Re: C++ Frequently Questioned Answers

"Andrei Alexandrescu (See Website For Email)" <>
Wed, 14 Nov 2007 16:48:41 CST
Jerry Coffin wrote:

In article <>,
[ ... ]

There's plenty of things that can be done, and lex/yacc are only the

I have no argument with that -- I just pointed to them as the "most
obvious" examples, not (by any means) the only ones.

* Perl's black hole and white hole classes

It's late, so maybe I'm just tired and easily confused. While these
obvious applicability in dynamically typed languages, I'm not sure how
you apply them in a statically-typed language like C++.
For any others looking on: with a dynamically typed language, you
specify the type of an object, so the compiler can't tell whether the
object will really implement the method you use. C++, however, is
statically typed, so the compiler always knows what type an object is,
and if you try to call a method that doesn't exist in that type, the
code doesn't compile.
Of course, you can work around that with something like a
reinterpret_cast. Most of us would just say: "don't do that", but if
want more, tagged types would seem like the obvious way to go. With
in place, either of the behaviors above becomes trivial.

This is a confusion. Black and white holes are useful in statically-
typed languages.


struct Employee
  virtual string name() = 0;
  virtual string ssn() = 0;

Then BlackHole<Employee> generates:

struct BlackHole<Employee> : Employee
  virtual string name() { return string(); }
  virtual string ssn() { return string(); }

And WhiteHole<Employee> generates:

struct WhiteHole<Employee> : Employee
  virtual string name() { throw runtime_error(typeid(*this).name() +
    string(" does not implement name()"); }
  virtual string ssn() { throw runtime_error(typeid(*this).name() +
    string(" does not implement ssn()"); }


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