Re: [boost] [interprocess] difference between resize & reserve

From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2006-08-11 12:09:25

Hi Dmitry,

Dmitry Smirnov wrote:
> Hi, Ion!
> 
> It seems I found the difference between "resize" and "reserve"
> 
> After changing resize to reserve in following code 
> m_objSendVectorStream.tellp() will return -1 instead of size of data 
> packet :-(
 >
 >
 > m_objSendVectorStream.resize( QueueSettings::maxMessageSize() );
 >
 > m_objSendVectorStream.seekp( 0 );
 > {
 > boost::archive::defOArchive objOArchive( m_objSendVectorStream );
 > objOArchive << objData;
 > }
 >

In theory, "reserve()" means "reserve memory to avoid reallocations" but 
does _not_ insert characters in the stream, so the stream size will be 
zero. I mean:

-> "resize(resize_size)" called "resize(resize_size)" in the underlying 
vector, so vector was zero-filled (vector.size() == resize_size) and 
thus, the result was that '0' characters were introduced in the stream 
and the position was reset to 0.

->"reserve(reserve_size)" calls "reserve(reserve_size)" in the vector, 
so memory is allocated but no characters are introduced in the vector 
(vector.size() == 0), and the position is reset to zero, so you would be 
in the end of the stream (position 0 == eof since there are no 
characters). The formatted stream is unchanged but you guarantee no 
allocations.

So:

basic_vectorstream<...> vstream;
vstream.resize(1000);

was equivalent to inserting 1000 '0' characters in the stream and to 
reset the position. I wanted to avoid this, because the only way to 
write to the stream should be though stream functions.

basic_vectorstream<...> vstream;
vstream.reserve(1000);

should be equivalent to just creating the vectorstream (so there are no 
characters in the stream).

I need to investigate if tellp returning -1 in this case is correct 
(should it return 0 or eof?)

My intention was to make this equivalent to creating a file.:

std::fstream file("file, std::ios_base::in | std::ios_base::out);
file.tellp() == ?

Hmmm,you were just telling me what the difference is or you consider 
this a bug?

Regards,

Ion