failed to open a stream

[Gary Wessle]

Hi

please help with this.

std::fstream iofs( f.c_str(), std::ios::in|st d::ios::out );
std::cout << f << '\n' << iofs.is_open() << std::endl;

puts out
*************** *************** *************** *************** ****
pair_status/myPair
0
*************** *************** *************** *************** ****

and the file is not created, why is_open() is reporting a failure?
how can I create the file if it is not there besides what I have
already done?

many thanks

[Gary Wessle]

Gary Wessle <ph****@yahoo.c omwrites:

Gary Wessle <ph****@yahoo.c omwrites:

John Harrison <jo************ *@hotmail.comwr ites:

John Harrison wrote:
typo

>
std::fstream iofs( f.c_str(), std::ios::in|st d::ios::out );
if (iofs.is_open() )
if (!iofs.is_open( ))

{
iofs.open( f.c_str(), ios_base::in | ios_base::out |
ios_base::app );
}
>
john
>
No I got that very wrong (maybe I should read th emanual) here's a
better attempt.
>
std::fstream iofs( f.c_str(), std::ios::in|st d::ios::out );
if (!iofs.is_open( ))
{
iofs.open( f.c_str(), ios_base::in | ios_base::out |
ios_base::trunc );
}
>
That will open an existing file for reading and writing, but if it
doesn't exist it will create a new file for reading and writing.
>
john

thank you.

that did create the file, and stepping through with gdb shows that
the if block is opening the file, but few lines down the code I have

iofs << "reversals" << " " << val << std::endl;

that is not printing out any thing to the file that just been created.

whats wrong with this, I am using linux FC5.

thanks

the file which as created has the right permissions.
-rw-r--r-- 1 fred fred 0 Mar 1 10:52 myPair

ok, very strange as it is but I don't know how to fix it.

after the file is created with the above setup, I have

while( !found_it && getline( iofs, line ) )
{
std::stringstre am ss( line );
ss >word;
if( word == lookup )
{
found_it = true;
ss >val;
m_pair_status[lookup] = val;
iofs.close();
return val;
}
}

iofs << "reversals" << " " << val << std::endl;

the while block does not get executed in this case, but it does
something to the stream that makes the line after the while block fail
to print any thing in the open stream.

hummm

[red floyd]

Gary Wessle wrote:

John Harrison <jo************ *@hotmail.comwr ites:

>>
std::fstream iofs( f.c_str(), std::ios::in|st d::ios::out );
if (!iofs.is_open( ))
{
iofs.open( f.c_str(), ios_base::in | ios_base::out |
ios_base::trunc );
}

iofs << "reversals" << " " << val << std::endl;

that is not printing out any thing to the file that just been created.

whats wrong with this, I am using linux FC5.

thanks

I think you need a iofs.clear() inside the braces, before the second
open call.

[Gary Wessle]

Gary Wessle <ph****@yahoo.c omwrites:

John Harrison <jo************ *@hotmail.comwr ites:

John Harrison wrote:

typo
>
>>
>std::fstream iofs( f.c_str(), std::ios::in|st d::ios::out );
>if (iofs.is_open() )
if (!iofs.is_open( ))
>
>{
> iofs.open( f.c_str(), ios_base::in | ios_base::out |
> ios_base::app );
>}
>>
>john

No I got that very wrong (maybe I should read th emanual) here's a
better attempt.

std::fstream iofs( f.c_str(), std::ios::in|st d::ios::out );
if (!iofs.is_open( ))
{
iofs.open( f.c_str(), ios_base::in | ios_base::out |
ios_base::trunc );
}

That will open an existing file for reading and writing, but if it
doesn't exist it will create a new file for reading and writing.

john

thank you.

that did create the file, and stepping through with gdb shows that
the if block is opening the file, but few lines down the code I have

iofs << "reversals" << " " << val << std::endl;

that is not printing out any thing to the file that just been created.

whats wrong with this, I am using linux FC5.

thanks

the file which as created has the right permissions.
-rw-r--r-- 1 fred fred 0 Mar 1 10:52 myPair

[David Harmon]

On 01 Mar 2007 09:26:47 +1100 in comp.lang.c++, Gary Wessle
<ph****@yahoo.c omwrote,

std::fstream iofs( f.c_str(), std::ios::in|st d::ios::out );

if (!iofs) perror(f.c_str( ));

