C++ Logo


Advanced search

Re: Accessing data members in constructors' function try bocks

From: Frank B. Brokken <f.b.brokken_at_[hidden]>
Date: Wed, 18 Jan 2023 18:18:52 +0100
Dear Edward Catmur, you wrote:
> On Wed, 18 Jan 2023 at 16:02, Frank B. Brokken via Std-Discussion <

> > ... reaches the constructor's function-try-block handler.
> This seems to me a simple corollary of [class.cdtor], particularly
> paragraph 1. I'd even question whether normative wording is necessary under
> [except.handle]; this could probably be a note.

Hi Edward,

Thanks for the fast reply to my question.

11.9.5 [class.cdtor] states:

    ... For an object with a non-trivial destructor, referring to any
    non-static member or base class of the object after the destructor
    finishes execution results in undefined behavior.

However, that's not the situation I'm encountering. In the example an
exception is thrown from the constructor's body and so no destructor is
called. But before the exception is thrown its d_value has received a value,
which is then displayed in its function-try-block.

> > But why would referring to members of basic data types like 'int' be
> > undefined?
> If the data member is a member of a virtual base class, the vtable
> (speaking concretely) and the offsets it contains necessary to compute the
> address of that member may no longer be valid.

True, but that's also not the situation encountered in my example.

> There is an issue that [class.cdtor] is actually more restrictive here than
> [basic.life], meaning that operations that are allowed on an object fully
> outside its lifetime (principally, referring to a non-virtual base class)
> can no longer be performed once it is under construction or destruction.

But that, too, is not the situation in my Int class: there's no base class
either. It's just the plain Int class having a single int data member. So I'm
still struggling with the question why referring to its d_value data member
in the constructor's function-try-block implies undefined behavior

> I'm not sure if we've got around to opening a CWG issue there.

Well, maybe I posted my question to the wrong list, but since
https://isocpp.org/about/contact specifies that questions about the standard
should be posted to this list, and since my question is about an item in the
C++ standard and not so much about C++ itself I posted the question here. I
tried several compilers with my example, and so far all resulting programs
show the line 'value = 5'. But please advise if I should address the question
to 'StackOverflow C++'

Kind regards,

    Frank B. Brokken
    (+31) 6 5353 2509
    PGP Key Fingerprint: DF32 13DE B156 7732 E65E  3B4D 7DB2 A8BE EAE4 D8AA

Received on 2023-01-18 17:18:56