Is there a way to save a copy of std::cout

[Adrian]

Is it possible to save a copy of cout the same way you can save a copy
of stdout in C (I know C version is portable, but unix portable is
good enough for me).

I have to use a library which closes stdout on init because its used
to mostly in background processes but I need it for an interactive
process. Library cant be changed unfortunately.

I tried the following but the C++ version doesn't work, I guess
because it only shares the underlying streambuf.

#ifdef C_VERSION
#include <stdio.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
int copy_of=dup(fileno(stdout));
fprintf(stdout, "This is before on original\n");
fclose(stdout);
fprintf(stdout, "This is after on original\n");

stdout=fdopen(copy_of, "w");

fprintf(stdout, "This is after on original after copy\n");

// Output:
// This is before on original
// This is after on original after copy

return 0;
}
#else
#include <iostream>

int main(int argc, char *argv[])
{
std::ostream copy_of(std::cout.rdbuf());

std::cout << "This is before on original\n";
copy_of << "This is before on copy\n";
fclose(stdout);
std::cout << "This is after on original\n";

copy_of << "This is after on copy\n";

// Output:
// This is before on original
// This is before on copy

return 0;
}
#endif

[Victor Bazarov]

Adrian wrote:

Is it possible to save a copy of cout the same way you can save a copy
of stdout in C (I know C version is portable, but unix portable is
good enough for me).

(a) What is "save a copy of cout"? Output streams are not copyable
objects, so the answer is most likely "no, unless you mean something else".

(b) "Unix portable" is an oxymoron. If it's only on one OS, it's not
"portable".

(c) If you need Unix-specific solution, you should ask in a Unix
newsgroup, don't you think?

[..]

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[Adrian]

On Aug 20, 1:33 pm, Victor Bazarov <[e-mail]:

(a) What is "save a copy of cout"? Output streams are not copyable
objects, so the answer is most likely "no, unless you mean something else".

(b) "Unix portable" is an oxymoron. If it's only on one OS, it's not
"portable".

(c) If you need Unix-specific solution, you should ask in a Unix
newsgroup, don't you think?

Good to see you are as acerbic as ever ;-)

a) Thanks so its not copyable
b) Not really - there are many flavours of unix
c) I didnt ask for one but would accept one :-)

Another question then, if the FILE * stdout has been closed/redirected
is there any way to revert std::cout back to state it had before the
closing/redirection

Or if I set stdout back to a valid FILE * will std::cout use it or is
it not defined if std::cout uses stdout.

Adrian

[Victor Bazarov]

Adrian wrote:

[..]
Another question then, if the FILE * stdout has been closed/redirected
is there any way to revert std::cout back to state it had before the
closing/redirection

I don't see anything that would allow you to manipulate 'cout', except
making a different stringbuf and stuffing it into 'cout'.

Or if I set stdout back to a valid FILE * will std::cout use it or is
it not defined if std::cout uses stdout.

I do not know of any connection between 'std::cout' and FILE* or between
'std::cout' and 'stdout'. It is quite possible that internally 'cout'
does output to 'stdout' or they both use the same mechanism (common
denominator), but IIRC it's not mandated by the Standard. I can easily
be wrong, of course, I've not done much work with streams. Let's hope
others will chime in...

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[heiki.ojasild]

Is it possible to save a copy of cout the same way you can save a copy

of stdout in C (I know C version is portable, but unix portable is
good enough for me).

Not directly, as you have been answered earlier. But you can create
stringstream object and output evreything you output to cout into
stringstream object too. And then you are free to do whatever you want
with your data in stringstream.

[heiki.ojasild]

Is it possible to save a copy of cout the same way you can save a copy

of stdout in C (I know C version is portable, but unix portable is
good enough for me).

There is a way to implement something similiar:

#include <sstream>
#include <iostream>

using namespace std;

int main() {
stringstream ss;
cout << "This is a message." << endl;
ss << "This is a message." << endl;
return 0;
}

[heiki.ojasild]

Is it possible to save a copy of cout the same way you can save a copy

of stdout in C (I know C version is portable, but unix portable is
good enough for me).

