throwing dtors...

[Chris M. Thomasson]

Is it every appropriate to throw in a dtor? I am thinking about a simple
example of a wrapper around a POSIX file...
_______________ _______________ _______________ _______________ ____________
class file {
FILE* m_handle;

public:
// [...];

~file() /* throw() */ {
int const status fclose(m_handle );
if (status) {
/* shi% hit the fan:

http://www.opengroup.org/onlinepubs/007908775/xsh/fclose.html

/*

// [what now?]
}
}
};
_______________ _______________ _______________ _______________ ____________


How to properly handle `EAGAIN' in dtor? Well, what about any error for that
matter? I am a C programmer and only code C++ for fun, and some in-house
projects. If I were really going to create C++ application and release it
into the wild, well, how would you advise me to handle the case above? I am
interested in how throwing in a dtor effects dynamic destruction... Would
something like the following be legal?



<pseudo code!!!!>
_______________ _______________ _______________ _______________ ___
struct throw_from_dtor {
int const m_status;

public:
throw_from_dtor (int const status)
m_status(status ) {}

int get_status() const { return m_status; }
};

class file {
FILE* m_handle;

public:
// [ctor];

~file() {
int const status = fclose(m_handle );
if (status) {
throw throw_from_dtor (status);
}
}
};


int main() {
file* f = new file();
try {
delete f;
} catch(throw_fro m_dtor const& e) {
// handle error from `e.get_status() '
delete f;
}
return 0;
}
_______________ _______________ _______________ _______________ ___


?


or what about using smart pointer...



int main() {
std::auto_ptr<f ilef;
try {
f.reset(new file());
} catch(throw_fro m_dtor const& e) {
// handle error from `e.get_status() '
}
}



?




Please keep in mind that refusing to not handle an error from `fclose' could
resule is HORRIBLE things down the road... Think massive data lost...
Perhaps __permanent__ data-! OUCH!!!

;^/

[Chris M. Thomasson]

Re: throwing dtors...


"Chris M. Thomasson" <no@spam.invali dwrote in message
news:k4XEk.1619 9$hX5.2021@news fe06.iad...

Is it every appropriate to throw in a dtor? I am thinking about a simple
example of a wrapper around a POSIX file...
_______________ _______________ _______________ _______________ ____________

[...]

_______________ _______________ _______________ _______________ ____________
>
>
How to properly handle `EAGAIN' in dtor? Well, what about any error for
that matter? I am a C programmer and only code C++ for fun, and some
in-house projects. If I were really going to create C++ application and
release it into the wild, well, how would you advise me to handle the case
above? I am interested in how throwing in a dtor effects dynamic
destruction... Would something like the following be legal?
>
>
>
<pseudo code!!!!>
_______________ _______________ _______________ _______________ ___
struct throw_from_dtor {
int const m_status;
>
public:
throw_from_dtor (int const status)
m_status(status ) {}
>
int get_status() const { return m_status; }
};
>
class file {
FILE* m_handle;
>
public:
// [ctor];
>
~file() {
int const status = fclose(m_handle );
if (status) {
throw throw_from_dtor (status);

^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^

// ummmm! well, stupid me forgot to store `errno' into the
exception!!!!!!



throw throw_from_dtor (errno);



// sorry about the non-sense! ;^(...

}
}
};

int main() {
file* f = new file();
try {
delete f;
} catch(throw_fro m_dtor const& e) {
// handle error from `e.get_status() '
delete f;
}
return 0;
}
_______________ _______________ _______________ _______________ ___
>
>
?
>
>
or what about using smart pointer...
>
>
>
int main() {
std::auto_ptr<f ilef;
try {
f.reset(new file());
} catch(throw_fro m_dtor const& e) {
// handle error from `e.get_status() '
}
}
>
>
>
?
>
>
>
>
Please keep in mind that refusing to not handle an error from `fclose'
could resule is HORRIBLE things down the road... Think massive data
lost... Perhaps __permanent__ data-! OUCH!!!
>
;^/

[Chris M. Thomasson]

Re: throwing dtors...

"Chris M. Thomasson" <no@spam.invali dwrote in message
news:k4XEk.1619 9$hX5.2021@news fe06.iad...

