Re: [Boost-users] [qi][lambda] Get wrong reaction of the qi::rule if I use lambda for the action

Subject: Re: [Boost-users] [qi][lambda] Get wrong reaction of the qi::rule if I use lambda for the action
From: boost_mailinglist_at_[hidden]
Date: 2011-08-26 05:44:56

Am 24.08.2011 10:52, schrieb Thomas Heller:
> On Monday, August 15, 2011 01:11:04 PM boost_mailinglist_at_[hidden] wrote:
>> 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.
>
> Reformatting code to explain the effect ...
>
>> #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']
>                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> This will be evaluated eagerly and print "The good one " at rule construction
> time and return std::cout again ....
>                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> ... to be picked up the operator overloads of the lambda library to create a
> lazy function object (aka lambda)
>
>>            | char_("b")
>>              [std::cout<<  "The bad one "<<  boost::lambda::_1<<  '\n']
>
> Same here.
>
>>            >>  '}'
>>       );
>>
>>       return 0;
>> }
>
> P.S.: Use phoenix for maximum satisfaction. Same rules apply.
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>

Thank you for the clear explanation.