Re: Why it is not good code for constructor

From:

"Ben Voigt [C++ MVP]" <rbv@nospam.nospam>

Newsgroups:

microsoft.public.vc.language

Date:

Wed, 26 Dec 2007 14:12:48 -0600

Message-ID:

<OXkm1t$RIHA.4740@TK2MSFTNGP02.phx.gbl>

"Alexander Grigoriev" <alegr@earthlink.net> wrote in message
news:OgcmXl%23RIHA.5976@TK2MSFTNGP05.phx.gbl...

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.

Since the memory was allocated in C::C, it is most likely C::~C that is
responsible for freeing it, but C::~C will not be called, and a function-try
handler cannot access the pointer, while a catch handler in local scope will
not be executed.

If B::~B is freeing the pointer, then that would be correct usage of RAII
and safe. But the example being discussed makes it clear this isn't the
case, C is responsible for freeing the memory.

"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