作为使用 Boost.Spirit 的更大语法的第一阶段，我试图解析“true”和“false”以产生相应的布尔值，true并且false.
我正在使用 Spirit.Lex 对输入进行标记，并对整数和浮点文字（包括以宽松的科学记数法表示的文字）、公开int和float属性有一个有效的实现。
令牌定义
#include <boost/spirit/include/lex_lexertl.hpp>
namespace lex = boost::spirit::lex;
typedef boost::mpl::vector<int, float, bool> token_value_type;
template <typename Lexer>
struct basic_literal_tokens : lex::lexer<Lexer> {
basic_literal_tokens() {
this->self.add_pattern("INT", "[-+]?[0-9]+");
int_literal = "{INT}";
// To be lexed as a float a numeric literal must have a decimal point
// or include an exponent, otherwise it will be considered an integer.
float_literal = "{INT}(((\\.[0-9]+)([eE]{INT})?)|([eE]{INT}))";
literal_true = "true";
literal_false = "false";
this->self = literal_true | literal_false | float_literal | int_literal;
}
lex::token_def<int> int_literal;
lex::token_def<float> float_literal;
lex::token_def<bool> literal_true, literal_false;
};
测试浮点文字的解析
我的实际实现使用 Boost.Test，但这是一个独立的示例。
#include <string>
#include <iostream>
#include <cmath>
#include <cstdlib>
#include <limits>
bool parse_and_check_float(std::string const & input, float expected) {
typedef std::string::const_iterator base_iterator_type;
typedef lex::lexertl::token<base_iterator_type,
token_value_type
> token_type;
typedef lex::lexertl::lexer<token_type> lexer_type;
basic_literal_tokens<lexer_type> basic_literal_lexer;
base_iterator_type input_iter(input.begin());
float actual;
bool result = lex::tokenize_and_parse(input_iter, input.end(),
basic_literal_lexer,
basic_literal_lexer.float_literal,
actual);
return result && std::abs(expected - actual) < std::numeric_limits<float>::epsilon();
}
int main(int argc, char *argv[]) {
if (parse_and_check_float("+31.4e-1", 3.14)) {
return EXIT_SUCCESS;
} else {
return EXIT_FAILURE;
}
}
解析“真”和“假”
我的问题是在尝试解析“true”和“false”时。这是我正在使用的测试代码（在删除 Boost.Test 部分之后）：
bool parse_and_check_bool(std::string const & input, bool expected) {
typedef std::string::const_iterator base_iterator_type;
typedef lex::lexertl::token<base_iterator_type,
token_value_type
> token_type;
typedef lex::lexertl::lexer<token_type> lexer_type;
basic_literal_tokens<lexer_type> basic_literal_lexer;
base_iterator_type input_iter(input.begin());
bool actual;
lex::token_def<bool> parser = expected ? basic_literal_lexer.literal_true
: basic_literal_lexer.literal_false;
bool result = lex::tokenize_and_parse(input_iter, input.end(),
basic_literal_lexer, parser,
actual);
return result && actual == expected;
}
但编译失败：
boost/spirit/home/qi/detail/assign_to.hpp: In function ‘void boost::spirit::traits::assign_to(const Iterator&, const Iterator&, Attribute&) [with Iterator = __gnu_cxx::__normal_iterator<const char*, std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, Attribute = bool]’:
boost/spirit/home/lex/lexer/lexertl/token.hpp:434: instantiated from ‘static void boost::spirit::traits::assign_to_attribute_from_value<Attribute, boost::spirit::lex::lexertl::token<Iterator, AttributeTypes, HasState>, void>::call(const boost::spirit::lex::lexertl::token<Iterator, AttributeTypes, HasState>&, Attribute&) [with Attribute = bool, Iterator = __gnu_cxx::__normal_iterator<const char*, std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, AttributeTypes = boost::mpl::vector<int, float, bool, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, HasState = mpl_::bool_<true>]’
... backtrace of instantiation points ....
boost/spirit/home/qi/detail/assign_to.hpp:79: error: no matching function for call to ‘boost::spirit::traits::assign_to_attribute_from_iterators<bool, __gnu_cxx::__normal_iterator<const char*, std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, void>::call(const __gnu_cxx::__normal_iterator<const char*, std::basic_string<char, std::char_traits<char>, std::allocator<char> > >&, const __gnu_cxx::__normal_iterator<const char*, std::basic_string<char, std::char_traits<char>, std::allocator<char> > >&, bool&)’
boost/spirit/home/qi/detail/construct.hpp:64: note: candidates are: static void boost::spirit::traits::assign_to_attribute_from_iterators<bool, Iterator, void>::call(const Iterator&, const Iterator&, char&) [with Iterator = __gnu_cxx::__normal_iterator<const char*, std::basic_string<char, std::char_traits<char>, std::allocator<char> > >]
我对此的解释是 Spirit.Qi 不知道如何将字符串转换为布尔值——当然不是这样吗？以前有没有其他人这样做过？如果是这样，怎么做？