C/C++ :: Defining Member Function In Different CPP File

I am work on building a simple parse tree and the layout of my code look like this:

Headers
pt_node.hiterator.hparsetree.h

Source files
node.cppparsetree.cppmain.cpp

I am still relatively new to C++ , and have been advised to include function definition for the member function of both pt_node class and iterator class in the node.cpp file

I particular I have declare the following iterator.h:

inline bool operator ==(const tree_iterator& rhs);

which is defined in node.cpp as:

inline bool tree_iterator::operator==(const tree_iterator& rhs) {
return (node ==rhs.node);
}

However on building I receive the following error:

undefined reference to `dnkmat001::tree_iterator::operator==(dnkmat001::tree_iterator const&)'

Why is this occurring and what measure can I take to fix my code