[Boost-users] spirit::qi and Phoenix placeholder

Subject: [Boost-users] spirit::qi and Phoenix placeholder
From: Olivier Austina (olivier.austina_at_[hidden])
Date: 2013-10-04 16:12:39

I am reading Spirit tutorial, at
http://www.boost.org/doc/libs/1_54_0/libs/spirit/doc/html/spirit/qi/tutorials.html.

I am new to boost. It is not clear to me what the placeholder are and when
using them. This is some of them:

*Spirit.Qi specific Phoenix placeholders*
_1, _2... , _N

Nth attribute of p
_val

The enclosing rule's synthesized attribute.
_r1, _r2... , _rN

The enclosing rule's Nth inherited attribute.
_a, _b... , _j

The enclosing rule's local variables (_a refers to the first).
_pass

Assign false to _pass to force a parser failure.

For example in this code :

template <typename Iterator>bool parse_numbers(Iterator first,
Iterator last, std::vector<double>& v){
    using qi::double_;
    using qi::phrase_parse;
    using qi::_1;
    using ascii::space;
    using phoenix::push_back;
    using phoenix::ref;

    bool r = phrase_parse(first, last,

        //  Begin grammar        (
            *double_[push_back(ref(v), _1)]
                >> *(',' >> double_[push_back(ref(v), _1)])*
        )
        ,
        //  End grammar
        space);

    if (first != last) // fail if we did not get a full match
return false;
    return r;}

why using _1, (using qi::_1) instead of _2 (using qi::_2).

Which parts of boost should I read before reading spirit.
Any suggestion is welcome. Thank you.

Regards
Olivier