C++ :: Calling A Function From A Header To Another Header?

I'm doing a compiler, and I'm using writing everything in headerfiles.

at the moment I have lexer.h, token.h and for this case I need reservedWords.h

reservedWords.h is made up of enums showing the needed words

in token.h I have a function called reservedWords reservedLookup(string str) which returns a reservedWord (enum)

now in lexer.h I have a function called Token* getNextToken() in which I need to make use of the function reservedLookup found in token.h

note that all of them are classes apart from reservedwords as in

class Lexer
{
public: .....
}
class token
{
public .....
}

how I can call that function?

I tried declaring reservedWords reservedLookup (string str) BUT obviously it's directing me as Lexer::reservedLookup and not as Token::reservedLookup

When I tried using Token::reservedLookup it gave me

E:Universitycompilerslexer.h|65|error: cannot declare member function 'Token::reservedLookup' within 'Lexer'|

N.B I use namespace std, that's why I didnot write std::string str