问题
I have to find variables in a sentence and replace them by their value. Variables can be written in different forms, like $varName, or $(varName) for example.
I'd like to have a struct VariableHolder to have easy access to both :
struct VariableHolder
{
string name; // contains "varName"
string fromFile; // contains "$(varName)" or "$varName"
void setName(ustring n) { name = n; }
}
Obviously, I'd like to avoid doing multiple passes and calling multiple parsers. What I have so far is this :
BOOST_FUSION_ADAPT_STRUCT(VariableHolder,
(ustring, fromFile)
)
// variableName is another parser that returns a string
qi::rule<Iterator, VariableHolder()> variable %=
(qi::char_("$")
>> (variableName[phoenix::bind(&VariableHolder::setName, qi::_val, qi::_1)]
| (qi::char_("(")
>> variableName[phoenix::bind(&VariableHolder::setName, qi::_val, qi::_1)]
>> qi::char_(")")))
);
Which doesn't work. The name is correctly set, but the fromFile variable only contains a "$", and never anything else.
So, my questions to you fine folk :
- My idea was to only adapt part of the structure with BOOST_FUSION_ADAPT_STRUCT, and fill the rest with semantic actions. Stupid idea, or am I just doing it wrong?
Is there a way to bind a semantic action and still get the output? Like
char_[doSomething] // Can this both call doSomething, and parse a char?
回答1:
Addressing the questions:
- My idea was to only adapt part of the structure with
BOOST_FUSION_ADAPT_STRUCT
, and fill the rest with semantic actions. Stupid idea, or am I just doing it wrong?
Not unthinkable. Not my recommendation (see Boost Spirit: "Semantic actions are evil"?). But yeah you're doing it wrong:
You want lit("$")
instead of char_("$")
if you don't want it to be part of the exposed attribute. In fact, '$'
will do here
Is there a way to bind a semantic action and still get the output? Like
char_[doSomething] // Can this both call doSomething, and parse a char?
Yes. You're doing it right now, and it works because of operator%=
you used instead of operator=
(see the docs: http://www.boost.org/doc/libs/1_61_0/libs/spirit/doc/html/spirit/qi/reference/nonterminal/rule.html#spirit.qi.reference.nonterminal.rule.expression_semantics).
However, it seems you are really trying to use the same input twice (raw as fromFile
and "cooked" as name
?) it backfires, because the auto-rule attribute propagation also overwrites name
with the value you want for fromFile
.
The only quick way out, here, is to use SA only. I'd suggest making VariableHolder
's constructor responsible for details though.
Side note: it looks a bit as if the optional parentheses suggest an expression grammar. If so, make that explicit in the grammar, instead of hardcoding a special case in the rule for a
variableName
. If not, carry on :)
Here's an attempted fix:
Live On Coliru
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>
namespace qi = boost::spirit::qi;
namespace phoenix = boost::phoenix;
struct VariableHolder {
std::string name; // contains "varName"
std::string fromFile; // contains "$(varName)" or "$varName"
};
template <typename It, typename Skipper = qi::ascii::space_type> struct P : qi::grammar<It, VariableHolder(), Skipper> {
P() : P::base_type(start) {
auto _name = phoenix::bind(&VariableHolder::name, qi::_val);
auto _fromFile = phoenix::bind(&VariableHolder::fromFile, qi::_val);
variableName = qi::alpha >> +qi::alnum;
variable = '$' >> (variableName | '(' >> variableName >> ')');
start = qi::as_string [ qi::raw [
variable [ _name = qi::_1 ]]
] [ _fromFile = qi::_1 ];
BOOST_SPIRIT_DEBUG_NODES((start)(variable)(variableName))
}
private:
qi::rule<It, std::string(), Skipper> variable;
qi::rule<It, VariableHolder(), Skipper> start;
// lexemes
qi::rule<It, std::string()> variableName;
};
int main() {
using It = std::string::const_iterator;
P<It> const p;
for (std::string const input : {
"$foo1",
"$(bar2)"
})
{
It f = input.begin(), l = input.end();
VariableHolder data;
bool ok = qi::phrase_parse(f, l, p, qi::ascii::space, data);
if (ok) {
std::cout << "Parse success: " << data.name << " (source: '" << data.fromFile << "')\n";
} else {
std::cout << "Parse failure ('" << input << "')\n";
}
if (f != l) {
std::cout << "Remaining unparsed input: '" << std::string(f, l) << "'\n";
}
}
}
Prints
Parse success: foo1 (source: '$foo1')
Parse success: bar2 (source: '$(bar2)')
来源:https://stackoverflow.com/questions/38877702/boostspirit-parsing-into-structure-and-reusing-parts-of-it