Re: Why it is not good code for constructor

From:

"Bo Persson" <bop@gmb.dk>

Newsgroups:

microsoft.public.vc.language

Date:

Thu, 27 Dec 2007 17:55:15 +0100

Message-ID:

<5ti3n4F1d7verU1@mid.individual.net>

George wrote:
:: Thanks Alexander,
::
::
:: How about the case that we succeed in new[] and enters in B's
:: constructor and some other code in B's constructor causes the
:: exception in B's constructor? :-)
::
:: I think in this situation, B's constructor should catch the
:: exception, delete[] the memory and re-throw the exception. Do you
:: think it is a good approach?

No!

The right question to ask is WHY in the world is C creating an array,
and passing it to B?!

If B needs a buffer, it could create it all by itself. And be
responsible for it!

Bo Persson

::
::
:: regards,
:: George
::
:: "Alexander Grigoriev" wrote:
::
::: Are you sure argument of B() is evaluated no immediately before
::: B() is called? If, suppose, the base classes are constructed in
::: B, A order, then if A() throws, ~B() will be called.
:::
::: "Ben Voigt [C++ MVP]" <rbv@nospam.nospam> wrote in message
::: news:%23K3hjF9RIHA.5524@TK2MSFTNGP05.phx.gbl...
::::
:::: "George" <George@discussions.microsoft.com> wrote in message
:::: news:75F1E950-242F-46F3-9500-5BAA7FAC2AB6@microsoft.com...
::::: Hello everyone,
:::::
:::::
::::: Here is a sample from Dr. Dobb C++. In the analysis, the code
::::: is bad below.
:::::
::::: But I do not think the code is bad,
:::::
::::: 1. if bad_alloc is thrown in new int[], we just catch it and
::::: write some log;
::::: 2. if there are any exception in B's constructor, we will also
::::: be in catch
::::: block and we could also write some log.
:::::
::::: Why it is bad code? Any comments?
:::::
::::: (I do not agree that there is resource leak, since if we met
::::: with bad_alloc
::::: in new int[], there is no memory allocated at all, so no root of
::::: memory/resource leak).
::::
:::: The code you provided isn't sufficient to guarantee that.
::::
:::: What if:
::::
:::: class C : public B, public A
:::: {
:::: ...
:::: };
::::
:::: now if new int[n] completes ok, but the constructor for A
:::: throws, the array is leaked with no chance to free it.
::::
:::::
:::::
::::: http://www.ddj.com/cpp/184401297
:::::
::::: [Code]
::::: C::C(int)
::::: try
::::: : B(new int[n]) // horrible!
::::: {
::::: ...
::::: }
::::: catch(Error &e)
::::: {
:::::
::::: }
::::: [/Code]
:::::
:::::
:::::
::::: thanks in advance,
::::: George

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