C / C++ / MFC

4-Jan-06 0:06

I come from a java background where Exceptions are heavily used. I started using try/catch(...) in C++, but not sure what all can be done in the catch block.
1. What does the elipsis (...) mean? What else can be put here?
2. If we want to catch an exception of a known type, can we specify the exception in the catch here?
3. If so, how do you define an exception class? Is there an interface that we should implement?
4. When we have the elipsis(...) how do you get the details of the exception?
5. Are there simple examples anywhere that demonstrate exceptions in C++ for Java programmers?

thanks!

Zdeslav Vojkovic4-Jan-06 0:41

4-Jan-06 0:41

1. it means that any possible exception type will be caght there
2. yes, like this: catch (MyException& e)
& operator is not necessary, but it is a good programming practice regarding performance
in c++ you can also catch integral types, like: catch (long x)
3. any class/type can be exception tpye, but maybe you would like to have look at the 'exception' class, which is a base class for all exeption classes in standard c++ library
4. as far as i know, there is no portable way to do it.
5. these are not specifically for java developers but it is not that hard.
http://cis.stvincent.edu/swd/except/except.html
http://cplus.about.com/od/beginnerctutorial/l/aa122202a.htm

there is also a lot of in-depth articles on exceptions at Guru of the Week[^] and here on CodeProject

Chintoo7234-Jan-06 1:17

4-Jan-06 1:17

Zdeslav Vojkovic wrote:
yes, like this: catch (MyException& e)
& operator is not necessary, but it is a good programming practice regarding performance

The example[^] here uses a pointer to the exception object, whereas you are using a reference. What is correct? or does it depend on what is being "thrown", but you dont always have cotnrol over what is thrown, right? When it is a pointer, is it considered as an integral datatype exception?

Zdeslav Vojkovic wrote:
in c++ you can also catch integral types, like: catch (long x)

Does this work like this:

try {<br />
  throw 5;<br />
}catch(long x) {<br />
 printf("%d", x);<br />
}<br />

Does this print 5?

Zdeslav Vojkovic wrote:
as far as i know, there is no portable way to do it.

Can you tell me how you would obtain the exception info when using Microsoft Visual C++?

Zdeslav Vojkovic wrote:
5. these are not specifically for java developers but it is not that hard.

Sure, I will go throught them.

thanks!

Zdeslav Vojkovic4-Jan-06 2:02

4-Jan-06 2:02

Britley wrote:
The example[^] here uses a pointer to the exception object, whereas you are using a reference. What is correct?

if an exception is allocated on the heap (using 'new' operator), it must be caught through a pointer. however, this is not a good practice because it is not obvious to the client code how to handle it, and it complicates the deleting of the allocated exception instance (who will call delete on it). exceptions shoud be stack allocated, thrown, and caught by reference. please note that the example is for an MFC client, and MFC implements somewhat specific idiom, where exceptions are allocated on the heap and you have to call Delete() method of the CException class to destroy the instance.

Britley wrote:
Does this work like this:

no, because 5 is an int, not a long, but this does:

try {<br />
throw 5L;<br />
}catch(long x) {<br />
printf("%d", x);<br />
}

Britley wrote:
Can you tell me how you would obtain the exception info when using Microsoft Visual C++?

when you catch exception with (...) you can't do that. anyway, in this case you should always rethrow the exception, because it is not possible to correctly handle an exception without knowing its type. you can retrieve some exception info, but all the ways i know are OS specific, and not supported by standard c++. for more details, look here:
http://www.codeproject.com/cpp/exceptionhandler.asp
http://www.codeproject.com/debug/XCrashReportPt2.asp
and lots of other exception handling articles on codeproject

what you can do is to catch specific exceptions. example:
catch(exception& e)
this will catch all the exceptions of type 'exception' and of all types derived from it . exceptions are usually modelled in hierarchies, so you can handle multiple exception types in one handler.

Chintoo7234-Jan-06 2:37

4-Jan-06 2:37

Thanks for the great information.

Zdeslav Vojkovic wrote:
if an exception is allocated on the heap (using 'new' operator), it must be caught through a pointer.

Why must? Why cannot it be accessed through reference? I mean, you catch what is thrown. So if you throw the object by dereferencing the pointer, you may catch it through reference. Isnt this correct?

Zdeslav Vojkovic wrote:
however, this is not a good practice because it is not obvious to the client code how to handle it, and it complicates the deleting of the allocated exception instance (who will call delete on it).

I can see the complication, which doesnt exist in Java due to its automatic garbage collection.

Zdeslav Vojkovic wrote:
exceptions shoud be stack allocated, thrown, and caught by reference.

This is what puzzles me. If the exception is stack allocated, should we make sure the catch statement is in the same scope(braces) as that of the exception variable? Otherwise, how is the stack cleaned up when the control moves out of the scope?

Zdeslav Vojkovic wrote:
MFC implements somewhat specific idiom, where exceptions are allocated on the heap and you have to call Delete() method of the CException class to destroy the instance

I see. What is the equivalent for the C++ "exception" class to free the object? Btw, this Delete method of CException might destroy the instance to free up resources, but is the object itself freed too (perhaps through 'delete this')?

thanks!

Zdeslav Vojkovic4-Jan-06 3:38

4-Jan-06 3:38

Britley wrote:
Why must? Why cannot it be accessed through reference?

because T* can't be treated as T, while T& can (they are interchangeable).