Is it every appropriate to throw in a dtor? I am thinking about a simple
example of a wrapper around a POSIX file...
_______________ _______________ _______________ _______________ ____________

[...]

_______________ _______________ _______________ _______________ ____________

[...]

how would you advise me to handle the case above? I am interested in how
throwing in a dtor effects dynamic destruction... Would something like the
following be legal?
>

[...]

I am doing some experimenting, and found that throwing from a dtor
apparently leaves the object fully intact wrt the memory that makes it up so
that proper disaster cleanup can indeed be performed... For example, the
following program goes into infinite loop:
_______________ _______________ _______________ _______________ __________
#include <cstdio>

struct throw_on_dtor {};

class foo {
public:
~foo() {
throw throw_on_dtor() ;
}
};

int main(void) {
foo* f = new foo();
retry:
try {
delete f;
} catch (throw_on_dtor const& e) {
std::puts("thro w_on_dtor caught!");
goto retry;
}
return 0;
}

_______________ _______________ _______________ _______________ __________




So, AFAICT, throwing from a dtor will complicate some odd complications.
However, they can be worked out for sure. This fact that a dtor can throw
will need to be CLEARY documented indeed. The above highly crude technique
can be used to solve the fact when a file close is interrupted by a signal
(e.g., `EINTR'). It can also be used to handle `EAGAIN'... Although, it
seems eaiser to use placement new when your dealing with a class that can
throw from its dtor, so that the catch block can actually free memory
without running the dtor again like delete does... Something like:

_______________ _______________ _______________ _______________ __________
#include <cstdio>
#include <cstdlib>
#include <new>

struct throw_on_dtor {};

class foo {
public:
~foo() {
throw throw_on_dtor() ;
}
};

int main(void) {
foo* f = new (std::malloc(si zeof(*f))) foo();
if (f) {
retry:
try {
f->~foo();
std::free(f);
} catch (throw_on_dtor const& e) {
std::puts("thro w_on_dtor caught! Handling Error...");
std::free(f);
}
}
return 0;
}
_______________ _______________ _______________ _______________ __________



Humm... The placement new soultion looks like a good match for throwing
dtors indeed!



Also, a class which throws from dtors could contain a dtor counter and/or
flag to detect how many times, if any, the dtor has been invoked; something
like:



class foo {
unsigned m_dtor_invoke; // = 0
bool m_dtor_throw; // = false;
public:
~foo() {
++m_dtor_invoke ;
if (! m_dtor_throw) {
m_dtor_throw = true;
throw throw_on_dtor() ;
}
}
};



Any thoughts? BTW, try not to flame me too harshly! I am trying to avoid the
user explicitly calling a close function... Or, is that a great idea wrt
dealing with any class that has a dtor which calls an API that can fail
_AND_ such failure indicates something important?


;^(...

[Chris M. Thomasson]

Re: throwing dtors...


"Chris M. Thomasson" <no@spam.invali dwrote in message
news:aoXEk.1620 4$hX5.2055@news fe06.iad...

"Chris M. Thomasson" <no@spam.invali dwrote in message
news:k4XEk.1619 9$hX5.2021@news fe06.iad...

>Is it every appropriate to throw in a dtor? I am thinking about a simple
>example of a wrapper around a POSIX file...
>______________ _______________ _______________ _______________ _____________

[...]

>______________ _______________ _______________ _______________ _____________

[...]
>

>how would you advise me to handle the case above? I am interested in how
>throwing in a dtor effects dynamic destruction... Would something like
>the following be legal?
>>

[...]
>
I am doing some experimenting, and found that throwing from a dtor
apparently leaves the object fully intact wrt the memory that makes it up
so that proper disaster cleanup can indeed be performed... For example,
the following program goes into infinite loop:
_______________ _______________ _______________ _______________ __________
#include <cstdio>
>
struct throw_on_dtor {};
>
class foo {
public:
~foo() {
throw throw_on_dtor() ;
}
};
>
int main(void) {
foo* f = new foo();
retry:
try {
delete f;
} catch (throw_on_dtor const& e) {
std::puts("thro w_on_dtor caught!");
goto retry;
}
return 0;
}
>
_______________ _______________ _______________ _______________ __________
>
>
>
>
So, AFAICT, throwing from a dtor will complicate some odd complications.

