Calling operator. on placeholder inside Lambda expression?

[pheres]

Hi,

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

#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!

[pheres]

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

#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

[brewbuck]

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.

[pheres]

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.