Boost Qi Composing rules using Functions - c++

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. The original parser works fine, but once I started to use the composing functions, my parsers no longer seem to work.
The general language is of the form:
A B: C
There are subsets of the language where A, B, or C must be specific types, such as A is an int while B and C are floats. Here is the parser I used for that sub language:
using entry = boost::tuple<int, float, float>;
template <typename Iterator>
struct sublang : grammar<Iterator, entry(), ascii::space_type>
{
sublang() : sublang::base_type(start)
{
start = int_ >> float_ >> ':' >> float_;
}
rule<Iterator, entry(), ascii::space_type> start;
};
But since there are many subsets, I tried to create a function to build my parser rules:
template<typename AttrName, typename Value>
auto attribute(AttrName attrName, Value value)
{
return attrName >> ':' >> value;
}
So that I could build parsers for each subset more easily without duplicate information:
// in sublang
start = int_ >> attribute(float_, float_);
This fails however and I'm not sure why. In my clang testing, parsing just fails. In g++, it seems the program crashes.
Here's the full example code: http://coliru.stacked-crooked.com/a/8636f19b2e9bff8d
What is wrong with the current code and what would be the correct approach for this problem? I would like to avoid specifying the grammar of attributes and other elements in each sublanguage parser.

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<typename AttrName, typename Value>
auto attribute(AttrName attrName, Value value) {
return boost::proto::deep_copy(attrName >> ':' >> value);
}
fixes it: Live On Coliru
Alternatively
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.
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']
;
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