^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^ ^

let me rephrase:

So, AFAICT, throwing from a dtor will _create_ some odd complications.. .




Humm... I am now thinking that instead of throwing from dtor, all error
handling should be preformed within dtor... However, what if user wants to
be informed of any failure case within dtor? Should I provide a simple
callback function to inform user of such condition? Something like:
_______________ _______________ _______________ ______________
class file {
FILE* m_handle;
bool (*m_fp_on_dtor_ error) (file&, int);

public:
foo(fp_on_dtor_ error fp = NULL)
: m_fp_on_dtor_er ror(fp) {
[...]
};

~foo() {
retry:
if (! fclose(m_handle )) {
if (m_fp_on_dtor_e rror) {
if (m_fp_on_dtor_e rror(*this, errno)) {
goto retry;
}
}
}
}
};
_______________ _______________ _______________ ______________




Humm... I need ADVISE!

;^o

[anon]

Re: throwing dtors...

Chris M. Thomasson wrote:

Is it every appropriate to throw in a dtor? I am thinking about a simple
example of a wrapper around a POSIX file...

Take a look here:

http://www.parashift.com/c++-faq-lite/dtors.html#faq-11.13

_______________ _______________ _______________ _______________ ____________
class file {
FILE* m_handle;
>
public:
// [...];
>
~file() /* throw() */ {
int const status fclose(m_handle );
if (status) {
/* shi% hit the fan:

http://www.opengroup.org/onlinepubs/007908775/xsh/fclose.html

/*
>
// [what now?]
}
}
};
_______________ _______________ _______________ _______________ ____________
>
>
How to properly handle `EAGAIN' in dtor? Well, what about any error for
that matter? I am a C programmer and only code C++ for fun, and some
in-house projects. If I were really going to create C++ application and
release it into the wild, well, how would you advise me to handle the
case above? I am interested in how throwing in a dtor effects dynamic
destruction... Would something like the following be legal?
>

How would you handle it in C?

>
>
<pseudo code!!!!>
_______________ _______________ _______________ _______________ ___
struct throw_from_dtor {
int const m_status;
>
public:
throw_from_dtor (int const status)
m_status(status ) {}
>
int get_status() const { return m_status; }
};
>
class file {
FILE* m_handle;
>
public:
// [ctor];
>
~file() {
int const status = fclose(m_handle );
if (status) {
throw throw_from_dtor (status);
}
}
};
>
>
int main() {
file* f = new file();
try {
delete f;
} catch(throw_fro m_dtor const& e) {
// handle error from `e.get_status() '
delete f;
}
return 0;
}
_______________ _______________ _______________ _______________ ___
>
>
?
>
>
or what about using smart pointer...
>
>
>
int main() {
std::auto_ptr<f ilef;
try {
f.reset(new file());
} catch(throw_fro m_dtor const& e) {
// handle error from `e.get_status() '
}
}
>
>
>
?
>

These two mains are almost the same (at least they are doing the same thing.

>
>
>
Please keep in mind that refusing to not handle an error from `fclose'
could resule is HORRIBLE things down the road... Think massive data
lost... Perhaps __permanent__ data-! OUCH!!!
>

What can you do when fclose fails?

[Chris M. Thomasson]

Re: throwing dtors...

"anon" <anon@no.invali dwrote in message
news:gc1n78$kks $1@news01.versa tel.de...

Chris M. Thomasson wrote:

>Is it every appropriate to throw in a dtor? I am thinking about a simple
>example of a wrapper around a POSIX file...

>
Take a look here:
http://www.parashift.com/c++-faq-lit...html#faq-11.13

Okay; I will give it a look.

[...]

How would you handle it in C?

[...]

What can you do when fclose fails?

