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From: =?ISO-8859-1?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>
Subject: Re: noexcept-specification vs returning a prvalue
Organization: A noiseless patient Spider
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Xref: x330-a1.tempe.blueboxinc.net comp.std.c++:127


On 2011-04-23 18:21, Nikolay Ivchenkov wrote:
>
> Consider the following example:
>
>      #include<iostream>
>
>      struct C
>      {
>          C();
>          C(C const&) noexcept(false);
>      };
>
>      C f() noexcept(true);
>
>      int main()
>      {
>          std::cout<<  (int)noexcept(f());
>      }
>
> Here f is declared with non-throwing exception-specification, though
> copy constructor of its return type does not have non-throwing
> exception-specification. What is the value of expression noexcept(f())?

I think the FDIS says that noexcept(f()) shall evaluate to "true" in
this example. My interpretation is based on the way how I read the
meaning of "A function is said to allow" in [except.spec] p. 8
(complemented by p. 12) to refer to exceptions that are possibly
invoked by the function implementation including its return statement.

The return statement includes the construction and possible move/copy
of the return value of f() as explained in [stmt.return] p. 2:

"A return statement can involve the construction and copy or move of a
temporary object (12.2)."

so the tagging of f with noexcept(true) means that we are sure that
the function call including the construction and possible copy [move
is not possible here] of the return value) does no throw an exception.
It seems to me that this does not include the destruction of the
temporary, so extending the definition of C like

      struct C
      {
          C();
          C(C const&) noexcept(false);
          ~C noexcept(false);
      };

should change the value of noexcept(f()) to false, because the
noexcept operator also considers the destruction of the temporary of
the complete function call expression.

It could be helpful, if [except.spec] p. 8 and/or p. 12 would be
clearer in this regard.

HTH & Greetings from Bremen,

Daniel Krügler




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