C / C++ / MFC

George_George wrote:

The reason why we can not write an exception safe copy assignment for class Widget, is because (suppose we assign t1_, then assign t2_) if T1::operator=() does not throw exception, and T2::operator=() throws exception, then since all operations on T1 may throw, it means we can not find any operations on T1 which could rollback the original status to T1 without exception thrown (if exception is thrown during rollback, means rollback is not successful, and T1 will be in an in-consistent status which is different from original status -- which violates the strong exception safe guideline).

CPallini wrote:

it maybe also impossible to roll back because T1 class public interface simply doesn't allow it.

George_George wrote:

To rollback, we can simply make assignment again.

<br />
try<br />
{<br />
temp = this->tx;<br />
this->tx = rhs.tx;<br />
}<br />
catch (...)<br />
{<br />
  this->tx = temp; // rolback  <br />
  throw;<br />
}<br />

George_George wrote:

the statement "cannot in general roll back the change already made to t1_", why can not?

George_George wrote:

If there is exception thrown in assignment operator

George_George wrote:

assignment is enough.

George_George wrote:

Why "probably", CPallini?

George_George wrote:

I quote what you said, and you should be sure about what you mean.

led mike wrote:

At least I'm pretty sure it isn't on the plant I come from

led mike wrote:

Have you read Meyers?

led mike wrote:

is rolling back an inherent feature of an assignment operator

CPallini wrote:

You're interpretation is correct, but is not the whole scenario: it maybe also impossible to roll back becauseT1 class public interface simply doesn't allow it.

led mike wrote:

Ok and rolling back would be equal to inconsistent, not a solution to it.