Re: [Boost-users] [iostream] Overloaded bitshift operators

From: Jonathan Turkanis (turkanis_at_[hidden])
Date: 2008-03-05 12:14:39

Robert Dailey wrote:
> On second thought, even though I've found the operators they don't do 
> what I expected them to do in my tests. So, I have the following stream, 
> which is a seekable device:
> 
> boost::iostreams::stream<MyStreamDevice> IOStream;
> 
> void foo()
> {
>     char data;
>     IOStream >> data;
> }
> 
> I expected here that MyStreamDevice::read() would be called with a 
> request for 1 byte, 

Boost.Iostreams provides buffering automatically, so I would expect that 
the request to read would ask for a full buffer of data. The buffer size 
can be set at construction (http://tinyurl.com/2ptvu2), but is never 
zero because of the need for filters to be able to put back characters. 
Try specifying a buffer size of 0 and a putback buffer size of 1 if you 
want a small buffer.

What behavior are you observing?

> since the sizeof( data ) is 1 byte. I want my stream 
> to act as binary, so the bitshift operators should attempt to "fill" the 
> right operand with as much as it can. For longs, this would be 4 bytes 
> for example.

Standard iostreams do not support binary i/o directly. When you attempt 
to read a long from a stream, the stream attempts to interpret the first 
few characters in the stream as a formatted number, depending on the 
locale. For example:

     #include <assert>
     #include <stringstream>

     int main()
     {
       std::istringstream in("10000");
       long x;
       in >> x;
       assert(x == 10000);
     }

If you want to do binary i/o, you should not use the overloaded bitshift 
operators at all. Use the unformatted operations (such as istream::get, 
istream::read, streambuf::sgetc, streambuf::sgetn) to read one or more 
characters, then examine the characters carefully, taking into account 
sign and byte order.

You can see some examples here: 
http://www.boost.org/boost/iostreams/filter/gzip.hpp. Look for the 
functions read_uint8 and read_unit32, and the functions that call them.

> Do I have to overload my own generic bitshift operators for this 
> behavior or does Boost.Iostreams supply some overloaded bitshift 
> operators that provide such functionality?

-- 
Jonathan Turkanis
CodeRage
http://www.coderage.com