Hi,
I've heard that if you've declared a variable (such as a double or
an int) and not initialize it, then the result of printing out its
value is undefined.
I've also heard that "undefined behavior" can mean just about
anything, such as "flying monkeys shooting out of your nose." Sure,
that's an exaggeration, but normally I interpret that to mean that the
program can crash (or cease running) erratically, or even corrupt
data.
So my question is: Although I can never safely predict the printed
output of an uninitialized int or double, is it still safe (or legal)
to do so? In other words, if I run this program:
#include <iostream>
int main(int argc, char ** argv)
{
int i;
double d;
std::cout << "i = " << i << std::endl; // safe?
std::cout << "d = " << d << std::endl; // safe?
return 0;
}
I may not be able to predict what will print out, but is there any
chance that the program can crash because of those lines?
If you're curious why I ask this, it's because in some code I'm
working through there is a structure with ints -- some of which are
never used nor initialized. However, this structure (will all its
ints) is getting written out to disk (and later read back in). But at
no time are the values of these uninitialized ints used for logic in
the code.
Because the code is writing out uninitialized values (and later
reading them in), is there a possibility that the program can self-
destruct (or corrupt anything) just because those ints weren't
initialized?
Thanks in advance.
-- Jean-Luc
14  1439 
On Fri, 25 Jul 2008 09:04:07 -0700, jl_post wrote:
Hi,
I've heard that if you've declared a variable (such as a double or
an int) and not initialize it, then the result of printing out its value
is undefined.
I've also heard that "undefined behavior" can mean just about
anything, such as "flying monkeys shooting out of your nose."
In fact the commonly acknowledged consequence is that your fridge defrosts.
Sure,
that's an exaggeration, but normally I interpret that to mean that the
program can crash (or cease running) erratically, or even corrupt data.
So my question is: Although I can never safely predict the printed
output of an uninitialized int or double, is it still safe (or legal)
to do so?
It's certainly "legal" in the sense that it won't prevent your program
compiling.
In other words, if I run this program:
#include <iostream>
int main(int argc, char ** argv)
{
int i;
double d;
std::cout << "i = " << i << std::endl; // safe?
std::cout << "d = " << d << std::endl; // safe?
return 0;
}
I may not be able to predict what will print out, but is there any
chance that the program can crash because of those lines?
You're ok. The *value* of the variable i may well be undefined, but
i is nonetheless an int; and outputting an int - any int, whatever its
value - should never crash your program. Ditto double, etc.
--
Lionel B jl*****@hotmail .com wrote:
[..] So my question is: Although I can never safely predict the printed
output of an uninitialized int or double, is it still safe (or legal)
to do so? In other words, if I run this program:
#include <iostream>
Add
#include <ostream>
to make it completely standard-compliant.
>
int main(int argc, char ** argv)
{
int i;
double d;
std::cout << "i = " << i << std::endl; // safe?
std::cout << "d = " << d << std::endl; // safe?
return 0;
}
I may not be able to predict what will print out, but is there any
chance that the program can crash because of those lines?
[..]
I have never seen 'int' behave that way, but I can attest that an
uninitialised 'double' can cause the system to throw a hardware
exception, and I've seen it happen.
Search the archives for "uninitiali zed value trap exception" or
something like that, and you can hopefully find others' encounters with
pointers and perhaps even integral values.
Generally speaking, any *use* of an uninitialised object has undefined
behaviour.
V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask
On Jul 25, 6:04 pm, jl_p...@hotmail .com wrote:
I've heard that if you've declared a variable (such as a
double or an int) and not initialize it, then the result of
printing out its value is undefined.
Anything you do with its *value* is undefined behavior. (You
can still take its address, or assign to it.) With the
exception of unsigned char and char.
I've also heard that "undefined behavior" can mean just about
anything, such as "flying monkeys shooting out of your nose."
Sure, that's an exaggeration, but normally I interpret that to
mean that the program can crash (or cease running)
erratically, or even corrupt data.
In non-priviledged mode, under a modern general purpose OS,
that's what it normally means. On systems without a priviledged
mode (or in kernel code)... I have seen it require the disk to
be reformatted.
So my question is: Although I can never safely predict the
printed output of an uninitialized int or double, is it still
safe (or legal) to do so?
No. It's undefined behavior.
In other words, if I run this program:
#include <iostream>
int main(int argc, char ** argv)
{
int i;
double d;
std::cout << "i = " << i << std::endl; // safe?
std::cout << "d = " << d << std::endl; // safe?
return 0;
}
I may not be able to predict what will print out, but is there
any chance that the program can crash because of those lines?
Of course. It's not likely with int, on most modern machines
(but there is at least one where it is a distinct possibility).
With double, it's possible on every Windows or Unix machine I
know.
If you're curious why I ask this, it's because in some code
I'm working through there is a structure with ints -- some of
which are never used nor initialized. However, this structure
(will all its ints) is getting written out to disk (and later
read back in). But at no time are the values of these
uninitialized ints used for logic in the code.
Because the code is writing out uninitialized values (and
later reading them in), is there a possibility that the
program can self- destruct (or corrupt anything) just because
those ints weren't initialized?
Formally, yes, and any good debugging system will complain. (I
know Purify does, because I've had to deal with the same
problem.) Why don't you just initialize the structs.
And how are you writing them out? If you're just copying the
bits of a struct to disk, then you have no guarantee of being
able to read the data in the future.
