Error: '…' does not name a type

Most solutions to this problem talk about the headers. But the first reference below mentioned something that inspire me and solved my problem --  circular inclusion. 

There are many references about it online but I like this one:
http://stackoverflow.com/questions/625799/resolve-header-include-circular-dependencies

The way to think about this is to "think like a compiler".

Imagine you are writing a compiler. And you see code like this.

// file: A.h
class A {
  B _b;
};

// file: B.h
class B {
  A _a;
};

// file main.cc
#include "A.h"
#include "B.h"
int main(...) {
  A a;
}

When you are compiling the .cc file (remember that the .cc and not the .h is the unit of compilation), you need to allocate space for object A. So, well, how much space then? Enough to store B! What's the size of B then? Enough to store A! Oops.

......you can read on.

ref:
http://stackoverflow.com/questions/3961103/error-does-not-name-a-type (inspired by this)
http://stackoverflow.com/questions/2133250/does-not-name-a-type-error
http://stackoverflow.com/questions/8470822/error-x-does-not-name-a-type

http://stackoverflow.com/questions/625799/resolve-header-include-circular-dependencies