Re: Guarantee of side-effect free assignment

From:

jdennett@acm.org (James Dennett)

Newsgroups:

comp.std.c++

Date:

Sun, 7 Oct 2007 00:21:28 GMT

Message-ID:

<pAVNi.205750$zz2.13007@newsfe12.phx>

Alf P. Steinbach wrote:

From discussions in [comp.lang.c++] and [comp.lang.c++.moderated], as
well as articles on the net about concurrency in C++, I'm reasonably
sure that given

  #include <iostream>
  #include <ostream>

  struct S { S(){ throw 123; } int foo(){ return 666; } };

  int main()
  {
      S* p = 0;

      try
      {
          p = new S();
      }
      catch( ... )
      {}

      if( p ) { std::cout << p->foo() << std::endl; }
  }

there is no guarantee that this code will not end up in a call to
p->foo() with an invalid pointer p, i.e., that might well happen.

Surely that couldn't have been the committee's intention?

I wouldn't imagine so.

Why isn't assignment treated as a function call?

It doesn't need to be. The assignment cannot occur until the
new value is known, which means that the "new" operator
has returned its result, which means that the object has been
constructed. If the constructor throws, there's no value
from "new" above, and the assignment cannot occur; p will
remain null.

-- James

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