问题
Hi I am very new to boost spirit library. Can you please let me know why below code is not compile?
When I add "scientificNumber" rule to my grammar it dont compile. What can be the reason? I have added "scientificNumber" rule to make it possible to parse scientific notaion like "12E10". I dont know if this was the right way to do it.
namespace qi = boost::spirit::qi;
namespace phx = boost::phoenix;
typedef boost::function<double()> Value;
#define BINARY_FUNCTOR(name, op) \
struct name \
{ \
name(Value x, Value y): x_(x), y_(y) {} \
double operator()() { return x_() op y_(); } \
Value x_, y_; \
};
BINARY_FUNCTOR(ADD, +)
BINARY_FUNCTOR(SUB, -)
BINARY_FUNCTOR(MUL, *)
BINARY_FUNCTOR(DIV, /)
struct LIT
{
LIT(double x): x_(x) {}
double operator()() { return x_; }
double x_;
};
struct NEG
{
NEG(Value x): x_(x) {}
double operator()() { return -x_(); }
Value x_;
};
struct SQRT
{
SQRT(Value x): x_(x){}
double operator()() {return sqrt(x_()); }
Value x_;
};
struct SCIENTIFIC
{
SCIENTIFIC(std::wstring x): x_(x){}
double operator()() {return boost::lexical_cast<double>(x_); }
std::wstring x_;
};
// expression grammar definition
template <typename It, typename Skipper=boost::spirit::qi::space_type>
struct parser : boost::spirit::qi::grammar<It, Value(), Skipper>
{
parser() : parser::base_type(expression)
{
using namespace qi;
expression =
term [_val = _1]
>> *( ('+' >> term [_val = phx::construct<ADD>(_val, _1)])
| ('-' >> term [_val = phx::construct<SUB>(_val, _1)])
);
term =
factor [_val = _1]
>> *( ('*' >> factor [_val = phx::construct<MUL>(_val, _1)])
| ('/' >> factor [_val = phx::construct<DIV>(_val, _1)])
);
factor =
double_ [_val = phx::construct<LIT>(_1)]
| scientificNumber [_val = phx::construct<SCIENTIFIC>(_1)]
| '(' >> expression [_val = _1] >> ')'
| ('-' >> factor [_val = phx::construct<NEG>(_1)])
| ('+' >> factor [_val = _1])
| (string("SQRT") >> '(' >> expression [_val = phx::construct<SQRT>(_1)] >> ')');
scientificNumber = lexeme[+(boost::spirit::qi::digit) >> lit('E') >> lit('-') >> +(boost::spirit::qi::digit)];
BOOST_SPIRIT_DEBUG_NODE(expression);
BOOST_SPIRIT_DEBUG_NODE(term);
BOOST_SPIRIT_DEBUG_NODE(factor);
}
private:
boost::spirit::qi::rule<It, std::wstring , Skipper> scientificNumber;
qi::rule<It, Value(), Skipper> expression, term, factor;
};
int main()
{
std::wstring::const_iterator beginExpression(testExp.begin());
std::wstring::const_iterator endExpression(testExp.end());
typedef std::wstring::const_iterator It;
parser<It , boost::spirit::qi::space_type> expressionParser;
Value logicExpression;
phrase_parse(beginExpression,endExpression,expressionParser,boost::spirit::qi::space,logicExpression);
}
Can you please also let me know what is the boost::spirit::qi::grammar
回答1:
If you just search for the first line that contains error, you'll see the following comment:
// If you are seeing a compilation error here stating that the
// fourth parameter can't be converted to a required target type
// then you are probably trying to use a rule or a grammar with
// an incompatible skipper type.
if (f(first, last, context, skipper))
This is bingo: it tells you exactly what is wrong. It thinks std::wstring is the skipper.
qi::rule<It, std::wstring, Skipper> scientificNumber; // huh?