Well, it depends on the error:

http://www.opengroup.org/onlinepubs/007908775/xsh/fclose.html

For instance, fclose can get interrupted by a signal. In this case, you need
to reissue the operation; e.g:
_______________ _______________ _______________ _______________ __
struct file {
FILE* handle;
};

int file_close(
struct file* const this
) {
int status;
do {
status = fclose(this->handle);
} while (status == EOF && errno == EINTR);
return status;
}
_______________ _______________ _______________ _______________ __




Or, what if it returns `EAGAIN', well, this certainly needs to be handled.
However, it would be better to let the application to handle this, not do it
implicitly within the `file_close()' function. There are many ways to handle
this. That's not the problem. The problem is when a retarded program does
not handle it! IHO, any program that does not explicitly handle errors from
`fclose()' is severely broken and VERY dangerous. Let me give you a
example... Imagine a C++ wrapper around a C FILE... Fine. Imagine the dtor
looks like this:



class file {
FILE* m_handle;

public:
~file() throw() {
fclose(m_handle );
}
};



Fine... Now, a user needs to copy a file to a disk, and destroy the
original. Okay. It creates two file objects (e.g., src and dest:)



{
file src(...);
file dest(...);

// Then it performs the copy operation:

[copy src to dest]
}



Now the code-block goes out of scope, and no exceptions were thrown during
the copy process, HOWEVER, the call to `fclose()' in the dest object
failed!!!! Well, the user thinks everything is fine because the completely
retarded ignorant moron file object did not report the fuc%ing error! So the
user operates in ignorance and happily destroys the original file thinking
that the file was COMPLETELY copied onto the disk! WRONG! The file was
partially copied because `fclose()' failed to do its thing and properly
flush the buffers, or whatever... Now, the missing file data is LOST
__forever__! OUCH!!!


This is why its ESSENTIAL to report and handle errors from `fclose()'... If
`fclose()' fails, you can't be so sure that the file is 100% coherent...


Any thoughts?

[Chris M. Thomasson]

Re: throwing dtors...

"anon" <anon@no.invali dwrote in message
news:gc1n78$kks $1@news01.versa tel.de...

Chris M. Thomasson wrote:

>Is it every appropriate to throw in a dtor? I am thinking about a simple
>example of a wrapper around a POSIX file...

>
Take a look here:
http://www.parashift.com/c++-faq-lit...html#faq-11.13

[...]

Well, AFAICT, it seems like C++ destructors are really not all that good for
gracefully handling critical shutdown operations in dtors, such as
`fclose()'. Where am I going wrong? It seems like the only way to avoid
throwing from a dtor would be doing something along the lines of:


// example on how to handle EINTR or perhaps even EAGAIN
_______________ _______________ _______________ _______________ __________
class file {
FILE* m_handle;

// returns true to retry; or false to continue...
bool (*m_fp_on_fclos e_dtor) (int);

public:
file(bool (*fp_on_fclose_ dtor) (int) = NULL, ...)
: m_fp_on_fclose_ dtor(fp_on_fclo se_dtor) {
// [...];
}

~file() throw() {
while (fclose(m_handl e) == EOF) {
if (m_fp_on_fclose _dtor && !
m_fp_on_fclose_ dtor(errno)) {
break;
}
}
}
};




static bool on_fclose_dtor( int status) {
switch (status) {
case EINTR:
return true;

case EAGAIN:
sleep(1);
return true;

default:
std::terminate( );
}
}


int main() {
{
file f(on_fclose_dto r, ...);
// [...];
}
return 0;
}
_______________ _______________ _______________ _______________ __________





Now, if a signal is thrown, or if the operation would block, the operation
will at least try to gracefully complete. All other errors KILL the program
dead... Sounds good to me.




However, this looks like a horrible hack. There has to be a MUCH between way
indeed!

:^o

[Paavo Helde]

Re: throwing dtors...

"Chris M. Thomasson" <no@spam.invali dkirjutas:

Is it every appropriate to throw in a dtor? I am thinking about a
simple example of a wrapper around a POSIX file...
_______________ _______________ _______________ _______________ ___________
_ class file {
FILE* m_handle;
>
public:
// [...];
>
~file() /* throw() */ {
int const status fclose(m_handle );
if (status) {
/* shi% hit the fan:

http://www.opengroup.org/onlinepubs/007908775/xsh/fclose.html

/*
>
// [what now?]
}
}
};
_______________ _______________ _______________ _______________ ___________
_
>
>
How to properly handle `EAGAIN' in dtor? Well, what about any error
for that matter? I am a C programmer and only code C++ for fun, and
some in-house projects. If I were really going to create C++
application and release it into the wild, well, how would you advise
me to handle the case above? I am interested in how throwing in a dtor
effects dynamic destruction... Would something like the following be
legal?

