Who's designing the error messages of g++? Anyways, here is a translation:
error: request for member ‘foo’ in ‘((Test*)this)->Test::_bar’,
which is of non-class type ‘C_bar*’
means: in class Test, you've written _bar.foo, but _bar is a pointer; you need to write _bar->foo, that is, change the dot to an arrow.
Happy C++ coding!