我怎么可能假设一个“默认值"?使用boost :: spirit解析时? [英] How might I assume a "default value" when parsing using boost::spirit?

问题描述

假设我有一个语法定义如下:

Let's say I have a grammar defined to something like:

some_rule := a b [c [d]]

其中，c和d是可选的，如果未指定，则默认为特定值(假设为14).如果未提供该值，我可以将其默认设置为14吗?我希望生成的std::vector总是大小为4.

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.

我来的最接近的东西是这样的:

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];

然后，对于未显示的可选值，它将获得0(我相信).然后，我将连续的0最终更改为14.这不仅是严重的错误，而且还不够优雅.有更好的方法吗?

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?

推荐答案

看来您可以使用boost::qi::attr辅助解析器来做到这一点.

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];

尽管如此，我仍然不确定这是否是最好的方法.

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

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