我尝试用Boost Spirit实现一种使用一种编程语言的Lexer.
我得到一个令牌的值,我得到一个bad_get异常：
terminate called after throwing an instance of ‘boost::bad_get’
what(): boost::bad_get: Failed value get using boost::get Aborted
在这样做时我得到这个例外：
std::string contents = "void"; base_iterator_type first = contents.begin(); base_iterator_type last = contents.end(); SimpleLexer<lexer_type> lexer; iter = lexer.begin(first,last); end = lexer.end(); std::cout << "Value = " << boost::get<std::string>(iter->value()) << std::endl;
我的词法分类是这样定义的：
typedef std::string::iterator base_iterator_type;
typedef boost::spirit::lex::lexertl::token<base_iterator_type,boost::mpl::vector<unsigned int,std::string>> Tok;
typedef lex::lexertl::actor_lexer<Tok> lexer_type;
template<typename L>
class SimpleLexer : public lex::lexer<L> {
private:
public:
SimpleLexer() {
keyword_for = "for";
keyword_while = "while";
keyword_if = "if";
keyword_else = "else";
keyword_false = "false";
keyword_true = "true";
keyword_from = "from";
keyword_to = "to";
keyword_foreach = "foreach";
word = "[a-zA-Z]+";
integer = "[0-9]+";
litteral = "...";
left_parenth = '(';
right_parenth = ')';
left_brace = '{';
right_brace = '}';
stop = ';';
comma = ',';
swap = "<>";
assign = '=';
addition = '+';
subtraction = '-';
multiplication = '*';
division = '/';
modulo = '%';
equals = "==";
not_equals = "!=";
greater = '>';
less = '<';
greater_equals = ">=";
less_equals = "<=";
whitespaces = "[ \\t\\n]+";
comments = "\\/\\*[^*]*\\*+([^/*][^*]*\\*+)*\\/";
//Add keywords
this->self += keyword_for | keyword_while | keyword_true | keyword_false | keyword_if | keyword_else | keyword_from | keyword_to | keyword_foreach;
this->self += integer | litteral | word;
this->self += equals | not_equals | greater_equals | less_equals | greater | less ;
this->self += left_parenth | right_parenth | left_brace | right_brace;
this->self += comma | stop;
this->self += assign | swap | addition | subtraction | multiplication | division | modulo;
//Ignore whitespaces and comments
this->self += whitespaces [lex::_pass = lex::pass_flags::pass_ignore];
this->self += comments [lex::_pass = lex::pass_flags::pass_ignore];
}
lex::token_def<std::string> word,litteral,integer;
lex::token_def<lex::omit> left_parenth,right_parenth,left_brace,right_brace;
lex::token_def<lex::omit> stop,comma;
lex::token_def<lex::omit> assign,swap,addition,subtraction,multiplication,division,modulo;
lex::token_def<lex::omit> equals,not_equals,greater,less,greater_equals,less_equals;
//Keywords
lex::token_def<lex::omit> keyword_if,keyword_else,keyword_for,keyword_while,keyword_from,keyword_to,keyword_foreach;
lex::token_def<lex::omit> keyword_true,keyword_false;
//Ignored tokens
lex::token_def<lex::omit> whitespaces;
lex::token_def<lex::omit> comments;
};
有没有办法获得令牌的价值？
解决方法
您可以随时使用“默认”令牌数据(这是源迭代器类型的iterator_range).
std::string tokenvalue(iter->value().begin(),iter->value().end());
在boost库中研究了测试用例之后,我发现了一些事情：
>这是设计
>有一个更简单的方法
>在Lex语义动作(例如使用_1)和在Qi中使用词法分析器令牌时,更容易的方式自动化;该赋值将自动转换为Qi属性类型
>这个(确实)得到了文档中提到的“懒惰,一次性,评估”的语义
令牌是令牌数据是变体,它作为原始输入迭代器范围开始.只有在“a”强制分配之后,转换的属性才会被缓存在变体中.你可以看到过渡：
lexer_type::iterator_type iter = lexer.begin(first,last); lexer_type::iterator_type end = lexer.end(); assert(0 == iter->value().which()); std::cout << "Value = " << boost::get<boost::iterator_range<base_iterator_type> >(iter->value()) << std::endl; std::string s; boost::spirit::traits::assign_to(*iter,s); assert(1 == iter->value().which()); std::cout << "Value = " << s << std::endl;
如您所见,属性分配在这里被强制直接使用assign_to trait实现.
全程演示：
#include <boost/spirit/include/lex_lexertl.hpp>
#include <iostream>
#include <string>
namespace lex = boost::spirit::lex;
typedef std::string::iterator base_iterator_type;
typedef boost::spirit::lex::lexertl::token<base_iterator_type,boost::mpl::vector<int,std::string>> Tok;
typedef lex::lexertl::actor_lexer<Tok> lexer_type;
template<typename L>
class SimpleLexer : public lex::lexer<L> {
private:
public:
SimpleLexer() {
word = "[a-zA-Z]+";
integer = "[0-9]+";
literal = "...";
this->self += integer | literal | word;
}
lex::token_def<std::string> word,literal;
lex::token_def<int> integer;
};
int main(int argc,const char* argv[]) {
SimpleLexer<lexer_type> lexer;
std::string contents = "void";
base_iterator_type first = contents.begin();
base_iterator_type last = contents.end();
lexer_type::iterator_type iter = lexer.begin(first,last);
lexer_type::iterator_type end = lexer.end();
assert(0 == iter->value().which());
std::cout << "Value = " << boost::get<boost::iterator_range<base_iterator_type> >(iter->value()) << std::endl;
std::string s;
boost::spirit::traits::assign_to(*iter,s);
assert(2 == iter->value().which());
std::cout << "Value = " << s << std::endl;
return 0;
}