V2 are not intended to be used with the auto keyword. When attempting to assign a parser to an auto variable, the program may crash with a segmentation fault.

This issue arises because Spirit's underlying Proto expression templates hold references to temporaries. To work around this limitation, you can use the following methods:

• qi::copy(): Available in the Boost V2 trunk but not yet released. Copies the parser, creating a new object.
• boost::proto::deep_copy: Performs a deep copy of the parser, resolving any references to temporaries.
• BOOST_SPIRIT_AUTO: A macro defined as boost::proto::wrap().

For more information, refer to the StackOverflow discussion at https://stackoverflow.com/search?q=user:85371 deep_copy.

Note: This limitation does not exist in Boost Spirit X3, which has a more modern architecture.

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