Using Symbian OS String Descriptors

When I was new to Symbian, The first thing that I encountered is Symbian OS String handling and Manipulations. It was then day(s) of nightmare that struck me :). What I found that its 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 try to remember the things. I will not go here on the war of words whether having descriptors is a good approach or not, because be it be good or bad you have to use it :(. Another thing I will not discuss here is how you can do similar thing using plain C++ or how to do something you can do with plain C++ string manipulation techniques. This article is just regarding descriptors.

Background

The very first thing you need to do is to remember the descriptor hierarchy. This is very important as all of the 5 descriptors you are going to use derived from some classes and you must know from which class this derives to be able to successfully decide which particular descriptor to use in which scenario. I am not going to explain what a buffer descriptor and Heap Descriptor means, and also what is meant by modifiable and non modifiable descriptors. I believe you must be having enough information on what is meant by above terminologies. I will also not use 8 and 16 bit variant as functionality wise there is a similarity between these two and also not important in understanding the descriptors. The Symbian Descriptor Hierarchy looks something like. (Sorry for not providing fabulous pictures of Descriptor hierarchy). You can have a look at String and Descriptors for picture.

Usage of TPtrC<n>

If we want a literal meaning for this, then it will be "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 it just contains constructors and set methods, and since it derives directly from the TDesC it contains all the functionalities of TDesC.

The pointer will point to data in one of the two ways:
Create an empty TPtrC and then point it to some data using Set(...) functions.
Pass the data while constructing by using any one of the overloaded constructors. Let see the above statements with the help of few examples below.

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);

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 );

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 few examples on how to create 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), but 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 buffers data. Let see the first way to change 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 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.

_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 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 and HBufC to a function that takes TDesC & as parameter, you need to simply dereference the HBufC pointer.
the Size of Heap Descriptor buffer can be changed using ReAlloc function as oppose to TBufC.

// 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 case of TBufC and HBufC, so we know most of it. So let’s remember how we can create TPtr.
  Another TPtr.
  From TBufC, HBufC using Des() member function.
  From an explicit Pointer into the memory and specifying the max length.
  From and explicit Pointer into the memory and specifying the data as well as the maximum length.

// 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>