T x;<br />
T& ref = x; // valid<br />
T* ptr = x; // not valid

Britley wrote:
If the exception is stack allocated, should we make sure the catch statement is in the same scope(braces) as that of the exception variable

no, catch statement can be at any level higher than the current context. implementation of stack unwinding is left to compiler vendors, so there is no single answer for that. VC++ creates temporary object which will be thrown, and destroys it after the stack is cleaned up

Britley wrote:
What is the equivalent for the C++ "exception" class to free the object?

delete operator.

Britley wrote:
Btw, this Delete method of CException might destroy the instance to free up resources, but is the object itself freed too (perhaps through 'delete this')?

it depends. CException constructor takes a BOOL parameter which states whether it is an auto-delete exception. if set to true, Delete() does nothing (this is used for CException instances allocated on the stack). if it is set to false (for heap-allocated instances), Delete() must be called, and it will call 'delete this' internally.

Chintoo7234-Jan-06 3:45

4-Jan-06 3:45

Zdeslav Vojkovic wrote:
because T* can't be treated as T, while T& can (they are interchangeable).

I meant this:

<br />
try {<br />
  T *t = new T;<br />
  throw *t;<br />
}catch(T& te) {<br />
}<br />

Anything wrong, other than the memory leak, ofcourse?

Zdeslav Vojkovic wrote:
What is the equivalent for the C++ "exception" class to free the object

Zdeslav Vojkovic wrote:
delete operator.

but for objects created on stack I guess the compiler frees it eventually, so we dont need to call the delete operator. Why do you need to call Delete on the MFC CException again? May be its MFC specific logic...

thanks!

Zdeslav Vojkovic4-Jan-06 3:54

4-Jan-06 3:54

Britley wrote:
Anything wrong, other than the memory leak, ofcourse?

no, that's completely valid.

Britley wrote:
Why do you need to call Delete on the MFC CException again

actually you don't, but the problem is that if you call an MFC function which throws a CException*, you don't necessarilly know how it was allocated, so you can't call 'delete ex' because it will crash for stack allocated object, and if you fail to call it for heap allocated object you have a memory leak. Therefore, you must call Delete(), which knows whether to destroy the object or not. if you are 100% sure that it is stack-allocated, you can omit Delete() call, but still you can't be sure if next version of MFC will behave differently and then you would have to modify the code to avoid memory leak.

markkuk4-Jan-06 12:49

markkuk

4-Jan-06 12:49

Britley wrote:
What is the equivalent for the C++ "exception" class to free the object?

The C++ Standard Library throws objects of classes derived from std::exception, not pointers to them, and you should catch them by reference. You don't free the object.

Jared Parsons4-Jan-06 5:38

Jared Parsons

4-Jan-06 5:38

Zdeslav Vojkovic wrote:
3. any class/type can be exception tpye, but maybe you would like to have look at the 'exception' class, which is a base class for all exeption classes in standard c++ library

To avoid confusion since he is coming from a Java background: In C++ the class does not have to derive from Exception to be thrown. Any type can by thrown.

Jared Parsons
jaredp@beanseed.org
http://jaredparsons.blogspot.com/[^]

<color>Aljechin 4-Jan-06 0:45

<color>Aljechin

4-Jan-06 0:45

Britley wrote:
I started using try/catch(...) in C++, but not sure what all can be done in the catch block.

1. Anything that is holy, practically. Big Grin | :-D

Britley wrote:
1. What does the elipsis (...) mean?

(...) Catch any exception that occurs

Britley :->wrote:
What else can be put here?

A known exception type can be put here which has to be caught. Eg: catch(CFileException Cfe)

Britley wrote:
If so, how do you define an exception class? Is there an interface that we should implement?

There are a bunch of exceptions. You will not need to implement any interface. You can use any existing exception.

Britley wrote:
4. When we have the elipsis(...) how do you get the details of the exception?

There is one API call known as GetLastError for this purpose

Britley wrote:
5. Are there simple examples anywhere that demonstrate exceptions in C++ for Java programmers

<br />
TRY<br />
{<br />
CDatabase DB;<br />
CRecordset Rc;<br />
CString szSql = "SELECT * FROM MYTABLE WHERE AGE = 20"<br />
DB.ExecuteSql(szSql);<br />
}CATCH(CDatabaseException CDbe){ AfxMessageBox(CDBE->m_error));}<br />

I believe above things are right. I a newbie too, but have worked on a database module. I am sure you will get a bunch of replies and guidance. Smile | :)

Aljechin Alexander

Zdeslav Vojkovic4-Jan-06 0:58

4-Jan-06 0:58

Aljechin wrote:
Eg: catch(CFileException Cfe)

it is a bad practice to catch exceptions by value (except for integral types)

Aljechin wrote:
There is one API call known as GetLastError for this purpose

it will not return details of exception. it is a Win API to retrieve last Win32 error for the current thread.

also, microsoft specific TRY/CATCH should be avoided, and standard try/catch used instead, for reasons stated here[^]

connecting to server...

Yuwraj4-Jan-06 0:00

Yuwraj

4-Jan-06 0:00

Hi guies,

I am new to VC++.

I have to write a code which contain connect method and which connect's

to server.

Anyone can give me that procedure,Pl'z.(which contains connect method).

yuwraj

sunit54-Jan-06 0:49

sunit5

4-Jan-06 0:49

http://www.codeproject.com/internet/winsockintro02.asp

never say die

Zdeslav Vojkovic4-Jan-06 0:49

4-Jan-06 0:49

it would help if you could be more specific in your questions, what kind of server, etc.
if you are talking about connecting through sockets, have a look at this[^] article. also, try google, it should return thousands of hits on this topic

ThatsAlok4-Jan-06 1:33

ThatsAlok

4-Jan-06 1:33

http://www.codeproject.com/internet/server_client_sockets.asp[^]

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow

cheers,
Alok Gupta
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