Using Symbain OS String Descriptors

Introduction

When I was new to Symbian, The first thing I encountered was Symbian OS String handling and manipulations. It was then days of nightmare that struck me :). What I found was that it's a bit tricky to remember the descriptor stuffs, but once you get the trick it’s easy. No, I am not kidding..

So here I am trying to explain how I learned the basic Symbian OS string handling and tried to remember the things. I will not go into a war of words regarding whether having descriptors is a good approach or not, because whether it is good or bad you have to use it :(. Another thing I will not discuss here is how you can do similar things using plain C++ or how to do something with plain C++ string manipulation techniques. This article is just about descriptors.

The prerequisite of this article is a working knowledge of the Symbian OS.

Background

The very first thing you need to do is to remember the descriptor hierarchy. This is very important as all of the five descriptors you are going to use are derived from some classes and you must know from which class they are derived to be able to successfully decide which particular descriptor should be used and in which scenario. I am not going to explain what a buffer descriptor and Heap Descriptor mean, and also what is meant by modifiable and non modifiable descriptors. I believe you must be having enough information about what is meant by the above terminologies. I will also not use 8 and 16 bit variants, as functionality wise there is a similarity between these two and also these are not important in understanding the descriptors. The Symbian Descriptor Hierarchy looks fabulous. (Sorry for not providing fabulous pictures of Descriptor hierarchy). You can have a look at newlc for pictures.

Usage of TPtrC<n>

If you want a literal meaning for this, then it is a "Pointer to a data that cannot be manipulated". The basic thing to remember about TPtrC<n> is that it contains no manipulation functionalities of its own and that it just contains constructors and set methods, and since it is derived directly from TDesC it contains all the functionalities of TDesC.

The pointer will point to data in one of the two ways:

• Create an empty TPtrC<N> and then point it to some data using Set(...) functions.
• Pass the data while constructing by using any one of the overloaded constructors.

Let's see the above statements with the help of a few examples given below:

• Example 1:- Getting TPtrC from TBuf and TBufC :

  LIT(KText , "Test Code");
  TBufC<10> Buf ( KText ); OR(/) TBuf<10> Buf ( KText );
  // Creation of TPtr using Constructor
  TPtrC    Ptr (Buf);
  // Creation of TPtr using Member Function
  TPtrC    Ptr1;
  Ptr1.Set(Buf);

• Example 2:- Getting TPtrC from TText*:

  The example below uses TText16:

  TText * text = _S("Hello World\n");
  TPtrC ptr(text);
  // OR
  TPtrC Ptr1;
  Ptr1.Set(text);
  // To store only a part of TText we can use the following 
  // This descriptor pointer will store only Hello
  TPtrC   ptr4 ( text , 5 );

• Example 3:- Getting TPtrC from another TPtrC:

  You can easily assign one TPtrC to another.

  TText * text = _S("Hello World\n");
  TPtrC ptr(text);
  // Get TPtrC from another TPtrC
  TPtrC p1(ptr);
  // OR
  TPtrC p2;
  p2.Set(ptr);

• Example 4:- Getting TText * from TPtrC:

  We can get the TText * from TPtrC by using the Ptr() member.

  // Set the TPtrC
  _LIT(KText , "Test Code");
  TBufC<10> Buf ( KText ); 
  TPtrC    Ptr1 (Buf);
  // Get the TText *
  TText * Text1 = (TText *)Ptr1.Ptr();

Usage of TBufC<n>

The examples used to describe the working TPtrC gives some understanding about the usage of TBufC<n>, Nevertheless here are a few examples on how to create TBufC<n>.

• Example 5:- Instantiating the TBufC<N>:

  // Instantiating the TBufC from Literals
  _LIT(Ktext, "TestText");
  TBufC<10> Buf (Ktext);
  // or
  TBufC<10> Buf2;
  Buf2 = Ktext;
  // Creating a new TBufC with existing TBufC
  TBufC<10> Buf3(Buf2);

  TBufC<n> is used generally for text data. For binary data, explicit TBufC8<n> is used. Though TBufC<n> means that the data cannot be modified ('C' stands for Constant), there are two ways by which this data can be modified:

  • The data can be replaced by using assignment operator.
  • By using Des() function to construct a TPtr modifiable pointer descriptor for the buffer data.

  Let's see the first way to change the contents of TBufC<n>.

• Example 6:- Changing the contents of TBufC<N>:

  // Describe some literals for Testing example
  _LIT(Ktext , "Test Text");         
  _LIT(Ktext1 , "Test1Text");
  // Generate TPtrC
  TBufC<10> Buf1 ( Ktext );
  TBufC<10> Buf2 ( Ktext1 );
  // Change the contents of Buf2
  Buf2 = Buf1;
  // Create an Empty TBufC and assign it to Buf1
  TBufC<10> Buf3;
  Buf3 = Buf1;

  Another way of changing the contents of TBufC<n> is by using the Des() member. This member function returns TPtr modifiable pointer descriptor using members of the TPtr. The maximum length of the TPtr is the value of the TBufC<n> template parameter. All the manipulating functions are from TDesC base.

