I am writing a parser for a fairly complicated language in C++. The Parser class is given a list of tokens and it builds the AST. Though only a part of the parser is completed, the Parser.cpp file is already more than 1.5k lines and the class has around 25 functions. So, I plan to break the large Parser class into smaller classes such that I can have separate classes for parsing different language constructs. For example, I wish to have ExprParser class that parses expressions, a TypeParser class that parses types. It seems to be much cleaner. The problem is that the parsing functions must have access to a state that includes the position of the current token, and several parsing helper functions. In C#, it is possible to implement related functions in different classes using partial classes. Is there any specific design pattern or recommended way for this? 
Breaking large class into smaller classes when they need a common state?
design patterns;object oriented
Create a Scanner or Tokenizer class, which takes the input data (the text to be parsed) and holds the position of the current token or similar state. It can also provide some shared helper functions. Then provide a reference (or a shared pointer) to the Scanner object to all your individual xyzParser objects, so they can all access the same scanner. The scanner will be only responsible for accessing the data by basic tokenize functions, the individual parsers will be responsible for the actual parsing logic.This will work most easily as long as your scanner does not need to know which individual parsers exists. If the scanner actually needs to know this, you might consider to resolve the cyclic dependency by introducing abstract interface base classes, or by implementing some kind of call back or event mechanism, where the scanner can notify any kind of observers.