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William Payne
Guest
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 Posted: Sun Mar 07, 2004 10:30 pm Post subject: String corrupted after throw |
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Hello, I am making a binary tree and I've encountered a problem when I throw
an exception. Consider this code:
/* in inserter function for the tree */
char error_message[64];
sprintf(error_message, "%i already present in the tree.", n); /* n is of
type int */
cout << error_message << endl;
throw error_message;
/* in main() */
try
{
binary_tree.insert(7);
}
catch(const char* error_message)
{
cerr << error_message << endl;
}
When I print out the string I am about to throw in the inserter-function, I
get:
7 already present in the tree.
but when I catch the exception in main() and print out I get:
? al(?C y present in the tre"í
As you can see, it has become corrupted. What am I doing wrong? Must I
allocate the error message dynamically or?
/ WP
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Clark Cox
Guest
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| Posted: Sun Mar 07, 2004 11:58 pm Post subject: Re: String corrupted after throw |
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In article <c2g7on$gkq$1 (AT) news (DOT) island.liu.se>,
"William Payne" <mikas493_no_s_p_a_m_ (AT) student (DOT) liu.se> wrote:
| Quote: | char error_message[64];
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[snip]
| Quote: | throw error_message;
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[snip]
| Quote: | As you can see, it has become corrupted. What am I doing wrong?
|
You're throwing a pointer that points to a automatic variable local
to the block that is doing the throwing.
| Quote: | Must I allocate the error message dynamically or?
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You're throwing the value of the char*, but once you leave this
scope, the array no longer exists, so that dereferencing that pointer
results in undefined behavior. This is the same issue with trying to
return a pointer to a local variable.
Try throwing (and catching) a std::string -- or some subclass of
std::exception instead.
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Alf P. Steinbach
Guest
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| Posted: Mon Mar 08, 2004 12:05 am Post subject: Re: String corrupted after throw |
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* "William Payne" <mikas493_no_s_p_a_m_ (AT) student (DOT) liu.se> schriebt:
| Quote: | Hello, I am making a binary tree and I've encountered a problem
|
In the great tradition now explained to me by others, of first providing
a short answer to "the showstopping problem":
* The showstopping problem is that you're throwing a pointer to a local
variable that's gone out of scope when that pointer is used.
* The fix of the showstopping problem is to throw a
std::runtime_error exception, catch a 'std::exception const& x'.
* The main overall problem is C style instead of C++ style, but since
this is a rather subtle point don't think about it, just forget it.
| Quote: | when I throw
an exception. Consider this code:
/* in inserter function for the tree */
char error_message[64];
sprintf(error_message, "%i already present in the tree.", n); /* n is of
type int */
|
Don't use unsafe C formatting functions.
As a newbie only use type-safe C++ formatting such as e.g.
std::ostringstream.
| Quote: | cout << error_message << endl;
throw error_message;
|
Don't throw anything other than objects of classes derived from
std::exception.
| Quote: | /* in main() */
try
{
binary_tree.insert(7);
}
catch(const char* error_message)
{
cerr << error_message << endl;
}
When I print out the string I am about to throw in the inserter-function, I
get:
7 already present in the tree.
but when I catch the exception in main() and print out I get:
? al(?C y present in the tre"í
As you can see, it has become corrupted. What am I doing wrong? Must I
allocate the error message dynamically or?
|
No, leave that to std::exception.
--
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
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