Class destructors - clarify what is safe

[Brother Bill]

On page 325 of Programming in D, we may have a single 
parameterless destructor named ```~this()```.

A class may have members of string, File, other classes, etc.

When entering a class destructor, are any of the dynamically 
allocated class members safe to read?  That is, can we expect 
that a class instance entering its destructor is "fully intact", 
satisfying all its invariants, and all member variables have not 
undergone any destruction at this point?

I would expect that a class with strings, FILE handles, etc. 
would need these to do its housekeeping.  If we opened a file in 
the constructor, we should be able to close the file in the 
destructor.

As we can explicitly call destroy() on a class instance multiple 
times, do we need to be "careful" of only closing a file once, 
rather than closing a file multiple times, if that can cause 
issues.

What guarantees, if any, exist on entering a class destructor?