How to use modifiable buffer descriptor — TBuf

下面的内容摘自Symbian S60 3rd Edition SDK，先把英文原版贴在这里，等时间充足的时候我再把它翻译出来。How to use modifiable buffer descriptor — TBuf<TInt>《如何使用可修改的缓冲描述符TBuf<TInt>》

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);

---