Boost Qi Composing rules using Functions

Question

I\'m trying to define some Boost::spirit::qi parsers for multiple subsets of a language with minimal code duplication. To do this, I created a few basic rule building functions.

Answer 1

Quite simply: using auto with Spirit (or any EDSL based on Boost Proto and Boost Phoenix) is most likely Undefined Behaviour¹

Now, you can usually fix this using

• BOOST_SPIRIT_AUTO
• boost::proto::deep_copy
• the new facility that's coming in the most recent version of Boost (TODO add link)

In this case,

template
auto attribute(AttrName attrName, Value value) {
    return boost::proto::deep_copy(attrName >> ':' >> value);
}

fixes it: Live On Coliru

Alternatively

1. you could use qi::lazy[] with inherited attributes.

   I do very similar things in the prop_key rule in Reading JSON file with C++ and BOOST.

2. you could have a look at the Keyword List Operator from the Spirit Repository. It's designed to allow easier construction of grammars like:

   no_constraint_person_rule %=
       kwd("name")['=' > parse_string ]
     / kwd("age")   ['=' > int_]
     / kwd("size")   ['=' > double_ > 'm']
     ;
   

3. This you could potentially combine with the Nabialek Trick. I'd search the answers on SO for examples. (One is Grammar balancing issue)

---

¹ Except for entirely stateless actors (Eric Niebler on this) and expression placeholders. See e.g.

• Assigning parsers to auto variables
• undefined behaviour somewhere in boost::spirit::qi::phrase_parse
• C++ Boost qi recursive rule construction
• boost spirit V2 qi bug associated with optimization level

Some examples

• Define parsers parameterized with sub-parsers in Boost Spirit
• Generating Spirit parser expressions from a variadic list of alternative parser expressions