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The generated parser expects yylex to have the following prototype.
value_type* yylval, location_type* yylloc, type1 arg1, …) ¶value_type* yylval, type1 arg1, …) ¶Return the next token. Its kind is the return value, its semantic value and location (if enabled) being yylval and yylloc. Invocations of ‘%lex-param {type1 arg1}’ yield additional arguments.
Note that when using variants, the interface for yylex is the same,
but yylval is handled differently.
Regular union-based code in Lex scanner typically looks like:
[0-9]+ {
yylval->ival = text_to_int (yytext);
return yy::parser::token::INTEGER;
}
[a-z]+ {
yylval->sval = new std::string (yytext);
return yy::parser::token::IDENTIFIER;
}
Using variants, yylval is already constructed, but it is not
initialized. So the code would look like:
[0-9]+ {
yylval->emplace<int> () = text_to_int (yytext);
return yy::parser::token::INTEGER;
}
[a-z]+ {
yylval->emplace<std::string> () = yytext;
return yy::parser::token::IDENTIFIER;
}
or
[0-9]+ {
yylval->emplace (text_to_int (yytext));
return yy::parser::token::INTEGER;
}
[a-z]+ {
yylval->emplace (yytext);
return yy::parser::token::IDENTIFIER;
}