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comp.lang.c++

Inheriting from streambuf - xsputn

Raymond Martineau

11/9/2008 6:24:00 AM

I have the following code segment for a class intended to split output
between cout and a file:

class SplitStream : public std::streambuf
{
std::streambuf *x;

public:
SplitStream()
{
x = cout.rdbuf(this);
}

~SplitStream()
{
cout.rdbuf(x);
}

std::streamsize xsputn ( const char * s, std::streamsize n )
{
return x->xsputn(s, n);
}
};

When I try compiling it, I get an error message stating that
"x->xsputn(s, n)" is a call to a protected method (in spite of the
fact that it's being called from a derived class.) What is the actual
cause behind that error message?

The streambuf is working after a few code changed. The call from
xsputn was changed to sputn, along with additional methods overflow
and sync.

For those who want the error message:

a.cpp: In member function `virtual std::streamsize
SplitStream::xsputn(const char*, std::streamsize)':
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/streambuf.tcc:80:
error: `std::streamsize std::basic_streambuf<_CharT,
_Traits>::xsputn(const _CharT*, std::streamsize) [with _CharT = char,
_Traits = std::char_traits<char>]' is protected
a.cpp:45: error: within this context
3 Answers

Kai-Uwe Bux

11/9/2008 6:37:00 AM

0

Raymond Martineau wrote:

> I have the following code segment for a class intended to split output
> between cout and a file:
>
> class SplitStream : public std::streambuf
> {
> std::streambuf *x;
>
> public:
> SplitStream()
> {
> x = cout.rdbuf(this);
> }
>
> ~SplitStream()
> {
> cout.rdbuf(x);
> }
>
> std::streamsize xsputn ( const char * s, std::streamsize n )
> {
> return x->xsputn(s, n);
> }
> };
>
> When I try compiling it, I get an error message stating that
> "x->xsputn(s, n)" is a call to a protected method (in spite of the
> fact that it's being called from a derived class.) What is the actual
> cause behind that error message?

You are calling x->xsputn(), but you do not inherit from *x. Since
x->xsputn() is protected, you get an access violation.

You could call the xsputn() method of the actual base of SplitStream.

[snip]


Best

Kai-Uwe Bux

James Kanze

11/9/2008 12:59:00 PM

0

On Nov 9, 7:24 am, Raymond Martineau <bk...@ncf.ca> wrote:
> I have the following code segment for a class intended to
> split output between cout and a file:

> class SplitStream : public std::streambuf
> {
>         std::streambuf *x;

>         public:
>         SplitStream()
>         {
>                 x = cout.rdbuf(this);
>         }

>         ~SplitStream()
>         {
>                 cout.rdbuf(x);
>         }

>         std::streamsize xsputn ( const char * s, std::streamsize n )
>         {
>                 return x->xsputn(s, n);
>         }
> };

This looks basically like a filtering streambuf, a more or less
standard idiom. (See
http://kanze.james.neuf.fr/articles/flt... and
http://kanze.james.neuf.fr/articles/flt..., for example.)
In which case, the functions you absolutely have to override are
overflow and sync. (The default implementation xsputn will call
overflow, but you may want to override it as well, for
optimization reasons.)

There's code in Boost to do most of the work for you.

> When I try compiling it, I get an error message stating that
> "x->xsputn(s, n)" is a call to a protected method (in spite of
> the fact that it's being called from a derived class.)  What
> is the actual cause behind that error message?

The fact that you're trying to access a protected element in a
streambuf from which you haven't derived. You can access
protected streambuf members in a SplitStream object, but not
protected members in just anything.

> The streambuf is working after a few code changed. The call
> from xsputn was changed to sputn, along with additional
> methods overflow and sync.

So what is the problem. Overflow should forward to sputc, and
xsputn to sputn (and sync to pubsync, if it's relevant).

--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orientée objet/
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Raymond Martineau

11/9/2008 2:47:00 PM

0

On Sun, 9 Nov 2008 04:58:35 -0800 (PST), James Kanze
<james.kanze@gmail.com> wrote:

>> When I try compiling it, I get an error message stating that
>> "x->xsputn(s, n)" is a call to a protected method (in spite of
>> the fact that it's being called from a derived class.)  What
>> is the actual cause behind that error message?
>
>The fact that you're trying to access a protected element in a
>streambuf from which you haven't derived. You can access
>protected streambuf members in a SplitStream object, but not
>protected members in just anything.

Okay, that makes sense. Apparantly, C++ isn't Java, which allows that
form of access.