How to use modifiable buffer descriptor — TBuf<TInt>

Modifiable buffer descriptors are useful for holding strings or data and providing safe ways to access and modify that data.

For text data, it is usual to construct a TBuf<TInt> type and allow the appropriate variant, either a TBuf8<TInt> or a TBuf16<TInt> to be selected at build time.
For binary data, an explicit TBuf8<TInt> is used.
It is rare to use an explicit TBuf16<TInt>.

Although, the following notes refer to the build independent types; they are equally valid for the explicit 8 bit and 16 bit types.

Constructing a TBuf<TInt>

A modifiable buffer descriptor can be constructed in a number of ways:

as an empty buffer descriptor.
as an empty buffer descriptor but giving it a length.
by copying data from any other type of descriptor.
by copying data from another modifiable buffer descriptor of the same size.

The following code fragment constructs a TBuf<16> object. The buffer descriptor is uninitialised, i.e. it contains no data. The assignment operator or the Copy() function can be used to put data into the buffer descriptor after construction:

_LIT(KText,"Hello World!");
...
TBuf<16> buf1; // length of buf1 is 0
...
buf1 = KText;  // data assigned

The source descriptor is a literal which is converted to descriptor type.

The following code fragment constructs a TBuf<16> object and sets it length to 12. No data is assigned into the descriptor.

...
TBuf<16> buf1(12); // length of buf1 is 12
...

The following code fragment constructs a TBuf<16> object, initialised with the 12 characters making up the English language phrase "Hello World!".

_LIT(KText,"Hello World!");
...
TBuf<16> buf1(KText);

The following code fragment constructs a TBuf<16> object from another TBuf<16> object. This is, in effect, copy construction.

_LIT(KText,"Hello World!");
...
TBuf<16> buf1(KText);
TBuf<16> buf2(buf1);   // buf2 constructed from the data in buf1

In both of these cases, the resulting length of the descriptor is 12.

A non-modifiable buffer descriptor can also be constructed from 'C' style zero terminated string. However, this is rarely necessary but may make it easier to port legacy 'C' code.

Replacing data

Data within a modifiable buffer descriptor can be completely replaced through the assignment operator or by using the Copy() function.

_LIT(KText,"Hello World!");
_LIT(KNewText,"New text");
_LIT(KReplaced,"Replaced");
...
TBuf<16> buf1(KText);
TBuf<16> buf2;
...
buf2 = buf1;               // buf2 now contains "Hello World!"
...
buf2 = KNewText;           // buf2 now contains "New text".
                           // the literal is converted to a descriptor
                           // type.
buf2.Copy(KReplaced);      // buf2 content replaced using Copy()

Accessing and changing data

Once a modifiable buffer descriptor has been constructed, the functions in the base classes, TDesC and TDes, are available to be access and change the data.

_LIT(KText,"Hello World!");
...
TBufC<16> buf1(KText);
...
buf1.Length();

and

_LIT(KText,"Hello World!");
...
TBufC<16> buf1(KText);   // length is 12 
...
buf1.Delete(6,6);        // length is now 6, leaving "Hello" in
                         // the buffer

Illegal access causing an exception

The following code fragment raises a panic because of an attempt to assign too much data. The maximum length of the buffer descriptor is 16 but the length of the data to be copied is 31:

_LIT(KText,"Hello World! The sun is shining");
...
TBufC<16> buf1(KText);

The following code fragment raises a panic because of an attempt to delete data outside the boundary defined by the descriptor:

_LIT(KText,"Hello World!");
...
TBufC<16> buf1(KText);
buf1.Delete(99,1);

Related concepts

Literal Descriptors