cout << i++ << i--
is semantically equivalent to
operator<<(operator<<(cout, i++), i--);
<------arg1--------->, <-arg2->
$1.9/15- “When calling a function
(whether or not the function is
inline), every value computation and
side effect associated with any
argument expression, or with the
postfix expression designating the
called function, is sequenced before
execution of every expression or
statement in the body of the called
function. [ Note: Value computations
and side effects associated with
different argument expressions are
unsequenced. —end note ]
C++0x:
This means that the evaluation of the arguments arg1/arg2 are unsequenced (neither of them is sequenced before the other).
The same section in the draft Standard also states,
If a side effect on a scalar object is
unsequenced relative to either another
side effect on the same scalar object
or a value computation using the value
of the same scalar object, the
behavior is undefined.
Now there is a sequence point at the semicolon at the end of the full expression below
operator<<(operator<<(cout, i++), i--);
^ the interesting sequence point is right here
As is clear, evaluation of both arg1 and arg2 lead to side effect on the scalar variable ‘i’, and as we saw above, the side effects are unsequenced.
Therefore the code has undefined behavior. So what does that mean?
Here is how ‘undefined behavior’ is defined 🙂 in the Standard.
Permissible undefined behavior ranges
from ignoring the situation completely
with unpredictable results, to
behaving during translation or program
execution in a documented manner
characteristic of the environment
(with or without the issuance of a
diagnostic message), to terminating a
translation or execution (with the
issuance of a diagnostic message).
Many erroneous program constructs do
not engender undefined behavior; they
are required to be diagnosed.
Do you see correlation with @DarkDust’s response ‘The compiler is even allowed to set your computer on fire :-)’
So any output you get from such a code is really in the dreaded realm of undefined behavior.
Don’t do it.
Only thing that is defined about such code is that it helps OP and many of us get lots of votes (if answered correctly) 🙂