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Very Useful C++ Stream Class

By , 18 Oct 2009
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Introduction

Over the years of Windows programming, I have quite often needed a MemoryStream class to handle various text and binary data streaming tasks. I have also needed to implement my own IStream interface, which of course must be backed by a MemoryStream. So I wrote the CStream class provided in this article. Since then, I have used this class in almost every project I have done. Often, I find I need to build strings a bit at a time. CStream makes a perfect StringBuilder class. You'll never need to worry about memory allocation, no matter how large the stream grows because CStream allocates new memory as needed.

Overview

The key feature of CStream is that it takes care of growing the internal buffer as needed. And the key to high performance is using VirtualAlloc. This is a very fast memory allocation function created just for this sort of situation where memory is reallocated often.

CStream implements the COM IUnknown and IStream interfaces*. This means you can use it wherever an IStream is needed. In addition to the IStream interface, there are many helper/access methods which give access to the internal buffer for example.

CStream is implemented in Stream.h and Stream.cpp.

* The IStream interface is not fully implemented - only the most commonly used methods.

Using CStream

Suppose you need a StringBuilder because you need to build up a string a bit at a time. Here's the code:

CStream strm;

strm << "This is ";
strm << "an example ";
strm << "of a multibyte ";
strm << "string builder ";
strm << "using CStream";
// NULL terminate
strm << char(0);

// now get the pointer to the stream buffer, casting it to the
correct type

PSTR pszString = (PTSTR) strm.GetBuf();

You only need to declare the CStream and begin writing to it. You can use the overloaded << operator or CStream::Write. Be sure to write a final NULL terminator before calling CStream::GetBuf. GetBuf returns the internal buffer as a BYTE* which must be cast to our desired type in this case.

Binary data streaming is accomplished using CStream::Write, as shown in the following example:

CStream strm;
char szFilename[300];
GetModuleFileName( NULL, szFilename, sizeof(szFilename) );

CFile file( szFilename, CFile::modeRead );

BYTE buf[500];
UINT nRead;

while( nRead = file.Read( buf, sizeof(buf) ) )
{
   strm.Write( buf, nRead, NULL );
}

Useful Methods

Besides GetBuf, CStream also has these helper methods:

  • GetByte: Returns the Byte at the current position, with the option to advance the current position.
  • GetCurPos: Returns the current internal position.
  • GetEndPos: Returns the current end position. This is not the current total allocated size, but rather the highest place in the buffer which has been written to.
  • SetEndPos: Sets the current end position, resizing the stream if necessary.
  • SeekToBegin: Sets the internal position to the beginning of the stream.
  • SeekToEnd: Sets the internal position to the end of the stream.
  • Seek: Sets the internal position to the indicated position in the stream.
  • Reset: Resets the stream, optionally setting the size.

Conclusion

Text and data streaming are common things to do in programming. This stream class fills the need quite nicely. I hope you find it to be as useful as I have.

History

  • 8th October, 2009: Initial version

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

About the Author

Brett Goodman
Software Developer (Senior) EzTools Software
Australia Australia
I'm EzTools Software, a small software company in Perth, Western Australia. I've created some interesting and hopefully useful tools and technology, which you can see at www.eztools-software.com.

Comments and Discussions

 
QuestionWhy not use CMemMapFile ? Pinmemberhw7704112-Nov-09 6:10 
QuestionWhy not use std::stringstream? PinmemberOwen Lawrence20-Oct-09 5:04 
AnswerRe: Why not use std::stringstream? PinmemberBrett Goodman20-Oct-09 22:41 
GeneralRe: VirtualAlloc Pinmemberemilio_grv21-Oct-09 4:30 
Just let me clarify one point:
The memory is managed by the operating system in ANY CASE, and the operating system API to manange memory is differentiated for historical reasons into Local, Global and Virtual memory.
 
Although in the past they have completely different implementation (essentially because of a limitation of the 16 bits architecture), in today system (32 or 64 bits) all the memory is "virtual" and "local" or "global" is only a matter of visibility.
 
That said, std::stringstream, std::string and all STL containers refer to stl::allocator that in turn use malloc.
 
The way malloc is implemented is -typically- as a sub-allocator that does some extra checks about the way C uses the memory the system gives to it.
 
And it gets the memory from the system usually as LocalAlloc, that in turn falls through (since Windows2000) to VirtualAlloc.
That's the one and only "real" allocator existing in the system.
 
The difference in speed between STL (and whatever C management memory)and a direct invocation of VirtualAlloc is because -by doing so- you bypass all the extra checks that malloc does, that - especially in its debug implementation - are not so trivial.
 
Release version should be quite different (the disassembly of malloc is just a redirected CALL), but -if speed is very important- you can also (as another way out) implement an alternative compliant std::allocator based on VirtualAlloc instead of malloc, and use that allocator instead of the default one when using STL classes.
 
This makes whatever STL class to use VirtualAlloc as well.
 

2 bugs found.
> recompile ...
65534 bugs found.
D'Oh! | :doh:


GeneralRe: VirtualAlloc PinmemberBrett Goodman21-Oct-09 15:43 
GeneralRe: VirtualAlloc PinmemberStefan6329-Oct-09 0:34 
AnswerRe: Why not use std::stringstream? Pinmemberdomehead21-Oct-09 14:35 
GeneralRe: Why not use std::stringstream? PinmemberBrett Goodman21-Oct-09 15:44 
QuestionIsn't there an API for this? Pinmemberwaleri20-Oct-09 1:17 
GeneralNice. A suggestion though... PinmemberArman Z. Sahakyan19-Oct-09 17:03 
GeneralRe: Nice. A suggestion though... PinmemberRDABC20-Oct-09 3:53 
GeneralRe: Nice. A suggestion though... PinmemberBrett Goodman20-Oct-09 22:46 

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