• Example 7:- Changing the contents of TBufC<N> using Des():

  _LIT(Ktext , "Test Text");
  _LIT(KXtraText , "New:");
   
  TBufC<10> Buf1 ( Ktext );
  TPtr Pointer = Buf1.Des();
   
  // delete the last 4 characters 
  Pointer.Delete(Pointer.Length()-4, 4 );
   
  // the length should be changed now
  TInt Len = Pointer.Length();
   
  // Append the new one
  Pointer.Append(KXtraText);
   
  Len = Pointer.Length();
  // To completly changed the buffer we can use following
  _LIT(NewText , "New1");
  _LIT(NewText1 , "New2");
   
  TBufC<10> Buf2(NewText);
  // change the context
   
  Pointer.Copy(Buf2);
  // or Directly from literal
   
  Pointer.Copy(NewText1);
  // All the above changed actually changing the contents of Buf1

Working with Heap Descriptor HBufC

HBufC is the descriptor of choice when we don’t know the size of the data that we want to have in the descriptor. Here 'C' stands for constant that means the data is constant but it can also be changed in two ways as it was changed in the case of TBufC<n>. First using the assignment operator and another by using the modifiable pointer descriptor, i.e. TPtr using Des() member function. There are two things to remember while using the HBufC.

• If you need to pass HBufC to a function that takes TDesC & as a parameter, you need to simply dereference the HBufC pointer.
• The size of Heap Descriptor buffer can be changed using ReAlloc function as opposed to TBufC<N>.

• Example 8:- Usage of HBufC:

  // Creation of a Heap Descriptor . There are 2 ways to do it
  // 1st way use either New(), NewL(), or NewLC()  
  // Let see the example for this. This wills construct the HBufC with
  // a Data space for 15, but the current size is Zero.
  HBufC * Buf = HBufC::NewL(15);
           
  // 2nd way to used the Alloc(), AllocL(), or AllocLC() of
  // the existing descriptors. The new Heap descriptor is automatically 
  // initialized with the content of descriptor.
  _LIT (KText , "Test Text");
  TBufC<10>  CBuf = KText;
  HBufC * Buf1 = CBuf.AllocL();
   
  // Lets Check the size and Length . The size will be 18 
  // and length will be 9 
  TInt BufSize = Buf->Size();
  TInt BufLength = Buf->Length();             
           
  // Changing what HBufC is pointing to 
  _LIT ( KText1 , "Text1");
  // Change the buffer to Point to KText1 using assignment operator
  *Buf1 = KText1;
           
  // And now changing the Data through Modifiable pointer descriptor
  TPtr Pointer = Buf1->Des();
  Pointer.Delete( Pointer.Length() - 2, 2 );
   
  // All operations that were done in case of TBufC<n> 
  // is applicable here also.
  // here is one such operation      
  _LIT ( KNew, "New:");
  Pointer.Append( KNew );

Usage of TPtr

Ahaa, we are using it in place of TBufC<N> and HBufC, so we know most of it. So let’s remember how we can create TPtr.

• Another TPtr.
• From TBufC<N>, HBufC using Des() member function.
• From an explicit pointer to the memory and specifying the max length.
• From an explicit pointer to the memory and specifying the data as well as the maximum length.

• Example 9:- Usage of TPtr:

  // First lets see 2nd way of Getting TPtr
  _LIT(KText, "Test Data");
  TBufC<10> NBuf ( KText );
  TPtr     Pointer = NBuf.Des();
           
  // 1st way 
  TPtr     Pointer2 ( Pointer );
           
  //3rd way using a memory area , Data and max length 
  TText * Text = _S("Test Second");
  TPtr    Pointer3 ( Text ,11, 12);
           
  // Now we will see how to replace the data with TPtr ,it can 
  // be done completly by using assignment Operator or copy function
           
  _LIT(K1, "Text1");
  _LIT(K2, "Text2");
           
  Pointer2 = K1; // Data will be Text1
  Pointer.Copy(K2); // Data Will be Text2;
   
  // we can also change the length of the data or set it to zero
  Pointer2.SetLength(2); // Only Te will be there
  Pointer2.Zero(); // Make length of data as Zero 
   
  // Data can be altered using the delete function as it used to alter 
  // in the earlier examples.

Usage of TBuf<n>

In this the data doesn't remain constant. The operations, instantiation and assignment will be similar to that of the TBufC<n> along with the modification functions that can be applied to it as it was applied in the case of TPtr, like Copy, Delete, assignments etc.. I hope I don’t need to give examples for this section.

History

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