c++ compiling error with terminal

I wrote a piece of code with xcode 7.2 on a mac which compiles and works fine. However when i try to compile from terminal with command g++ -std=c++11 -o main.cpp an error occurs undefined symbols for architecture x86_64 functionname(std::_1::basic_istream<char,std::_1::char_traits<char> >&)"

where functionname is a function I created that has a file stream as a parameter.I am new to these kinds of compiling errors, so help would be much appreciated.