Symbian C++ - Substring operations on descriptors

Question

Hi

What is the preferred/easiest way to manipulate TDesC strings, for example to obtain a substring.

I will give you an example of my scenario.

RBuf16 buf;
...
CEikLabel label;
...
label->SetTextL(buf); // (SetTextL takes a const TDesC&)

I want to get a substring from buf. So do I want to manipulate the RBuf16 directly and if so what is the best way?

Is there a way to convert to const char* so I can just use standard C string manipulation.

Thanks in advance

Answer 1

Read descriptors.blogspot.com (scroll down once loaded).

You can use TDes::LeftTPtr, TDes::RightTPtr or TDes::MidTPtr which will give you a substring as a TPtr (i.e. a descriptor which manipulates the original data).

You can use the TDes::Copy function if you want to create a copy of your substring.

Answer 2

Best or not, I cannot comment, but I use the following methods to extract sub-strings from descriptors:

TDes::LeftTPtr()
TDes::MidTPtr()
TDes::RightTPtr()

or

TDesC::Left()
TDesC::Mid()
TDesC::Right()

with the difference between the two sets being that the former returns a new modifiable descriptor, the latter, a new non-modifiable descriptor, containing the sub-string.

While you develop on the Symbian platform, I would suggest highly to follow the Symbian conventions, and not to think of Symbian C++ from a standard C or C++ point of view all the time. Use the libraries that Symbian has specifically made available, instead of standard C/C++ libraries that Symbian may or may not directly support. Since the ultimate goal of an application developed on the Symbian is to run on a mobile device where reliability and robustness of applications matter the most, you should stick to what Symbian prefers and suggests.

Answer 3

If you want a substring from an RBuf16, that's straightforward - just use TDes16::MidTPtr

You can convert a const char* by doing this:

TPtr8 narrowBuf;

// Create a buffer with enough space to store every character, plus one for 
// a null terminator
narrowBuf.AllocL( buf.Length() + 1);

// TPtr8::Copy accepts a TDesC16 
narrowBuf.Copy( buf );

// Append a null terminator and return a pointer to the resultant data
const char* ptr = (const char*)narrowBuf.PtrZ();

However, what you now have a is a narrowed copy of the original data. If you want to now set the label again, you'll need to widen it (TDes16::Copy(const TDesC8&)) before use. Trying to make Symbian look like "normal" C++ with char* and wchar_t* doesn't really work - the use of descriptors is too entrenched in the OS.