Throwing from a dtor is not really advisable in C++. It can easily lead
to duplicate throws during stack unwinding, and calling terminate() as
the result.

The C++ RAII model is built up on the assumption that releasing the
resource always succeeds (or its failure can be ignored by upper levels).
If this is not the case, then the application logic becomes very complex
immediately, essentially you are back in C again.

In any case, I would suggest to move any activity which can fail out of
the destructor, into a separate member function which has to be called
explicitly before destroying of the object, possibly from inside a try-
catch block dealing with errors.

In regard of this example, for most applications, fclose() failing
indicates that the disk is full. What can you do about this? Try to
delete some random other files from the disk? For most applications I
believe a proper behavior would be to try to log the error somewhere,
then either continue or abort, depending on the application type.

If the file integrity is of the most importance, e.g. in case of a
database program, this has to be managed explicitly anyway by storing
something like transaction completion markers in the file itself, or
whatever. I bet this is not trivial.

hth
Paavo

[anon]

Re: throwing dtors...

Chris M. Thomasson wrote:

"anon" <anon@no.invali dwrote in message
news:gc1n78$kks $1@news01.versa tel.de...

>Chris M. Thomasson wrote:

>>Is it every appropriate to throw in a dtor? I am thinking about a
>>simple example of a wrapper around a POSIX file...

>>
>Take a look here:
>http://www.parashift.com/c++-faq-lit...html#faq-11.13

[...]
>
Well, AFAICT, it seems like C++ destructors are really not all that good
for gracefully handling critical shutdown operations in dtors, such as
`fclose()'. Where am I going wrong? It seems like the only way to avoid
throwing from a dtor would be doing something along the lines of:
>

Destructors have to cleanup its objects, and they should (must) not fail.

>
// example on how to handle EINTR or perhaps even EAGAIN
_______________ _______________ _______________ _______________ __________
class file {
FILE* m_handle;
>
// returns true to retry; or false to continue...
bool (*m_fp_on_fclos e_dtor) (int);
>
public:
file(bool (*fp_on_fclose_ dtor) (int) = NULL, ...)
: m_fp_on_fclose_ dtor(fp_on_fclo se_dtor) {
// [...];
}
>
~file() throw() {
while (fclose(m_handl e) == EOF) {
if (m_fp_on_fclose _dtor && !
m_fp_on_fclose_ dtor(errno)) {
break;
}
}
}
};
>
>

IMO This would be better:

class file {
FILE* m_handle;

public:
file()
{
// [...];
}

~file()
{
try
{
close_file();
}
// catch other exceptions
catch(...)
{
// log the error
}
}

void close_file()
{
// do whatever you can to close the file
// throw an exception in a case of an error
}

};

int main()
{
try
{
file obj;
// do stuff
obj.close_file( );
}
catch(...)
{
// log error
// try to repair the damage
}
}

>
Now, if a signal is thrown, or if the operation would block, the
operation will at least try to gracefully complete. All other errors
KILL the program dead... Sounds good to me.
>
>

With all other errors, your file will not be closed, and you have a
terminated program. Not very elegant solution ;)

[James Kanze]

Re: throwing dtors...

On Oct 2, 5:25 am, "Chris M. Thomasson" <n...@spam.inva lidwrote:

Is it every appropriate to throw in a dtor?

Sure. There are special cases where the only reason to have a
destructor is for it to throw.

All such cases are, however, special cases, and objects of those
types should only exist in special contexts (typically, as
temporaries in a single expression).

I am thinking about a simple example of a wrapper around a
POSIX file...

That one definitly shouldn't throw.

_______________ _______________ _______________ _______________ ____________
class file {
FILE* m_handle;

public:
// [...];

~file() /* throw() */ {
int const status fclose(m_handle );
if (status) {
/* shi% hit the fan:

http://www.opengroup.org/onlinepubs/007908775/xsh/fclose.html

/*
// [what now?]
}
}
};
_______________ _______________ _______________ _______________ ____________

If you get to the destructor and the file hasn't been closed,
it's an error. It should only happen in two cases: your
unwinding the stack as a result of another error (which will
result in the generated file being deleted, or at least marked
as invalid), or there is an error elsewhere in the code (which
should result in an assertion failure).

