Re: newbie here ask about qi::_val and semantic action

 Thanks! I made a mistake about the placeholder,  so the return value in
the following:


double print_action(double f)
{
    return f;  // this value will be assigned to the rule r's attribute d?
}

rule<std::string::const_iterator,  double() > r =
qi::double_[&print_action];

will be assigned to the rule  r's attribute d?


On Mon, Jul 9, 2018 at 3:03 PM, Henri Menke <[email protected]> wrote:

> On 09/07/18 17:38, Yi Leong wrote:
>
>> yeah, in the qi::double_[qi::_val = qi::_1] , the semantic action is a
>> lambda expression, but as the following:
>>
>> #include <iostream>
>> #include <string>
>>
>> #include <boost/spirit/include/phoenix.hpp>
>> #include <boost/spirit/include/qi.hpp>
>>
>> void print_action(double f)
>> {
>>    qi::_val = qi::_1; // it does not work....
>> }
>>
>>
> print_action already has a callable type, so you can use it in the
> semantic action without placeholders.
>
> The placeholders are useful if you have a oneline semantic action for
> which you don't want to write an extra callable.  See the docs for details:
> https://www.boost.org/libs/spirit/doc/html/spirit/qi/tutoria
> ls/semantic_actions.html
>
> #include <iostream>
> #include <string>
>
> #include <boost/spirit/include/phoenix.hpp>
> #include <boost/spirit/include/qi.hpp>
>
> double print_action(double f)
> {
>     std::cout << "Attribute value: " << f << "\n";
>     return f;
> }
>
>
> int main() {
>     std::string s = "1234.56789";
>     double d;
>
>     std::string::iterator first = s.begin();
>     std::string::iterator last = s.end();
>
>     namespace qi = boost::spirit::qi;
>     bool r = qi::phrase_parse(first, last, qi::double_ [&print_action],
> qi::space, d);
>
>     if (!r || first != last) {
>         std::cerr << "Error while parsing.  Rest: " << std::string(first,
> last) << "\n";
>     }
>     std::cout << "Success! " << d << "\n";
> }
>
> int main() {
>>      std::string s = "1234.56789";
>>      double d;
>>
>>      std::string::iterator first = s.begin();
>>      std::string::iterator last = s.end();
>>
>>      namespace qi = boost::spirit::qi;
>>      bool r = qi::phrase_parse(first, last, qi::double_ [&print_action],
>> qi::space, d);
>>
>>      if (!r || first != last) {
>>          std::cerr << "Error while parsing.  Rest: " <<
>> std::string(first, last) << "\n";
>>      }
>>      std::cout << "Success! " << d << "\n";
>>
>> }
>>
>> The above code gets compiling error, so  is the placeholder qi::_val
>> usable only in the lambda expression, right?
>>
>> On Mon, Jul 9, 2018 at 10:06 AM, Henri Menke <[email protected]
>> <mailto:[email protected]>> wrote:
>>
>>     Elaborate.  The following works.
>>
>>     #include <iostream>
>>     #include <string>
>>
>>     #include <boost/spirit/include/phoenix.hpp>
>>     #include <boost/spirit/include/qi.hpp>
>>
>>     int main() {
>>          std::string s = "1234.56789";
>>          double d;
>>
>>          std::string::iterator first = s.begin();
>>          std::string::iterator last = s.end();
>>
>>          namespace qi = boost::spirit::qi;
>>          bool r = qi::phrase_parse(first, last, qi::double_ [qi::_val =
>>     qi::_1], qi::space, d);
>>
>>          if (!r || first != last) {
>>              std::cerr << "Error while parsing.  Rest: " <<
>>     std::string(first, last) << "\n";
>>          }
>>          std::cout << "Success! " << d << "\n";
>>
>>     }
>>
>>
>>     On 09/07/18 03:40, Yi Leong wrote:
>>
>>         Hi,
>>         I set qi::_val in semantic action function , but it does not work.
>>         Is the placeholder _val only usable in lambda expression?
>>
>>
>>
>>
>>
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