c – 我无法获取令牌的字符串值

概述我尝试用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 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;}

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