How to properly handle `EAGAIN' in dtor? Well, what about any
error for that matter? I am a C programmer and only code C++
for fun, and some in-house projects. If I were really going to
create C++ application and release it into the wild, well, how
would you advise me to handle the case above?

Require an explicit close by the user, before the object is
destructed, and return a return code from that function.

FWIW: most of my file output is through a file wrapper class
whose destructor deletes the file if it is called before the
file is "committed" ; it also has an option for linking several
such wrappers, so that all of the files will be deleted unless
all have been successfully "committed" . (Also, my shutdown
routines flush cout, and generate an error if that fails.)

[...]

Please keep in mind that refusing to not handle an error from
`fclose' could resule is HORRIBLE things down the road...

Obviously. And since the error must be handled, you never count
on the destructor for the close. (In the normal case---it's
fine if you're cleaning up after another error, and are going to
delete the file anyway as a result of the other error.)

--
James Kanze (GABI Software) email:james.kan ze@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

[James Kanze]

Re: throwing dtors...

On Oct 2, 7:47 am, anon <a...@no.invali dwrote:

[...]

What can you do when fclose fails?

At the very least, you must notify the user. Generally, you'll
want to delete the file, and usually return an error code to the
OS.

About the only exception I can think of is a log file. If
closing that fails, you probably want to continue anyway,
ignoring the error. (Theoretically, you should log the
error:-).)

--
James Kanze (GABI Software) email:james.kan ze@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

[James Kanze]

Re: throwing dtors...

On Oct 2, 9:20 am, "Chris M. Thomasson" <n...@spam.inva lidwrote:

"Paavo Helde" <nob...@ebi.eew rote in message

[...]

For most applications I believe a proper behavior would be
to try to log the error somewhere, then either continue or
abort, depending on the application type.

What if the application needs to copy a file to disk and
destroy the original? If `fclose()' fails on the destination
file, well, the application won't know about it and will
continue on and destroy the source file. Well, the
destination file is by definition in a non-coherent state
because `fclose()' failed to "do its thing". Well, the lost
data is gone forever. A log file will only show why the data
was lost, it does not prevent it. In this case I bet the user
wished the application just terminated when the `fclose()'
failed. Or better, I bet the user would like to be able to
catch and explicitly handle this case...

This is why it is important in such cases to return an error
status (EXIT_FAILURE) from main. A typical idiom in Unix would
be something like:

program filename tempFile && mv tempFile filename

And all too often, the program won't flush standard out and
verify that the flush worked. (All to many programs totally
ignore the fact that output might fail.) With the result that
if you fill up the disk, you loose important data.

--
James Kanze (GABI Software) email:james.kan ze@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

[Paavo Helde]

Re: throwing dtors...

"Chris M. Thomasson" <no@spam.invali dkirjutas:

>
"Paavo Helde" <nobody@ebi.eew rote in message
news:Xns9B2B65D 8AFDA9nobodyebi ee@216.196.97.1 31...

>"Chris M. Thomasson" <no@spam.invali dkirjutas:
>>

>>Is it every appropriate to throw in a dtor? I am thinking about a
>>simple example of a wrapper around a POSIX file...
>>_____________ _______________ _______________ _______________ ___________
>>__

[...]

>>
>Throwing from a dtor is not really advisable in C++. It can easily
>lead to duplicate throws during stack unwinding, and calling
>terminate() as the result.
>>
>The C++ RAII model is built up on the assumption that releasing the
>resource always succeeds (or its failure can be ignored by upper
>levels). If this is not the case, then the application logic becomes
>very complex immediately, essentially you are back in C again.
>>
>In any case, I would suggest to move any activity which can fail out
>of the destructor, into a separate member function which has to be
>called explicitly before destroying of the object, possibly from
>inside a try- catch block dealing with errors.

>
I think I agree here. Since, IMVHO, at least attempting to gracefully
handle `fclose()', such as deferred retrying in the case of `EINTR' or
`EAGAIN', is extremely important. Therefore, it sure seems to make
sense to force the used to explicitly call a member function which
invokes `fclose()' and throws when a very bad error its encountered
(e.g., something other than EINTR or EAGAIN).
>

Exceptions are for communicating situations which cannot be dealt locally
to the upper levels in the application logic. If the error can be dealt
locally, there is no need for an exception.

