The tag parser is used in Visual Studio 2010 to populate the SQL database that supersedes the NCB file. All of the browsing features of VC++ rely in some way on results provided by the tag parser. These include Class View, Call Hierarchy, Go To Definition/Declaration, Get All References, Quick Search, the Navigation Bar and VCCodeModel.

A Fuzzy Parser
It is a fuzzy parser, which means that instead of trying to strictly recognize and validate the full C++ syntax (we have an excellent compiler front-end to do that) it lazily matches an input stream of tokens with some patterns. This parser doesn’t populate a symbol table during parsing, it has no notion of types apart from built-in ones, it doesn’t build a full macro context and its unit of translation is a single file (i.e. it doesn’t follow through #include directives). But nevertheless, the parser is able to deal with all of C++, C++/CLI and IDL.

High level of tolerance to incomplete code and errors.
The tag parser doesn’t try to make sense of every symbol or identifier in the source code. It will be satisfied with being able to recognize the different parts of a declaration and their positions in the source file. If a name in the type specification of a declaration couldn’t get resolved by our C++ compiler this would not prevent the tag parser from recognizing the declaration and it will show up in Class View as in the example below.

The tag parser is somewhat analogous to a human reader of the source code that would just be looking at one unique declaration without knowing much about the rest of the project. He may not know what most identifiers actually represent but he can tell with a high level of confidence what the declaration is and locate its subparts.

In addition to the tolerance to ‘semantically’ incorrect code which is a property of fuzzy parsers, the tag parser has heuristic based error recovery for the most common causes of erroneous code during editing. For example, it will try to detect incomplete declarations or unclosed body of functions definitions as shown in the snapshot below...

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Courtesy:- blog.msdn.com