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

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/tutorials/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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