I am not familiar with EAGAIN, but can't this be handled locally as well?


[...]

What if the application needs to copy a file to disk and destroy the
original? If `fclose()' fails on the destination file, well, the
application won't know about it and will continue on and destroy the

Apparently, it has to know about this in this case. So fclose() result
has to be somehow communicated to the calling application. Unfortunately
it cannot be done just by throwing an exception from the destructor.
Well, in principle there is the uncaught_except ion() function which one
could check in the dtor and decide whether to throw a new exception or if
there is one already in the progress, and so no new exception is needed.
However, there were some gotchas which made uncaught_except ion() mostly
useless IIRC.

Paavo

[Yannick Tremblay]

Re: throwing dtors...

In article <X5_Ek.12946$ex 3.3200@newsfe02 .iad>,
Chris M. Thomasson <no@spam.invali dwrote:

>"anon" <anon@no.invali dwrote in message
>news:gc1n78$kk s$1@news01.vers atel.de...

>Chris M. Thomasson wrote:

>>Is it every appropriate to throw in a dtor? I am thinking about a simple
>>example of a wrapper around a POSIX file...

>>
>Take a look here:
>http://www.parashift.com/c++-faq-lit...html#faq-11.13

>
>Okay; I will give it a look.
>
>[...]
>

>How would you handle it in C?

>[...]
>

>What can you do when fclose fails?

>
>Well, it depends on the error:
>
>http://www.opengroup.org/onlinepubs/...sh/fclose.html
>
>For instance, fclose can get interrupted by a signal. In this case, you need
>to reissue the operation; e.g:
>______________ _______________ _______________ _______________ ___
>struct file {
FILE* handle;
>};
>
>int file_close(
struct file* const this
>) {
int status;
do {
status = fclose(this->handle);
} while (status == EOF && errno == EINTR);
return status;
>}

That's precisely your answer:

Do exactly what you would do in C in your file_close method
and call it from your destructor:

File::~File()
{
if(0 != file_close()) {
// log the error
// or std::terminate?
}
}

int File::file_clos e()
{
int status = 0;
do {
status = fclose(handle);
} while (status == EOF && errno == EINTR);
return status;
}

>Or, what if it returns `EAGAIN', well, this certainly needs to be handled.

So handle EAGAIN. 2 ways:

1- in file_close()
2- if impossible in file_close() then you must require that the client do
it by explicitely calling file_close() and dealing with errors.

>Fine... Now, a user needs to copy a file to a disk, and destroy the
>original. Okay. It creates two file objects (e.g., src and dest:)
>
>{
file src(...);
file dest(...);
>
// Then it performs the copy operation:
>
[copy src to dest]
>}
>
>
>
>Now the code-block goes out of scope, and no exceptions were thrown during
>the copy process, HOWEVER, the call to `fclose()' in the dest object
>failed!!!! Well, the user thinks everything is fine because the completely
>retarded ignorant moron file object did not report the fuc%ing error! So the
>user operates in ignorance and happily destroys the original file thinking
>that the file was COMPLETELY copied onto the disk! WRONG! The file was
>partially copied because `fclose()' failed to do its thing and properly
>flush the buffers, or whatever... Now, the missing file data is LOST
>__forever__! OUCH!!!

That's a failure of the application

foo()
{
file src(...)
file dest(...)
dest.copy(src);

// Must ensure that dest was correctly flushed/sync/close *before*
// deleting src
if(0 == dest.file_close ())
{
src.delete();
} else {
// whatever is needed to recover
}
}

[Chris M. Thomasson]

Re: throwing dtors...

"Chris M. Thomasson" <no@spam.invali dwrote in message
news:k4XEk.1619 9$hX5.2021@news fe06.iad...

Is it every appropriate to throw in a dtor? I am thinking about a simple
example of a wrapper around a POSIX file...

[...]