There is a way to implement something similiar:

#include <sstream>
#include <iostream>

using namespace std;

int main() {
stringstream ss;
cout << "This is a message." << endl;
ss << "This is a message." << endl;
return 0;
}

[Gianni Mariani]

On Aug 21, 3:46*am, Adrian <[e-mail]:

Is it possible to save a copy of cout the same way you can save a copy
of stdout in C (I know C version is portable, but unix portable is
good enough for me).

- I don't know what you mean by copying stdout

However, std::ostream (the type of std::cout) has methods for getting
and setting a strambuf.

streambuf* rdbuf ( ) const;
streambuf* rdbuf ( streambuf* sb );

You can "create" a streambuf to do pretty much anything you want,
including I suspect what you're looking to do.

[Adrian]

On Aug 20, 4:51*pm, Gianni Mariani <[e-mail]:

- I don't know what you mean by copying stdout

Same as for duplicating a file descriptor

>
However, std::ostream (the type of std::cout) has methods for getting
and setting a strambuf.

streambuf* rdbuf ( ) const;
streambuf* rdbuf ( streambuf* sb );

You can "create" a streambuf to do pretty much anything you want,
including I suspect what you're looking to do.

No it isnt, how can I make the following code output everything
(without removing the fclose :-)
#include <iostream>
#include <cstdio>
#include <unistd.h>

int main(int argc, char *argv[])
{
int save_stdout=dup(fileno(stdout));
std::ostream save_cout(std::cout.rdbuf());

std::cout << "std::cout << before close\n";
fprintf(stdout, "fprintf() on stdout before close\n");

fclose(stdout);

std::cout << "std::cout << after close\n";
fprintf(stdout, "fprintf() on stdout after close\n");

stdout=fdopen(save_stdout, "w");
std::cout << "std::cout << after restore stdout\n";
fprintf(stdout, "fprintf() on stdout after restore\n");

std::cout.rdbuf(save_cout.rdbuf());
std::cout << "std::cout << after restore cout\n";
return 0;
}

[Thomas J. Gritzan]

Adrian schrieb:

On Aug 20, 4:51 pm, Gianni Mariani <[e-mail]:

>You can "create" a streambuf to do pretty much anything you want,
including I suspect what you're looking to do.

No it isnt, how can I make the following code output everything
(without removing the fclose :-)

Yes it is.
You have to create a *new* streambuf object an let cout use it.

You can derive a class based on streambuf that uses your duplicated
stdout, or use a filebuf like this:

#include <fstream>
#include <iostream>
#include <stdio.h>

int main()
{
fclose(stdout);

std::streambuf* oldbuf = std::cout.rdbuf();
std::filebuf newbuf;
newbuf.open("somefile", std::ios::out);
std::cout.rdbuf(&newbuf);

std::cout << "Some Text" << std::endl;

// set to original streambuf
std::cout.rdbuf(oldbuf);
}

You could also output to std::cerr or fix the buggy library. :-)

--
Thomas

[Adrian]

On Aug 21, 4:56 pm, "Thomas J. Gritzan" <[e-mail]>
wrote:

Adrian schrieb:
Yes it is.
You have to create a *new* streambuf object an let cout use it.

You can derive a class based on streambuf that uses your duplicated
stdout, or use a filebuf like this:

[snip code]
Thanks, but that not quite what I needed, this outputs to a file where
as I need to output to the original place that stdout/cout does ie the
terminal.

You could also output to std::cerr or fix the buggy library. :-)

Unfortunaley its a feature of the library :-) and same lib also
redirects cerr to logs.

Just digging around the standard I found in 27.3.1.3
"The object cout controls output to a stream buffer associated with
the object stdout, declared in
<cstdio(27.8.2)."

So shouldnt this mean that if I can duplicate/save stdout then cout
should work as a fprintf(stdout) should?

Adrian

[LR]

Adrian wrote:

So shouldnt this mean that if I can duplicate/save stdout

I went looking through what I think is a recent version of a draft
standard for C and it suggests that stdout doesn't have to be an lvalue.
So the code that I think was posted that showed:
stdout = [don't recall what was here]
might not be portable even to various flavors of *nix.

But I'm not sure what version of the C standard the C++ standard
references and it might say something different.

LR