Chapter 1

Chapter 5: Strings and Descriptors
Overview
I once looked up the word 'computer' in a big dictionary in my high school library. It said, 'one
who computes.' This view is somewhat old- fashioned: the truth is that most programmers
expend far more effort on processing strings than they do computing with numbers, so it's a
good idea to start getting to know Symbian OS APIs by looking at its string handling
facilities.
In C, string processing is inconvenient. You have an awkward choice of char*, char[],
and malloc() with which to contend, just to allocate your strings. You get some help from
such functions as strlen(), strcpy(), and strcat(), but little else. You have to pass
around awkward maximum-length parameters to functions such as strncpy()and
strncat() that modify strings with an explicit length limit. You have to add one and
subtract one for the trailing NULL at the end of every string. If you get your arithmetic slightly
wrong, you overwrite memory and produce bugs that are hard to track down. It's not much
fun.
In Java, life is much easier. There is a String class with nice syntax such as a + operator for
concatenating strings. Memory for strings looks after itself: new memory is allocated for new
strings and memory for old string values or intermediate results is garbage collected when
no longer needed.
In standard C++, a similarly useful string class is also available, though it came along quite a
while after the C++ language itself.
In Symbian OS, strings are implemented by descriptors. Descriptors provide a safe and
consistent mechanism for dealing with both strings and general binary data regardless of the
type of memory in which they reside.
Like the string classes in Java and standard C++, they're much more comfortable to work
with than C strings. However, Symbian OS doesn't take the same approach as either Java
or standard C++, because memory management is so important in Symbian OS. You have
to be fully aware of the memory management issues when you're using descriptors.
I'll start this chapter off with a discussion of descriptors and memory management. Because
C string handling gives you control of memory management, I'll compare descriptors and
their memory management with C strings and their memories.
Then I'll move on to what you can do with descriptors. You need to know about both the
concrete implementation classes and the two key abstract base classes, TDesC and TDes,
which include a large number of convenience functions. TDesC is a one-word class (just the
length), and its convenience functions are all const – that's what the C in TDesCstands for.
TDes derives from TDesC, and adds an extra word (maximum length), and nonconst
convenience functions.
Symbian OS is built to use 'wide' characters – 16 bits, using Unicode. I'll explain some of the
implications of this.
Finally, I'll look at descriptors' role in describing data – which is where they got their name.
Descriptors are fundamental to many data- related Application Programming Interfaces
(APIs), such as the interthread reading and writing used by the client-server framework and
reading and writing data to files.