Thank you all (e.g., anon, James Kanze and Paavo Helde) for your excellent
insight. After absorbing your input, I think the way to go would be
something like; thread-safety wrt the `file::m_handle ' member aside for a
moment:
_______________ _______________ _______________ _______________ _____
#if ! defined(FILE_DT OR_UNEXPECTED)
# define FILE_DTOR_UNEXP ECTED(mp_file) assert(false)
#endif


class file {
FILE* m_handle;


public:
struct error {
struct base {
int const m_status;
base(int const status) : m_status(status ) {}
};

struct bad_descriptor : public base {
bad_descriptor( ) : base(EBADF) {}
};

struct exceeds_offset : public base {
exceeds_offset( ) : base(EFBIG) {}
};

struct process_orphane d : public base {
process_orphane d() : base(EIO) {}
};

struct no_free_space : public base {
no_free_space() : base(ENOSPC) {}
};

struct pipe_not_for_re ading : public base {
pipe_not_for_re ading() : base(EPIPE) {}
};

struct non_existing_de vice : public base {
non_existing_de vice() : base(ENXIO) {}
};

struct already_closed : public base {
already_closed( ) : base(0) {}
};

struct unknown : public base {
unknown() : base(0) {}
};

static void throw_status(in t const status) {
assert(! status);
switch (status) {
case EBADF:
throw bad_descriptor( );
case EFBIG:
throw exceeds_offset( );
case EIO:
throw process_orphane d();
case ENOSPC:
throw no_free_space() ;
case EPIPE:
throw pipe_not_for_re ading();
case ENXIO:
throw non_existing_de vice();
default:
throw unknown();
}
}
};


private:
int prv_close() throw() {
int status;
do {
status = std::fclose(m_h andle);
} while (status == EOF && errno == EINTR);
return status;
}


public:
bool close(bool handle_eagain = true, unsigned backoff = 1) {
if (! m_handle) {
throw error::already_ closed();
}
retry:
int status = prv_close();
if (status == EOF) {
if (errno != EAGAIN) {
error::throw_st atus(errno);
} else if (handle_eagain) {
sleep(backoff);
goto retry;
}
return false;
}
m_handle = NULL;
return true;
}


file(/* [...] */) : m_handle(NULL) {
// [...];
}

// [...];

~file() throw() {
if (m_handle) {
if (prv_close() == EOF) {
FILE_DTOR_UNEXP ECTED(m_handle) ;
}
}
}
};
_______________ _______________ _______________ _______________ _____





And explicitly document that `file::close()' returns true if everything
worked, false if the file was non-blocking and the operation would have
blocked, or throws if shi% hit the fan. Then note that `file::close()' only
returns false when the `file::close(ha ndle_eagain)' parameter is false.


Although James Kanze seems to suggest that `file::close()' should return a
error code; perhaps the value from errno. However, this approach requires
the user to decipher the errno value and act accordingly. Where the
fine-grain exception thing my example code uses decodes errno into specific
exception types that the user can catch. AFAICT, I think the fine-grain
exceptions are more C++'ish than return specific error codes. The fact that
`file::close()' returns a simple bool when a `EAGAIN' is encountered should
be okay.


Now an application could do something like; taking my file-copy example:
_______________ _______________ _______________ _______________ _____
void foo() {
try {
file src(...);
file dest(...);
dest.copy(src);
src.close();
dest.close();
src.delete();
} catch (file::error::b ad_descriptor const& e) {
// [...];
} catch (file::error::e xceeds_offset const& e) {
// [...];
} catch (file::error::p rocess_orphaned const& e) {
// [...];
} catch (file::error::n o_free_space const& e) {
// [...];
} catch (file::error::p ipe_not_for_rea ding const& e) {
// [...];
} catch (file::error::n on_existing_dev ice const& e) {
// [...];
} catch (file::error::a lready_closed const& e) {
// [...];
} catch (file::error::u nknown const& e) {
// [...];
}
}
_______________ _______________ _______________ _______________ _____





or if the application does not care which "specific" error caused things to
go bad it could do:
_______________ _______________ _______________ _______________ _____
void foo() {
try {
file src(...);
file dest(...);
dest.copy(src);
src.close();
dest.close();
src.delete();
} catch (file::error::b ase& e) {
// [...];
}
}
_______________ _______________ _______________ _______________ _____





Does that look Kosher to you C++ gurus?


;^)