As per the documentation, you need to spell the attribute type as the return type of a function signature:
qi::rule<It, std::wstring(), Skipper> scientificNumber;
Now, it compiles and runs, see below, printing the output:
Success: true Value: 2.7e-09
IMPORTANT
However, using lexical_cast inside Boost Spirit to... parse a number (!?!) is a travesty. You could just have tested the original thing: it works with just the LIT (qi::double_) parser just as well. See http://ideone.com/mI1ESI
Again, see the documentation here: link
Full Code
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>
#include <boost/lexical_cast.hpp>
namespace qi = boost::spirit::qi;
namespace phx = boost::phoenix;
typedef boost::function<double()> Value;
#define BINARY_FUNCTOR(name, op) \
struct name \
{ \
name(Value x, Value y): x_(x), y_(y) {} \
double operator()() { return x_() op y_(); } \
Value x_, y_; \
};
BINARY_FUNCTOR(ADD, +)
BINARY_FUNCTOR(SUB, -)
BINARY_FUNCTOR(MUL, *)
BINARY_FUNCTOR(DIV, /)
struct LIT
{
LIT(double x): x_(x) {}
double operator()()
{
return x_;
}
double x_;
};
struct NEG
{
NEG(Value x): x_(x) {}
double operator()()
{
return -x_();
}
Value x_;
};
struct SQRT
{
SQRT(Value x): x_(x) {}
double operator()()
{
return sqrt(x_());
}
Value x_;
};
struct SCIENTIFIC
{
SCIENTIFIC(std::wstring x): x_(x) {}
double operator()()
{
return boost::lexical_cast<double>(x_);
}
std::wstring x_;
};
// expression grammar definition
template <typename It, typename Skipper=qi::space_type>
struct parser : qi::grammar<It, Value(), Skipper>
{
parser() : parser::base_type(expression)
{
using namespace qi;
expression =
term [_val = _1]
>> *(('+' >> term [_val = phx::construct<ADD>(_val, _1)])
| ('-' >> term [_val = phx::construct<SUB>(_val, _1)])
);
term =
factor [_val = _1]
>> *(('*' >> factor [_val = phx::construct<MUL>(_val, _1)])
| ('/' >> factor [_val = phx::construct<DIV>(_val, _1)])
);
factor =
double_ [_val = phx::construct<LIT>(_1)]
| scientificNumber [_val = phx::construct<SCIENTIFIC>(_1)]
| '(' >> expression [_val = _1] >> ')'
| ('-' >> factor [_val = phx::construct<NEG>(_1)])
| ('+' >> factor [_val = _1])
| (string("SQRT") >> '(' >> expression [_val = phx::construct<SQRT>(_1)] >> ')');
scientificNumber = lexeme[+(qi::digit) >> lit('E') >> lit('-') >> +(qi::digit)];
BOOST_SPIRIT_DEBUG_NODE(expression);
BOOST_SPIRIT_DEBUG_NODE(term);
BOOST_SPIRIT_DEBUG_NODE(factor);
}
private:
qi::rule<It, std::wstring(), Skipper> scientificNumber;
qi::rule<It, Value(), Skipper> expression, term, factor;
};
int main()
{
const std::wstring testExp = L"3E-10*(12-3)";
typedef std::wstring::const_iterator It;
It f(testExp.begin()), e(testExp.end());
parser<It, qi::space_type> expressionParser;
Value logicExpression;
bool ok = phrase_parse(f,e,expressionParser,qi::space,logicExpression);
std::cout << "Success: " << std::boolalpha << ok << "\tValue: " << logicExpression() << '\n';
}
回答2:
The compilation problem is right here:
boost::spirit::qi::rule<It, std::wstring , Skipper> scientificNumber;
It does not produce any attribute as you do not provide constructor. Here is the fix:
boost::spirit::qi::rule<It, std::wstring() , Skipper> scientificNumber;
As for the boost::spirit::qi::grammar , check this: http://www.boost.org/doc/libs/1_53_0/libs/spirit/classic/doc/grammar.html
And there are plenty of examples here:
https://stackoverflow.com/questions/tagged/boost-spirit
来源：https://stackoverflow.com/questions/16778399/boost-spirit-compile-issue