Boost Spirit optional parser and backtracking

问题

Why this parser leave 'b' in attributes, even if option wasn't matched?

using namespace boost::spirit::qi;

std::string str = "abc";

auto a = char_("a");
auto b = char_("b");
qi::rule<std::string::iterator, std::string()> expr;
expr = +a >> -(b >> +a);

std::string res;

bool r = qi::parse(
        str.begin(),
        str.end(),
        expr >> lit("bc"),
        res
);

It parses successfully, but res is "ab".

If parse "abac" with expr alone, option is matched and attribute is "aba".

Same with "aac", option doesn't start to match and attribute is "aa".

But with "ab", attribute is "ab", even though b gets backtracked, and, as in example, matched with next parser.

UPD

With expr.name("expr"); and debug(expr); I got

<expr>
  <try>abc</try>
  <success>bc</success>
  <attributes>[[a, b]]</attributes>
</expr>

回答1:

Firstly, it's UB to use the auto variables to keep the expression templates, because they hold references to the temporaries "a" and "b" ^[1].

Instead write

expr = +qi::char_("a") >> -(qi::char_("b") >> +qi::char_("a"));

or, if you insist:

auto a = boost::proto::deep_copy(qi::char_("a"));
auto b = boost::proto::deep_copy(qi::char_("b"));
expr = +a >> -(b >> +a);

Now noticing the >> lit("bc") part hiding in the parse call, suggests you may expect backtracking to on succesfully matched tokens when a parse failure happens down the road.

That doesn't happen: Spirit generates PEG grammars, and always greedily matches from left to right.

On to the sample given, ab results, even though backtracking does occur, the effects on the attribute are not rolled back without qi::hold: Live On Coliru

Container attributes are passed along by ref and the effects of previous (successful) expressions is not rolled back, unless you tell Spirit too. This way, you can "pay for what you use" (as copying temporaries all the time would be costly).

See e.g.

boost::spirit::qi duplicate parsing on the output
Understanding Boost.spirit's string parser
Boost spirit revert parsing

<a>
  <try>abc</try>
  <success>bc</success>
  <attributes>[a]</attributes>
</a>
<a>
  <try>bc</try>
  <fail/>
</a>
<b>
  <try>bc</try>
  <success>c</success>
  <attributes>[b]</attributes>
</b>
<a>
  <try>c</try>
  <fail/>
</a>
<bc>
  <try>bc</try>
  <success></success>
  <attributes>[]</attributes>
</bc>
Success: 'ab'

^[1] see here:

Assigning parsers to auto variables
Generating Spirit parser expressions from a variadic list of alternative parser expressions
boost spirit V2 qi bug associated with optimization level

回答2:

Quoting @sehe from this SO question

A string attribute is a container attribute and many elements could be assigned into it by different parser subexpressions. Now for efficiency reasons, Spirit doesn't rollback the values of emitted attributes on backtracking.

So, I've put optional parser on hold, and it's done.

expr = +qi::char_("a") >> -(qi::hold[qi::char_("b") >> +qi::char_("a")]);

For more information see mentioned question and hold docs

来源：https://stackoverflow.com/questions/26185900/boost-spirit-optional-parser-and-backtracking

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