Re: [boost] read 4096 bits stream class problem

From: Jonathan Turkanis (technews_at_[hidden])
Date: 2005-11-14 17:08:27

Hendra Hidayat wrote:
> Hi,
> I have a problem when using stream class from
> iostreams library. I have created a device class say
> GDevice using ios::device_tag and ios::input_seekable
> tag. In GDevice I  've declared and implemented read
> function inside GDevice  class.  I also create a
> stream class let's say GStream which derived from
> ios::stream<GDevice>. The problem was occured when I
> created object from GStream and call GStream's read
> method directly to read say n bytes data. The read
> method invoked GDevice read method not in n bytes data
> but always in 4096 bits. I dont understand where this
> number came from. I'm using boost 1.33, gcc compiler
> version 4.0.2 under x86-64 bit suse linux. Can
> somebody help me?

Instances of stream<> are buffered by default. This means that they read ahead
in increments of the buffer size, so that subsequent read operations will be
more efficient. You can change this behavior by passing a buffer size to the
constructor or open() function. (This doesn't work for the forwarding
constructors and open() overloads, which are provided as a convenience.)

For example,

   stream<GDevice> str;
   str.open(GDevice(), 200); // buffer size of 200
   str.close();
   ...
   str.open(GDevice(), 0); // (Almost) unbuffered

In the second commented example, the stream may still have a small input buffer
to allow a character to be put back.

I may give the user more explicit control of buffering in the next release,
perhaps using the parameters library to keep the interface simple.

> Faithfully yours
>
> Hendra Hidayat

-- 
Jonathan Turkanis
www.kangaroologic.com