How might I assume a "default value" when parsing using boost::spirit?

boost-spirit, c++

Solution

It looks like you can do this with the `boost::qi::attr` auxiliary parser.

int default_value = 14;

qi::rule<Iterator, int(),              ascii::space_type> some_optional_rule;
qi::rule<Iterator, std::vector<int>(), ascii::space_type> some_rule;

some_optional_rule %= int_ | attr(default_value);
some_rule          %= repeat(2)[int_] >> repeat(2)[some_optional_rule];

I'm still not sure if this is the best way to do this though.

Problem

Let's say I have a grammar defined to something like: ``` some_rule := a b [c [d]] ``` where `c`, and `d` are optional and default to a certain value (let's say 14) if not given. Can I get it to default to the 14 if the value isn't given? I want the produced `std::vector` to always be of size 4. The closest I've come is like the following: ``` qi::rule<Iterator, std::vector<int>(), ascii::space_type> some_rule; some_rule %= int_ >> int_ >> -int_ >> -int_; // ... some_other_rule = some_rule[&some_callback_for_int_vectors]; ``` which will then get 0 for the optional values that didn't show up (I believe). I then change consecutive 0s at the end into 14. Not only is this horribly wrong, but it's also just not elegant. Is there a better way to do this?

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