Re: Guarantee of side-effect free assignment
On Oct 7, 8:00 pm, jkherci...@gmx.net wrote:
[...]
The argument runs as follows: What we have is a composite expression
p = new S()
and the left hand side is a new expression. The "assignment", i.e., the
change of the lvalue p is a side-effect of the assignment expression. The
construction of the object is a side-effect of the new expression. The
order of the two side-effects in the evaluation of the composite expression
is not specified because there is no sequence point that separates them.
That is what grants the compiler the freedom to rewrite the expression as
p = operator new( sizeof( S ) );
new( p ) S();
Exactly. You've managed to say what I've been trying to say
more exactly, and in a lot less words. The only real counter
argument I can think of is to say that the "value" of a new
expression is a pointer to a fully constructed object; that the
value doesn't exist until the constructor terminates. IMHO,
that's really stretching it.
[...]
This reasoning assumes that the result of an expression is not
known (or or does not exist) until the side-effects of
evaluating the expression have taken place. I think that
guarantee is not made anywhere in the standard.
Historically, the interpretation has always been that this is
*not* the case, although in most of the actual cases I can think
of, the standard goes out of its way to ensure that any code
which can actually tell involves undefined behavior.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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