--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
"Lionel B" <me@privacy.net wrote in message
news:g6******** **@south.jnrs.j a.net...
You're ok. The *value* of the variable i may well be undefined, but
i is nonetheless an int; and outputting an int - any int, whatever its
value - should never crash your program. Ditto double, etc.
Not true. It's not true in theory for int, and definitely not true in
practice for double -- because IEEE floating-point, which most modern
computers use, has a notion of "signaling not-a-number" values that cause a
run-time error condition if accessed.
So even in the simple case where the implementation puts random bits into
uninitialized doubles and does no other checking, it is entirely possible
that those random bits will happen to be a signaling NaN value, which will
cause a run-time exception.
Even for int, there is no reason why an implementation cannot keep track of
whether variables have been initialized, and stop the program if it tries to
access an uninitialized variable. I am not aware of any such
implementations in widespread use today, but they have existed in the past.
On 2008-07-26 10:31:04 -0400, "Andrew Koenig" <ar*@acm.orgsai d:
"Lionel B" <me@privacy.net wrote in message
news:g6******** **@south.jnrs.j a.net...
>You're ok. The *value* of the variable i may well be undefined, but
i is nonetheless an int; and outputting an int - any int, whatever its
value - should never crash your program. Ditto double, etc.
Not true. It's not true in theory for int, and definitely not true in
practice for double -- because IEEE floating-point, which most modern
computers use, has a notion of "signaling not-a-number" values that cause a
run-time error condition if accessed.
My reading of IEEE-754 is that acessing a signaling NaN causes an
invalid operation exception by default, and the result of that
exception is just to return a quite NaN. Unless the program has
installed a trap handler, this is completely innocuous.
--
Pete
Roundhouse Consulting, Ltd. ( www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
( www.petebecker.com/tr1book)
Pete Becker wrote:
On 2008-07-26 10:31:04 -0400, "Andrew Koenig" <ar*@acm.orgsai d:
>Not true. It's not true in theory for int, and definitely not true in
practice for double -- because IEEE floating-point, which most modern
computers use, has a notion of "signaling not-a-number" values that
cause a
run-time error condition if accessed.
My reading of IEEE-754 is that acessing a signaling NaN causes an
invalid operation exception by default, and the result of that exception
is just to return a quite NaN. Unless the program has installed a trap
handler, this is completely innocuous.
In theory.
In practice a had a bug in some debug output a few days ago that
accidentally read a double from a union that was initialized as a
pointer. In many cases nothing strange happened. But under some
conditions the whole operating system immediately froze, even when
running in the debugger. I don't know what exception the random bits
caused, but obviously it was strange enough that nobody tested it before.
Marcel
Victor Bazarov wrote:
Generally speaking, any *use* of an uninitialised object has undefined
behaviour.
You should exclude the operations 'taking the address of' and 'creating
a reference to'. They are obviously allowed unless you dereference the
pointers, again except for an assignment.
Strictly speaking this applies to PODs only. However, since C++ objects
usually are valid after construction, they make less trouble unless your
constructor leaves uninitialized members. This should be up to very
basic libraries only. E.g. some std::vector implementations do not
initialize the elements in the range [size(),capacity ()[.
Marcel
On Jul 27, 12:03 am, Marcel Müller <news.5.ma...@s pamgourmet.org>
wrote:
Victor Bazarov wrote:
Generally speaking, any *use* of an uninitialised object has
undefined behaviour.
You should exclude the operations 'taking the address of' and
'creating a reference to'. They are obviously allowed unless
you dereference the pointers, again except for an assignment.
The actual language says that it is using the value of an
uninitialised object which has undefined behavior. Roughly
speaking (or perhaps exactly speaking), it is the lvalue to
rvalue conversion which triggers the undefined behavior. (You
can still assign to the object, for example.)
Strictly speaking this applies to PODs only.
What makes you say that? It applies to everything. (Of course,
a lot of non-POD's have user defined constructors, which take
care of the initialization. But the standard doesn't require
it.)
However, since C++ objects usually are valid after
construction, they make less trouble unless your constructor
leaves uninitialized members. This should be up to very basic
libraries only. E.g. some std::vector implementations do not
initialize the elements in the range [size(),capacity ()[.
Some? The standard doesn't allow them to initialize those
elements (and it doesn't allow you to access them).
--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
On Jul 26, 4:30*pm, Pete Becker <p...@versatile coding.comwrote :
On 2008-07-26 10:31:04 -0400, "Andrew Koenig" <a...@acm.orgsa id:
"Lionel B" <m...@privacy.n etwrote in message
news:g6******** **@south.jnrs.j a.net...
You're ok. The *value* of the variable i may well be undefined, but
i is nonetheless an int; and outputting an int - any int, whatever its
value - should never crash your program. Ditto double, etc.
Not true. *It's not true in theory for int, and definitely not true in
practice for double -- because IEEE floating-point, which most modern
computers use, has a notion of "signaling not-a-number" values that cause a
run-time error condition if accessed.
My reading of IEEE-754 is that acessing a signaling NaN causes an
invalid operation exception by default, and the result of that
exception is just to return a quite NaN. Unless the program has
installed a trap handler, this is completely innocuous.
http://blogs.msdn.com/oldnewthing/ar...3/8682463.aspx
seems to disagree. Or at least trap handlers seem to get enabled
more easily than you'd think on some OSs
--
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