[John Harrison]

Gary Wessle wrote:

Gary Wessle <ph****@yahoo.c omwrites:

>>Gary Wessle <ph****@yahoo.c omwrites:

>>>John Harrison <jo************ *@hotmail.comwr ites:
John Harrison wrote:

>typo
>
>
>>std::fstr eam iofs( f.c_str(), std::ios::in|st d::ios::out );
>>if (iofs.is_open() )
>
>if (!iofs.is_open( ))
>
>
>>{
> iofs.open( f.c_str(), ios_base::in | ios_base::out |
> ios_base::app );
>>}
>>
>>john

No I got that very wrong (maybe I should read th emanual) here's a
better attempt.

std::fstrea m iofs( f.c_str(), std::ios::in|st d::ios::out );
if (!iofs.is_open( ))
{
iofs.open( f.c_str(), ios_base::in | ios_base::out |
ios_base::trunc );
}

That will open an existing file for reading and writing, but if it
doesn't exist it will create a new file for reading and writing.

john

thank you.

that did create the file, and stepping through with gdb shows that
the if block is opening the file, but few lines down the code I have

iofs << "reversals" << " " << val << std::endl;

that is not printing out any thing to the file that just been created.

whats wrong with this, I am using linux FC5.

thanks

the file which as created has the right permissions.
-rw-r--r-- 1 fred fred 0 Mar 1 10:52 myPair

ok, very strange as it is but I don't know how to fix it.

after the file is created with the above setup, I have

while( !found_it && getline( iofs, line ) )
{
std::stringstre am ss( line );
ss >word;
if( word == lookup )
{
found_it = true;
ss >val;
m_pair_status[lookup] = val;
iofs.close();
return val;
}
}

iofs << "reversals" << " " << val << std::endl;

the while block does not get executed in this case, but it does
something to the stream that makes the line after the while block fail
to print any thing in the open stream.

hummm

It's not very strange, it's how it supposed to work.

When you cause an error on a stream (such as reading past the end of
while) the stream is in a error state. When a stream is in an error
state nothing further will work until you clear that error state. Add
the following code.

while( !found_it && getline( iofs, line ) )
{
...
}
iofs.clear(); // clear the error state
iofs << "reversals" << " " << val << std::endl;

john

[John Harrison]

red floyd wrote:

Gary Wessle wrote:

>John Harrison <jo************ *@hotmail.comwr ites:

>>>
std::fstrea m iofs( f.c_str(), std::ios::in|st d::ios::out );
if (!iofs.is_open( ))
{
iofs.open( f.c_str(), ios_base::in | ios_base::out |
ios_base::trunc );
}

iofs << "reversals" << " " << val << std::endl;

that is not printing out any thing to the file that just been created.

whats wrong with this, I am using linux FC5.

thanks

I think you need a iofs.clear() inside the braces, before the second
open call.

Yes you're right. I think this is a case where the standard said
something different to what was intended. But some library writers took
them at their word and so iofs.clear() is necessary before the second
attempt to open.

john

[P.J. Plauger]

"John Harrison" <jo************ *@hotmail.comwr ote in message
news:AQ******** *******@newsfe3-win.ntli.net...

red floyd wrote:

>Gary Wessle wrote:

>>John Harrison <jo************ *@hotmail.comwr ites:

>>>>
std::fstre am iofs( f.c_str(), std::ios::in|st d::ios::out );
if (!iofs.is_open( ))
{
iofs.open( f.c_str(), ios_base::in | ios_base::out |
ios_base::trunc );
}
iofs << "reversals" << " " << val << std::endl;

that is not printing out any thing to the file that just been created.

whats wrong with this, I am using linux FC5.

thanks

I think you need a iofs.clear() inside the braces, before the second open
call.

Yes you're right. I think this is a case where the standard said something
different to what was intended. But some library writers took them at
their word and so iofs.clear() is necessary before the second attempt to
open.

Nope. The C++ Standard captures the behavior of the old, original
iostreams package that accompanies cfront. *All* of us library writers
took them at their word and did the silly thing, since we had no
wiggle room to do the sensible thing instead. IIRC, there's a DR that
might still fix this for the next revision of the C++ Standard.

P.J. Plauger
Dinkumware, Ltd.
http://www.dinkumware.com