Re: GCC's statement expression extension

[Mark Mitchell <mark at codesourcery dot com>]

>>>>> "Per" == Per Bothner <per@bothner.com> writes:

    Per> Mark Mitchell <mark@codesourcery.com> writes:

    >> statement expressions ... don't have documented semantics in
    >> C++ (for example, when does a temporary created in the
    >> statement expression go away?),

    Per> Intuitively, it seems to me there is only one rational
    Per> answer: when you exit the (block of) the statmenet
    Per> expression.

That seems like the obvious right answer.  It just doesn't work very
well for:

  X x;
  X f();

  x = ({ f(); });

Oops.  The return value of f() got destroyed before we got a chance to
copy it.  (There are more complex examples with similar problems,
including binding things to references, which cause lifetime
extensions in C++.)

I *am* a pretty experienced Scheme programmer.  (And ML, and other
functional languages.)  The problem is that the basic programming
model in C++ is different, and programmers think about things
differently.  Scheme is tremendously useful as a foundational
programming language analysis tool; every course I've taken or taught
on programming language theory features Scheme, and many use it as a
foundation onto which other programming languages are mapped.  But,
it's not always enough to simply say "if this were Scheme, what would
it mean?".

--
Mark Mitchell                   mark@codesourcery.com
CodeSourcery, LLC               http://www.codesourcery.com