[RFC] Default constructed stringbufs and stringstreams

[Paolo Carlini <pcarlini at suse dot de>]

Hi everyone, hi Nathan,

I'm working on these ideas related to the short-string optimization and
just noticed 27.7.1.1/2, about basic_stringbuf default constructor: "The
function allocates no array object".

This has interesting implications, since our basic_stringbuf has a 
basic_string
*data member*!

Our current implementation *seems* ok, since, thanks to the empty string 
optimiz
we are not allocating anything, but implies, for instance, that in the 
seeks we
check that capacity != 0 (i.e., we don't look at the pointers, see 
below) and we
cannot tell a default constructed stringbuf from one constructed from an 
empty
basic_string.

If/when we have a short-string inside the string object, a default 
constructed
string will acquire a non-zero capacity, the implementation of 
basic_stringbuf
will be still possible, in principle, since, again, we don't allocate 
anything.

Now one of my (many) doubts: due to 27.7.1.1/2 shouldn't a default 
constructed
stringbuf have null get/put area pointers? In our current implementation 
this
is not happening and I believe we have a bug. This seems confirmed by 
27.7.1.1/1
which says that the base class is initialized by basic_streambuf() 
(27.5.2.1)

And there are also interesting implications for DR 453.

Lot of work for tomorrow, I guess... ;)

Paolo.

//////////

#include <sstream>
#include <cassert>

class my_stringbuf : public std::stringbuf
{
public:
 const char_type*
 pub_eback() const
 { return eback(); }

 const char_type*
 pub_gptr() const
 { return gptr(); }

 const char_type*
 pub_egptr() const
 { return egptr(); }

 const char_type*
 pub_pbase() const
 { return pbase(); }

 const char_type*
 pub_pptr() const
 { return pptr(); }

 const char_type*
 pub_epptr() const
 { return epptr(); }
};

void test01()
{
 my_stringbuf sbuf;

 assert( !sbuf.pub_eback() );
 assert( !sbuf.pub_gptr() );
 assert( !sbuf.pub_egptr() );

 assert( !sbuf.pub_pbase() );
 assert( !sbuf.pub_pptr() );
 assert( !sbuf.pub_epptr() );
}

int main()
{
 test01();
 return 0;
}