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Re: noexcept-specification vs returning a prvalue

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(second submission attempt after 48h).

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())?
>

The value is "true", and calling "f" results in a call to std::terminate if
the copy consructor of C actually throws.

I would say the author of "f" needs to make sure its return type either is
nothrow-copyable, or that all constructions it does by returning will not
throw.

In addition, this is not tied to the copy constructor, I think. The
following would be fine if "f"'s author knows that C's default construtor
would no throw, too:

 C f() noexcept(true) { return {}; /* note: no copy! */ }

Note that any implicit call to the copy constructor of "C" for a return
statement has to occur in the context of the caller. So to the noexcept
operator, it does not make a difference whether or not the copy constructor
has a throwing exception spec.

If C has a potentially throwing dtor, that makes a difference though,
because the dtor is invoked in the context of the caller (the noexcept test-
expression).


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noexcept-specification vs returning a prvalue  Nikolay Ivchenkov <tsoae@mail.ru> - 2011-04-23 10:21 -0600
  Re: noexcept-specification vs returning a prvalue  Daniel Krügler <daniel.kruegler@googlemail.com> - 2011-04-26 11:05 -0600
    Re: noexcept-specification vs returning a prvalue Johannes Schaub<schaub.johannes@googlemail.com> - 2011-04-27 10:51 -0600
  Re: noexcept-specification vs returning a prvalue  Johannes Schaub <schaub.johannes@googlemail.com> - 2011-04-26 11:04 -0600

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