On Fri, Mar 21, 2008 at 10:16 PM, Steven Watanabe wrote:

AMDG

Robert Dailey wrote:

...

Hi,

I have defined a global overloaded operator<<. I want to use this overloaded operator in std::for_each(), but it must be properly deduced since there are many global overloads for this operator. For example: <snip>

What am I doing wrong? Do I need to be using boost bind instead? If so, how can I use boost.bind to map a global operator? Will it correctly deduce which overload to use?

Unfortunately, Boost.Lambda does not handle user defined operators automatically. If you don't do this Lambda will try to return a Stream by value which apparently triggers SFINAE when Stream is abstract.

#include <iostream> #include #include <vector> #include <algorithm>

class Stream { public: virtual void abstract() = 0; // This is an abstract class (cannot instantiate) void write(char ch) { std::cout << ch; } };

class MyStream : public Stream { void abstract() {} };

Stream& operator<< ( Stream& stream, char data ) { stream.write( data ); return stream; }

namespace boost { namespace lambda {

template<> struct plain_return_type_2, Stream, char> { typedef Stream& type; };

} }

Stream& operator<< ( Stream& stream, std::vector<char>& buffer ) { using namespace boost::lambda; std::for_each( buffer.begin(), buffer.end(), var( stream ) << _1 ); return stream; }

int main() { MyStream s; std::string str("test1"); std::vector<char> b(str.begin(), str.end()); s << b; }

See http://www.boost.org/doc/html/lambda/extending.html

Will the plain_return_type_2 specialization also work for classes derived from Stream? Or do I need to specialize that structure for each and every single class, regardless of inheritance? Thanks.

On Fri, Mar 21, 2008 at 10:52 PM, Steven Watanabe wrote:

AMDG

Robert Dailey wrote:

...

Will the plain_return_type_2 specialization also work for classes derived from Stream? Or do I need to specialize that structure for each and every single class, regardless of inheritance? Thanks.

Yes, you would need to specialize for every derived class. The other alternative is to use an explicit return type.

http://www.boost.org/doc/html/lambda/le_in_details.html#lambda.overriding_de...

How about Boost.Bind? Could I use it to bind my operator? I would also expect it to choose the correct overload of the stream operator depending on the type of _1. I have not used Bind like this before, so I'm not sure if it is capable. If all else fails you've given me 2 alternatives concerning Boost.Lambda, so I could always go with that. Thanks for all the help.