boost spirit assign_to_* customization points - c++

Question

I'm trying to understand how boost spirit assign_to_* customization points work.
Here is an exemple I am using:
I have this parser in a rule in a grammar:
int_ >> lit(':') >> char_
And I want the result to be put in this struct:
struct IntAndChar{
int i;
char c;
};
(This is just an exemple to use the customization point so I won't use the BOOST_FUSION_ADAPT_STRUCT or semantic actions.)
I thought I could just define a specialization of assign_to_attribute_from_value but I only get the int this way and the second element is dropped.
Can someone give me a hint to understand how it works?

Answer 1

You don't want to assign to the attribute¹. Instead you wish to transform boost::fusion::vector2<int, char> into IntAndChar.
Therefore, let's start off telling spirit our type is not container-like:
template<>
struct is_container<IntAndChar, void> : mpl::false_ { };
Next, tell it how it can transform e between raw and cooked forms of our attributes:
template<>
struct transform_attribute<IntAndChar, fusion::vector2<int, char>, qi::domain, void> {
using Transformed = fusion::vector2<int, char>;
using Exposed = IntAndChar;
using type = Transformed;
static Transformed pre(Exposed&) { return Transformed(); }
static void post(Exposed& val, Transformed const& attr) {
val.i = fusion::at_c<0>(attr);
val.c = fusion::at_c<1>(attr);
}
static void fail(Exposed&) {}
};
That's it! There is one catch though. It won't work unless you trigger a transformation. The docs say:
It is invoked by Qi rule, semantic action and attr_cast, [...]
1. Using qi::rule (not very helpful)
So here's a solution using rule:
Live On Coliru
int main() {
using It = std::string::const_iterator;
qi::rule<It, boost::fusion::vector2<int, char>(), qi::space_type> rule = qi::int_ >> ':' >> qi::char_;
//qi::rule<It, IntAndChar(), qi::space_type> rule = qi::attr_cast(qi::int_ >> ':' >> qi::char_);
for (std::string const input : { "123:a", "-4 : \r\nq" }) {
It f = input.begin(), l = input.end();
IntAndChar data;
bool ok = qi::phrase_parse(f, l, rule, qi::space, data);
if (ok) std::cout << "Parse success: " << data.i << ", " << data.c << "\n";
else std::cout << "Parse failure ('" << input << "')\n";
if (f != l) std::cout << "Remaining unparsed input: '" << std::string(f, l) << "'\n";
}
}
Prints:
Parse success: 123, a
Parse success: -4, q
Of course this approach requires you to spell out boost::fusion::vector2<int, char> which is tedious and error-prone.
2. Using qi::attr_cast
You can use qi::attr_cast to trigger the transform:
qi::rule<It, IntAndChar(), qi::space_type> rule = qi::attr_cast<IntAndChar, boost::fusion::vector2<int, char> >(qi::int_ >> ':' >> qi::char_);
// using deduction:
qi::rule<It, IntAndChar(), qi::space_type> rule = qi::attr_cast<IntAndChar>(qi::int_ >> ':' >> qi::char_);
// using even more deduction:
qi::rule<It, IntAndChar(), qi::space_type> rule = qi::attr_cast(qi::int_ >> ':' >> qi::char_);
CAVEAT That should work. However, due to very subtle behaviour (bugs?) you need to deep-copy the Proto expression tree there, in order for it to work without Undefined Behaviour:
qi::rule<It, IntAndChar(), qi::space_type> rule = qi::attr_cast(qi::copy(qi::int_ >> ':' >> qi::char_));
Bringing it all together, we can even do without the qi::rule:
Live On Coliru
int main() {
using It = std::string::const_iterator;
for (std::string const input : { "123:a", "-4 : \r\nq" }) {
It f = input.begin(), l = input.end();
IntAndChar data;
bool ok = qi::phrase_parse(f, l, qi::attr_cast(qi::copy(qi::int_ >> ':' >> qi::char_)), qi::space, data);
if (ok) std::cout << "Parse success: " << data.i << ", " << data.c << "\n";
else std::cout << "Parse failure ('" << input << "')\n";
if (f != l) std::cout << "Remaining unparsed input: '" << std::string(f, l) << "'\n";
}
}
Prints
Parse success: 123, a
Parse success: -4, q
¹ (unless you want to treat IntAndChar as a container, which is a different story)