Assigning parsers to auto variables

Question

Are spirit parsers not meant to be used with auto?

A simple parser works fine when passed to qi::parse() inline, but crashes with segfault if passed via an auto variable:

#include <cstdio>
#include <string>
#include <boost/spirit/include/qi.hpp>

using namespace std;

namespace qi = boost::spirit::qi;

int main()
{
    string line = "[z]";

    auto bracketed_z = '[' >> +qi::char_('z') >> ']';

    auto p = line.cbegin();
    printf("%d", qi::parse(p, line.cend(), '[' >> +qi::char_('z') >> ']'));  // Works

    p = line.cbegin();
    printf("%d", qi::parse(p, line.cend(), bracketed_z));                    // Crashes
}

Reproes with g++-4.8 and VC13.

Update: an error was fixed in the original code (p was not re-initialized before the second call to parse()).

How can any c++ parser deal with `auto`? Parsers are meant to read particular types determined at runtime, `auto` types are resolved at compile time. So IMHO unless the `auto` results in a `boost::variant` or `boost::any` it's not possible. — πάντα ῥεῖ, Feb 25 '14 at 19:16
Note that the first iterator is passed by reference (IIRC). That is, `p` has been changed after the first call to `qi::parse`. — dyp, Feb 25 '14 at 20:27
@πάνταῥεῖ: in this simple example it all should be compile-time. — mstone, Feb 25 '14 at 21:39

score 12 · Accepted Answer · edited May 23 '17 at 12:08

12

Spirit Parsers are not designed to be used with auto in Spirit V2.

This is because the underlying Proto expression templates hold references to the temporaries.

You can use

qi::copy() (existing in the trunk after boost_1_55_0, not in any released version at this time)
boost::proto::deep_copy
or BOOST_SPIRIT_AUTO (first coined here)

I've written about these things more often on SO: https://stackoverflow.com/search?q=user%3A85371+deep_copy, specifically, this:

boost spirit V2 qi bug associated with optimization level

Boost Spirit X3 will not have this limitation.

edited May 23 '17 at 12:08

Community

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answered Feb 25 '14 at 21:59

sehe

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The error recently is too often encountered. Thus there is the NEED to have contemporary version of Boost.Spirit Qi library, which must be C++11/C++1y fully enabled (I think it should contain at least support for move-semantic). – Tomilov Anatoliy Feb 26 '14 at 10:41
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@Dukales this appears to be an opinion in need of citation :) If you mean you're eager to see Spirit X3, +1 – sehe Feb 26 '14 at 11:04

score 3 · Answer 2 · answered Feb 25 '14 at 21:58

3

Boost.Spirit uses expression templates and does not work with auto. A workaround is to use boost::proto::deep_copy:

auto bracketed_z = proto::deep_copy('[' >> +qi::char_('z') >> ']');

answered Feb 25 '14 at 21:58

Juraj Blaho

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Assigning parsers to auto variables

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