Re: Advice about deriving class from streambuf ...

Ulrich Eckhardt <>
12 Jan 2007 09:50:05 -0500
<> wrote:

I'm writing a set of classes intended to shim the way a midly exotic
operating environment--an extensible DBMS in this case--handles I/O.
The idea is that I have a set of classes already written that serialize
themseves into files in the orthodox fashion. That is I have:

class Synopses
 SIZE_T read ( std::istream * fin );
 SIZE_T write ( std::ostream * fout );

I don't see what is 'orthodox' about that, I find this style rather
peculiar. The points are:
- write() isn't a const operation
- the sense of the returntype is unclear
- the parameters are probably not allowed to be null pointers, so I would
expect references instead

class blobbuf : public streambuf
    blobbuf ( blobhandle * pBLOB,
                           blobtype aType,
                           blobmode aMode );

Same as above, the pointer to the blobhandle is probably not allowed to be
zero, right? Also, do you really mean a pointer to a handle or a handle?

class oblobstream : public ostream {


    oblobstream ( blobhandle * pBLOB,
                         blobtype aType ) :
                ostream(new blobbuf (pBLOB, aType, a)){}

The parameter 'a' is undefined, so I guess this code is rewritten instead of
cut'n'pasted into your posting. That means that it is unusable for
analysis. Anyhow, again I wonder why you are using 'new' here? Instead,
pass a null pointer to the baseclass and then call rdbuf() in the body of
your constructor giving it the pointer to a member blobbuf. You shouldn't
directly pass that pointer because at the time the base class is
constructed that member isn't constructed yet. I'm not 100% sure, but I
think that this particular case is guaranteed to work nontheless because
ostream won't touch the streambuffer yet.

Well, the constructor barfs. Ick! So, thinking it's my code, I rack up
the following on the old test harness.

  blobbuf * dbb = new blobbuf ( outClob, ascii, r );
  ostream * pos = new ostream ( dbb );

Gah, why would you use 'new' unless you had to? This just makes your code
fragile in the presence of exceptions and puts way too much burden on the
maintainer to keep track of allocated resources. Remember, this isn't Java
where you have garbage collection by default.

So perhaps overloading streambuf() was not the wisest of choices for a
first foray into C++. But any clues would be gratefully accepted.

You can simply overload the overflow() function and see it already work:

  int overflow( int c) {
    return c;

This should enable you to redirect any output to stdout. If that works, at
least you didn't make any mistakes in deriving the streambuffer. However, I
don't think you ever reach the point where it calls this function...

good luck


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