C++ correction

Hi /prog/

http://pastebin.com/meb2ac2d

Line 45, part of find_idea_id().

I want to give 'it' a default value, null pointer. It won't compile, can't understand how it should be typed differently.

Help me out :3

OP here again, another problem. I can't find any cause of error.

./Idea Registry/idea_registry.cc:87: error: default argument given for parameter 2 of ‘std::ostream& Idea_Registry::print_idea(const long unsigned int&, std::ostream&) const’
./Idea Registry/idea_registry.h:28: error: after previous specification in ‘std::ostream& Idea_Registry::print_idea(const long unsigned int&, std::ostream&) const’