Hi,
i heard a lot about "undefined behaviour" in this and other newsgroups
dealing with c/c++.
Is there a list where all cases with undefined behaviour in C++ are listed?
regards marbac
Jul 22 '05
48  3094 
Duane Hebert wrote: But the union shares the memory between the unsigned char[2] and the
short. The unsigned char was assigned a value, reading the short
after is just interpreting the same memory as a short no?
Yes but the standard says:
"In a union, at most one of the data members can be active at any time,
that is, the value of at most one of the data members can be stored in a
union at any time."
So an implementer may choose to do weird magic in the implementation of
a union. In other words, it is not a well-defined behaviour.
Regards,
Ioannis Vranos http://www23.brinkster.com/noicys
Default User wrote: Well, I think what Ron said was somewhat confusing. I already quoted the
C standard on the issue. It's implementation-defined, that means that
the standard doesn't impose any behavior, but the implementation has to
select a behavior and document it. So the behavior for a user is not
unspecified, you can check your documentation.
There's also the bit about the union members being structs with common
initial sequences.
Not in C. Some have said that is so in C++, but I haven't seen a quote
from the standard. Usually these things are the same between the
languages, but sometimes not. Obviously, the fact that all data is POD
in C but not in C++ may well be a factor.
You are making a mistake. Here we are talking about C++ and not C. :-)
Regards,
Ioannis Vranos http://www23.brinkster.com/noicys
Peter Koch Larsen wrote: No it would not. Elsewhere in this thread you have quoted the standard on
undefined behaviour - read it and tell me if you find an assert or a throw
kochs_exception would violate the standard.
If a standard library implementation performs range checking in vector's
operator[]() and throws an exception in case of out of range, isn't this
a platform-specific extension?
Regards,
Ioannis Vranos http://www23.brinkster.com/noicys
Rolf Magnus wrote: I agree that I wouldn't call it a system-specific extension. Throwing an
exception is just as much of a system specific extension as crashing
and output like "segmentati on fault, core dumped" or just ignoring the
faulty access.
I was not talking about a system-specific *behaviour*, but a system
specific *extension*, like WinMain() for example.
Regards,
Ioannis Vranos http://www23.brinkster.com/noicys
Ioannis Vranos wrote: Peter Koch Larsen wrote:
No it would not. Elsewhere in this thread you have quoted the
standard on undefined behaviour - read it and tell me if you find an
assert or a throw kochs_exception would violate the standard.
If a standard library implementation performs range checking in
vector's operator[]() and throws an exception in case of out of range,
isn't this a platform-specific extension?
No, it isn't. If the behaviour is undefined, anything can happen from
the standard point of view, and "anything" includes an exception being
thrown, the program crashing or the computer exploding. So the
implementation does just what the standard says it should do.
Which behaviour would not be an "extension" for you?
Rolf Magnus wrote: Ioannis Vranos wrote:
Peter Koch Larsen wrote:
No it would not. Elsewhere in this thread you have quoted the
standard on undefined behaviour - read it and tell me if you find an
assert or a throw kochs_exception would violate the standard.
If a standard library implementation performs range checking in
vector's operator[]() and throws an exception in case of out of range,
isn't this a platform-specific extension?
No, it isn't. If the behaviour is undefined, anything can happen from
the standard point of view, and "anything" includes an exception being
thrown, the program crashing or the computer exploding. So the
implementation does just what the standard says it should do.
Which behaviour would not be an "extension" for you?
Whether something is an extension is not answered by the standard but by
the vendor of my compiler/library. If the documentation of the vector class
in a particular library implementation advertises that an exception will be
thrown for out of range indices, then I would call it an extension.
Best
Kai-Uwe Bux
Ioannis Vranos wrote: You are making a mistake. Here we are talking about C++ and not C. :-)
I am not making a mistake. I responded to the reply to Ron Natalie's
assertions about C. I specifically said that it may be different in C++.
Try to keep up.
Brian Rodenborn
Rolf Magnus wrote: No, it isn't. If the behaviour is undefined, anything can happen from
the standard point of view, and "anything" includes an exception being
thrown, the program crashing or the computer exploding. So the
implementation does just what the standard says it should do.
Which behaviour would not be an "extension" for you?
Yes, accessing a vector out of range causes undefined behaviour, which
includes a throwing of an exception, which would be a system-specific
extension provided it is documented in the implementer's documentation.
Regards,
Ioannis Vranos http://www23.brinkster.com/noicys
"Default User" <fi********@boe ing.com.invalid > wrote in message news:40******** *******@boeing. com.invalid...
The C99 standard says virtually the same thing.
No, it does not. As a matter of fact, the passage you are quoting doesn't even exist
in C99.
6.2.6.1 / 7 specifically says it's unspecified what happens to bytes of other union members
not stored to. Other than that, the standard is mute (which makes it UNDEFINED behavior).
Further, 6.5 / 7 clearly makes it undefined behavior in C. In the general case, it's undefined behavior for C++ (there are some
specific outs).
6.5 / 7 of C99 appears almost word for word as 5 / 9 in the C++ standard.
I was wrong, it's undefined behavior in both languages (in most cases).
"Default User" <fi********@boe ing.com.invalid > wrote in message news:40******** *******@boeing. com.invalid... Duane Hebert wrote:
"Ron Natalie" <ro*@sensor.com > wrote in message news:40******** *************** @news.newshosti ng.com...
C says it's unspecified, there is no need for the implementation to have a specific
behavior. In the general case, it's undefined behavior for C++ (there are some
specific outs).
I've heard this before but you mean basically that:
Well, Ron said that but he was incorrect. I already quoted the C
standard on the issue. It's implementation-defined, not unspecified.
That means that an implementation has to select a behavior and document
I was only incorrect in saying that C said the behavior was unspecified. I have
since found that C99 has the same text that C++ does. The following text (this is from
C99, but it's essentially the same in C++):
An object shall have its stored value accessed only by an lvalue expression that has one of
the following types:73)
- a type compatible with the effective type of the object,
- a qualified version of a type compatible with the effective type of the object,
- a type that is the signed or unsigned type corresponding to the effective type of the
object,
- a type that is the signed or unsigned type corresponding to a qualified version of the
effective type of the object,
- an aggregate or union type that includes one of the aforementioned types among its
members (including, recursively, a member of a subaggregate or contained union), or
- a character type. This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
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On Mon, 07 Feb 2005 21:28:30 GMT, Keith Thompson <kst-u@mib.org>
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>"Romeo Colacitti" <wwromeo@gmail.com> writes:
>> Chris Torek wrote:
>>> In article <4205BD5C.6DC8@mindspring.com>
>>> pete <pfiland@mindspring.com> wrote:
>
>>> >If you have
>>> > int array;
>>> >then
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I have a general idea of the different kinds of behaviour described by the
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(1) Well-defined behaviour:
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(Jist: The code will work perfectly.)
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