Why Do Auto Variables Cause Segfaults When Used with Boost Spirit Parsers?

Assigning Parsers to Auto Variables

When using Boost Spirit, it's essential to understand the implications of assigning parsers to auto variables.

The original code snippet highlights a peculiar behavior: passing a parser to qi::parse() inline works, but passing it via an auto variable leads to a segfault. This apparent discrepancy arises due to a limitation of Spirit V2 parsers.

Underlying Proto expression templates within Spirit Parsers hold references to temporaries, which can cause issues when assigned to auto variables. The auto variable itself is a temporary, resulting in its lifetime being shorter than the referenced parser's memory. Consequently, dereferencing the parser can lead to unspecified behavior, including segfaults.

To mitigate this issue, Boost Spirit V2 provides several solutions:

• Use qi::copy(), available in the latest trunk versions of Boost after Boost_1_55_0.
• Employ boost::proto::deep_copy.
• Leverage BOOST_SPIRIT_AUTO, as introduced in the provided link.

Alternatively, Boost Spirit X3 has addressed this limitation, allowing for more flexible parser handling.

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