April 1998

Manipulating memory with TMemoryStream

In the March issue of C++Builder Developer's Journal, the article "File I/O" explained the TFileStream class and how you can use this class to read and write to files on disk. Sometimes, however, you need to manipulate a chunk of data in memory rather than on disk. The good news is that TFileStream has a cousin called TMemoryStream designed for exactly this purpose. In this article, we'll show you how to use TMemoryStream to treat memory the same way you treated a file in our March article. We'll demonstrate how to use the same functions to read and write to memory as you would when performing file I/O.

OK, but why?

Here's another example: Sometimes you need to read a file from disk, manipulate the file in some way (encryption/decryption, for instance), and then write the file out again. Writing the file to memory and making the modifications there is much faster than reading the file, modifying data, then writing the file every time changes are necessary.

Finally, if you've ever performed operations on string data and then copied those strings to a TMemo, you know how slow this process can be. As an alternative, you can load the strings into memory and work with them there, copying the result to TMemo only when the data has been fully altered. We'll show you an example in just a bit, but for now let's take a closer look at what TMemoryStream offers.

TMemoryStream basics

Table A: Important TMemoryStream properties and methods

Property	Description
Position	The current value of the stream position indicator.
Position	is a read/write property.
Size	The number of bytes of data currently in the stream.
Method CopyFrom()	Copies a specified number of bytes from a stream to this stream.
LoadFromFile()	Fills the memory stream with the contents of the specified file.
LoadFromStream()	Fills the memory stream from another stream (either file or memory).
Read()	Reads a specified number of bytes from the stream to the specified memory location (such as an array).
Write()	Writes a specified number of bytes from a memory location to the stream.
SaveToFile()	Saves the contents of the memory stream to a disk file.
SaveToStream()	Saves the contents of the memory stream to another stream.
Seek()	Moves the file-position indicator by the specified amount from the start of the file, from the end of the file, or from the current position.
SetSize()	Allocates the specified amount of memory for the stream.

We covered the general concept of streams last month in "File I/O." The TMemoryStream properties and methods work identically to the TFileStream properties and methods discussed in that article. Rather than cover that ground again, let's take a look at some of the methods particular to TMemoryStream.

The Loadxxx and Savexxx methods simply make it easy to load or save a stream to and from either a file on disk or another memory stream. For example, you could load a file on disk, manipulate the data in memory, and save it back to disk, as follows:

TMemoryStream* stream = new TMemoryStream;
stream->LoadFromFile("myfile.dat");
// do some stuff to the stream
stream->SaveToFile("myfile.dat");
delete stream;

TMemoryStream* stream = new TMemoryStream;
stream->LoadFromFile(OpenDialog1->FileName);
stream->Position = 0;
for (unsigned int i=0;i<stream->Size;i++) {
  char c;
  stream->Read(&c, 1);
  c += 32;
  stream->Position--;
  stream->Write(&c, 1);
}
// file is scrambled in memory 
// later... descramble and display
stream->Position = 0;
for (unsigned int i=0;i<stream->Size;i++) {
  char c;
  stream->Read(&c, 1);
  c -= 32;
  stream->Position--;
  stream->Write(&c, 1);
}
stream->Position = 0;
Memo1->Lines->LoadFromStream(stream);

stream->Position--;

Listing A: STREAMU.H

#ifndef StreamUH
#define StreamUH
//------------------------------------
	---------#include <vcl\Classes.hpp>
#include <vcl\Controls.hpp>
#include <vcl\StdCtrls.hpp>
#include <vcl\Forms.hpp>
#include <vcl\Dialogs.hpp>
//-------------------------------
	--------------class TForm1 : public TForm
{
__published:  // IDE-managed Components
  TMemo *Memo1;
  TLabel *Label1;
  TButton *Button3;
  TLabel *Label2;
  TOpenDialog *OpenDialog1;
  TGroupBox *GroupBox1;
  TButton *Button1;
  TButton *Button2;
  void __fastcall Button1Click(TObject *Sender);
  void __fastcall Button2Click(TObject *Sender);
  void __fastcall Button3Click(TObject *Sender);
  void __fastcall FormCreate(TObject *Sender);
private:  // User declarations
public:   // User declarations
  __fastcall TForm1(TComponent* Owner);
};
//-------------------------------
	--------------extern TForm1 *Form1;
//---------------------------------------------#endif

#include <vcl\vcl.h>
#pragma hdrstop

#include "StreamU.h"
//-----------------------------
	----------------#pragma resource "*.dfm"
TForm1 *Form1;
//--------------------------------------
	-------__fastcall TForm1::TForm1(TComponent* Owner)
  : TForm(Owner)
{
}
//---------------------------------------------
void __fastcall 
TForm1::Button1Click(TObject *Sender)
{
  int start = GetTickCount();
  Memo1->Lines->
    LoadFromFile(OpenDialog1->FileName);
  char* buff = Memo1->Lines->GetText();
  Screen->Cursor = crHourGlass;
  for (unsigned int i=0;i<strlen(buff);i++) {
    char c = buff[i];
    c += 32;
    buff[i] = c;
  }
  for (unsigned int i=0;i<strlen(buff);i++) {
    char c = buff[i];
    c -= 32;
    buff[i] = c;
  }
  Memo1->Lines->Text = buff;
  Screen->Cursor = crDefault;
  char buff2[20];
  sprintf(buff2, "%.02f seconds",
    (GetTickCount() - start)/1000.0);
  Label2->Caption = buff2;
}
//---------------------------------------------
void __fastcall TForm1::Button2Click(TObject *Sender)
{
  TMemoryStream* stream = new TMemoryStream;
  stream->LoadFromFile(OpenDialog1->FileName);
  Screen->Cursor = crHourGlass;
  stream->Position = 0;
  for (unsigned int i=0;i<stream->Size;i++) {
    char c;
    stream->Read(&c, 1);
    c += 32;
    stream->Position--;
    stream->Write(&c, 1);
  }
  stream->Position = 0;
  for (unsigned int i=0;i<stream->Size;i++) {
    char c;
    stream->Read(&c, 1);
    c -= 32;
    stream->Position--;
    stream->Write(&c, 1);
  }
  stream->Position = 0;
  Memo1->Lines->LoadFromStream(stream);
  Screen->Cursor = crDefault;
  delete stream;
  char buff[20];
  sprintf(buff, "%.02f seconds", 
    (GetTickCount() - start)/1000.0);
  Label2->Caption = buff;
}
//---------------------------------------------
void __fastcall 
TForm1::Button3Click(TObject *Sender)
{
  if (OpenDialog1->Execute()) {
    Button1->Enabled = true;
    Button2->Enabled = true;
    Memo1->Text = 
      "File Selected: " + OpenDialog1->FileName;
  }
}
//---------------------------------------------
void __fastcall 
TForm1::FormCreate(TObject *Sender)
{
  Button1->Enabled = false;
  Button2->Enabled = false;
}