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From: Rem on 21 Feb 2010 13:21
I have a third party class X that stores and loads its objects using
std::ostream and std::istream classes. At the same time I'm using X
objects inside an environment that only allows me to save chunks of
memory c-style. What I would like to do then is to store X instance in
std::iostream instance and from that get the underlying array of
chars. But the problem is I don't know how to determine to size of the
buffer needed - there is no method of std::istream that returns the
number of chars used, is it?


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From: Michael Doubez on 21 Feb 2010 21:35
On 22 f�v, 07:21, Rem <therealr...(a)gmail.com> wrote:
> I have a third party class X that stores and loads its objects using
> std::ostream and std::istream classes. At the same time I'm using X
> objects inside an environment that only allows me to save chunks of
> memory c-style. What I would like to do then is to store X instance in
> std::iostream instance and from that get the underlying array of
> chars.

Do you mean, you want to use a X has a sink for your iostream ?
A minimal implementation is creating a structure inheriting streambuf
and redefining the underflow() member function.

> But the problem is I don't know how to determine to size of the
> buffer needed - there is no method of std::istream that returns the
> number of chars used, is it?

std::istream::gcount() returns the number of chars extracted by the
last unformatted read().
std::istream::tellg() may or may not be accurate depending on how you
use your stream.

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Michael


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From: Bart van Ingen Schenau on 21 Feb 2010 21:35
On Feb 22, 7:21 am, Rem <therealr...(a)gmail.com> wrote:
> I have a third party class X that stores and loads its objects using
> std::ostream and std::istream classes. At the same time I'm using X
> objects inside an environment that only allows me to save chunks of
> memory c-style. What I would like to do then is to store X instance in
> std::iostream instance and from that get the underlying array of
> chars. But the problem is I don't know how to determine to size of the
> buffer needed - there is no method of std::istream that returns the
> number of chars used, is it?

Not in std:;istream no, but that class is designed to be used as a
base-class.
You could use (the deprecated) strstream or std::stringstream classes
to let X load/store itself to/from respectively a memory buffer or a
std::string object.

If your problem is knowing where to continue in the buffer after X
hasa consumed its portion, you could extract the remainder of the
stream into a std::string object using getline().

Bart v Ingen Schenau


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From: Ulrich Eckhardt on 21 Feb 2010 21:35
Rem wrote:
> I have a third party class X that stores and loads its objects using
> std::ostream and std::istream classes. At the same time I'm using X
> objects inside an environment that only allows me to save chunks of
> memory c-style. What I would like to do then is to store X instance in
> std::iostream instance and from that get the underlying array of
> chars. But the problem is I don't know how to determine to size of the
> buffer needed - there is no method of std::istream that returns the
> number of chars used, is it?

Have you taken a look at std::stringstreams? Those read/write from a string
instead of a file. If that doesn't work or doesn't perform well enough, you
could also write a streambuffer that directly uses the underlying C-style
API.


Uli

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From: Davis King on 22 Feb 2010 05:57
On Feb 22, 1:21 am, Rem <therealr...(a)gmail.com> wrote:
> I have a third party class X that stores and loads its objects using
> std::ostream and std::istream classes. At the same time I'm using X
> objects inside an environment that only allows me to save chunks of
> memory c-style. What I would like to do then is to store X instance in
> std::iostream instance and from that get the underlying array of
> chars. But the problem is I don't know how to determine to size of the
> buffer needed - there is no method of std::istream that returns the
> number of chars used, is it?

Have you looked at the std::ostringstream and std::istringstream
objects? I think they might be just what you are looking for. They
allow you to get a copy of the data in the stream in the form of a
std::string.

Cheers,
Davis


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