How to use the non-modifiable pointer descriptor — TPtrC

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本文介绍了Symbian系统中TPtrC描述符的多种构造方法及其用途，包括引用常量字符串或数据、构建从其他描述符、内存指针及长度等，并展示了如何通过这些描述符访问数据。

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 the non-modifiable pointer descriptor — TPtrC《如何使用不可修改的指针描述符TPtrC》

How to use the non-modifiable pointer descriptor — TPtrC

Non-modifiable pointer descriptors are useful for referencing constant strings or data; for example, accessing strings built into ROM resident code, or passing a reference to data in RAM which must not be modified through that reference.

For text data, it is usual to construct a TPtrC type and allow the appropriate variant, either a TPtrC8 or a TPtrC16 to be selected at build time.
For binary data, an explicit TPtrC8 is used.
It is rare to use an explicit TPtrC16.

Constructing a `TPtrC`

A non-modifiable pointer descriptor can be constructed in a number of ways:

any other descriptor.
another non-modifiable pointer descriptor.
a pointer into memory and specifying the length of the data.
a zero terminated string.

The following code fragment constructs a TPtrC to represent the data already represented by any other type of descriptor.

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

_LIT(KText,"Hello World!"); ... TBufC<16> buf1(KText); // buf1 is the existing descriptor ... TPtrC ptr(buf1); // data in buf1 now accessible through ptr

The following code fragment constructs a TPtrC to represent the data already represented by another TPtrC.

_LIT(KText,"Hello World!"); ... TBufC<16> buf1(KText); // buf1 is the existing descriptor ... TPtrC ptr1(buf1); // data in buf1 now accessible through ptr1 TPtrC ptr2(ptr1); // data also accessible through ptr2

Although rarely used (possibly in porting legacy 'C' code), the following code fragment defines a constant ‘C’ style non-Unicode string and then constructs the non-modifiable pointer descriptor for that string. The explicit 8 bit variant is used here. The descriptor is separate from the data it represents.

const TText8* cstr = (TText8*)"Hello World!"; ... TPtrC8 ptr(cstr); ... ptr.Length(); // The length is 12. ptr.Ptr(); // The address of the descriptor's data, // i.e. 'C' string.

The following code fragment shows construction from a pointer into memory and a length. The example assumes that both the pointer and the length have valid values:

TUint8* memptr; TInt length; ... TPtrC8 ptr(memptr,length);

Accessing data

Once a non-modifiable pointer descriptor has been constructed, the functions in the base class, TDesC, are available to access the data.

For example, given a pointer descriptor labelled ptr:#

_LIT(KText,"Hello World!"); ... TBufC<16> buf1(KText); // buf1 is the existing descriptor ... TPtrC ptr(buf1); // data in buf1 now accessible through ptr ... ptr.Length(); // returns the length of the data (i.e. 12)