Re: Exception specifications unfortunate, and what about their future?

David Abrahams <>
Fri, 12 Dec 2008 18:53:51 CST
on Thu Dec 11 2008, DeMarcus <> wrote:

David Abrahams wrote:

on Thu Nov 20 2008, DeMarcus <> wrote:

The authors neglect this fact with following argument: "People often
suggest switching from dynamically checked exception specifications to
statically checked ones, as provided in Java and other languages. In
short, that just trades one set of problems for another; users of
languages with statically checked exception specifications seem to
equally often suggest switching to dynamically checked ones."

I don't blame the authors, don't take me wrong, but to me it seems like
the exception specifications need to be fixed when two of the most
renowned C++ programmers recommend us to avoid them.

Yes; they should be removed.

I think I agree. The best way to solve this issue may actually be, as
suggested earlier in this post, to deprecate exception specifications
and later reintroduce them in a way that will not affect code not using

I think statically checked exception specifications (the same way as in
Java) should be implemented because of three things.

1. Prefer *compile time* check before run-time.

Some things just don't need to be checked at compile-time. Can you
imagine having to declare every derived class that can come out of a
factory function?

  struct Base { virtual ~Base(); };
  std::auto_ptr<Base> factory() emits(Derived1, Derived2, ... DerivedN);
It amounts to the same thing for correctness, maintainability, coupling,
etc.: a nightmare.

To me exception specifications are used by the compiler as a consistency
check; a higher order of return values if you wish.

What do you mean by "to me...?" What compiler uses them that way?

A factory is not part of the language but rather a user defined design
pattern that may be arbitrary complex. Of course there must be a way to
disable exception specifications for such situations.

I don't think you understand me at all. Let me try again:

A language change that required you to change this legal C++03:

       Base* f();


       Base* f() emits(Derived1, Derived2, ... DerivedN);

   where the emits(...) clause enumerates all the possible dynamic types of
   the expression "*f()"

is very similar to a language change that required you to change this
legal C++03:

       void f();


       void f() throws(E1, E2, E3, .. EN);

   where the throws(...) clause enumerates all the possible dynamic types
   of the thrown exception.

Dave Abrahams
BoostPro Computing

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