2011/8/15 <boost_mailinglist@gmx.de>
Hello,

if I execute the following example, I get the following output "The bad one The good one a" but I only expected "The good one a".

Why does it happens?  Boost 1.47.0 was used.


#include <boost/config/warning_disable.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/lambda/lambda.hpp>

#include <iostream>

int main()
{
   using boost::spirit::qi::char_;
   using boost::spirit::qi::parse;

   char const *first = "{a}", *last = first + std::strlen(first);

   parse(first, last, '{' >> char_("a")[std::cout << "The good one " << boost::lambda::_1 << '\n'] |
       char_("b")[std::cout << "The bad one " << boost::lambda::_1 << '\n'] >> '}');

   return 0;
}

Because  (std::cout << "The good one ") & (std::cout << "The bad one ") are not lambda expressions so they are executed immediately, and it seems Boost.Lambda just happened to work here.

The corrected version would be:

#include <boost/config/warning_disable.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>

#include <iostream>

int main()
{
   using boost::spirit::qi::char_;
   using boost::spirit::qi::parse;
   using boost::spirit::qi::_1;
   using boost::phoenix::val;

   char const *first = "{a}", *last = first + std::strlen(first);

   parse(first, last, '{' >> char_("a")[std::cout << phx::val("The good one ") << _1 << '\n'] |
       char_("b")[std::cout << phx::val("The bad one ") << _1 << '\n'] >> '}');

   return 0;
}


HTH