Calling operator. on placeholder inside Lambda expression?

This is a discussion on Calling operator. on placeholder inside Lambda expression? within the C++ Programming forums, part of the General Programming Boards category; Hi, after searching the web I think something like this is not be possible because operator. can't be overloaded in ...

  1. #1
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    Calling operator. on placeholder inside Lambda expression?

    Hi,

    after searching the web I think something like this is not be possible because operator. can't be overloaded in c++:

    Code:
    #include <map>
    #include <ostream>
    #include <algorithm>
    #include <boost/lambda/lambda.hpp>
    
    typedef std::map<int, dem::EntityViewer> AgentDatabase;
    
    std::ostream&
    operator<<(std::ostream& os, const AgentDatabase& adb)
    {
    	std::for_each(adb.begin(), adb.end(), os << boost::lambda::_1.first << " ");
    	return os;
    }
    But maybe someone knows a nice workaround preventing me from writing a custom fuctor, which would force me to scatter code which belongs together.

    Thank you in advance!

  2. #2
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    I got it! I'had to use lambda::bind.

    Code:
    #include <boost/lambda/lambda.hpp>
    #include <boost/lambda/bind.hpp>
    
    using boost::lambda::bind;
    using boost::lambda::_1;
    using boost::lambda::constant;
    
    typedef std::map<int, std::string> MapType;
    
    std::ostream&
    operator<<(std::ostream& os, const MapType& map)
    {
    	std::for_each(map.begin(), map.end(), os
    		<< constant("key: ")
    	        << bind(&MapType::value_type::first, _1)
    		<< " value: " << bind(&MapType::value_type::second, _1)
    		<< "\n"
    //		<< std::endl
    		);
    	return os;
    }
    
    int main(int argc, char** argv)
    {	
    	MapType myMap;
    	myMap[0] = "nothing";
    	myMap[1] = "at least a bit";
    	myMap[500] = "a lot";
    	
    	std::cout << myMap << std::endl;
    }
    But can anybody tell me, why the compile fails, if I comment in the blue line?
    The error with g++ is:

    **** Build of configuration Debug for project ttest ****

    make all
    Building file: ../main.cpp
    Invoking: GCC C++ Compiler
    g++ -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"main.d" -MT"main.d" -o"main.o" "../main.cpp"
    ../main.cpp: In function ‘std::ostream& operator<<(std::ostream&, const MapType&)’:
    ../main.cpp:28: error: no match for ‘operator<<’ in ‘boost::lambda::operator<<(const boost::lambda::lambda_functor<T>&, const B&) [with Arg = boost::lambda::lambda_functor_base<boost::lambda:: bitwise_action<boost::lambda::leftshift_action>, boost::tuples::tuple<boost::lambda::lambda_functor <boost::lambda::lambda_functor_base<boost::lambda: :bitwise_action<boost::lambda::leftshift_action>, boost::tuples::tuple<boost::lambda::lambda_functor <boost::lambda::lambda_functor_base<boost::lambda: :bitwise_action<boost::lambda::leftshift_action>, boost::tuples::tuple<boost::lambda::lambda_functor <boost::lambda::lambda_functor_base<boost::lambda: :bitwise_action<boost::lambda::leftshift_action>, boost::tuples::tuple<std::basic_ostream<char, std::char_traits<char> >&, boost::lambda::lambda_functor<boost::lambda::ident ity<const char (&)[6]> >, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type> > >, boost::lambda::lambda_functor<boost::lambda::lambd a_functor_base<boost::lambda::action<2, boost::lambda::function_action<2, boost::lambda::detail::unspecified> >, boost::tuples::tuple<const int std::pair<const int, std::basic_string<char, std::char_traits<char>, std::allocator<char> > >::* const, const boost::lambda::lambda_functor<boost::lambda::place holder<1> >, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type> > >, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type> > >, const char (&)[9], boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type> > >, boost::lambda::lambda_functor<boost::lambda::lambd a_functor_base<boost::lambda::action<2, boost::lambda::function_action<2, boost::lambda::detail::unspecified> >, boost::tuples::tuple<std::basic_string<char, std::char_traits<char>, std::allocator<char> > std::pair<const int, std::basic_string<char, std::char_traits<char>, std::allocator<char> > >::* const, const boost::lambda::lambda_functor<boost::lambda::place holder<1> >, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type> > >, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type> >, B = char [2]](((const char (&)[2])"\012")) << std::endl’
    ../main.cpp:21: note: candidates are: std::ostream& operator<<(std::ostream&, const MapType&)
    make: *** [main.o] Error 1

  3. #3
    Officially An Architect brewbuck's Avatar
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    Quote Originally Posted by pheres View Post
    But can anybody tell me, why the compile fails, if I comment in the blue line?
    The error with g++ is:
    Don't you love compiler errors when there's complicated metaprogramming going on?

    I've run into this before. I think it's a bug in boost::lambda. Just use "\n" instead.

  4. #4
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    Quote Originally Posted by brewbuck View Post
    Don't you love compiler errors when there's complicated metaprogramming going on?
    It makes for a nice office game: The one who takes longest to read it loud without errors has to fix it.

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