Laboratory Exercises, C++ Programming

Strings and Streams 23
3 Strings and Streams
Objective: to practice using the string class and the stream classes.
Read:
• Book: strings, streams, function templates.
1 Class string
1.1 Introduction
In C, a string is a null-terminated array of characters. This representation is the cause of many
errors: overwriting array bounds, trying to access arrays through uninitialized or incorrect
pointers, and leaving dangling pointers after an array has been deallocated. The
library contains some useful operations on C-strings, such as copying and comparing strings.
C++ strings hide the physical representation of the sequence of characters. A C++ string
object knows its starting location in memory, its contents (the characters), its length, and the
length to which it can grow before it must resize its internal data buffer (i.e., its capacity).
The exact implementation of the string class is not defined by the C++ standard. The
specification in the standard is intended to be flexible enough to allow different implementations,
yet guarantee predictable behavior.
The string identifier is not actually a class, but a typedef for a specialized template:
typedef std::basic_string string;
So string is a string containing characters of the type char. There is another standard string
specialization, wstring, which describes strings of characters of the type wchar t. wchar t is the
type representing “wide characters”. The standard does not specify the size of these characters;
usually it’s 16 or 32 bits (32 in gcc). In this lab we will ignore all “internationalization” problems
and assume that all characters fit in one byte. Strings in C++ are not limited to the classes string
and wstring; you may specify strings of “characters” of any type, such as int. However, this is
discouraged: there are other sequence containers that are better suited for non-char types.
Since we’re only interested in the string specialization of basic string, you may wonder why
we mention basic string at all? There are two reasons: First, std::basic string is all the
compiler knows about in all but the first few translation passes. Hence, diagnostic messages only
mention std::basic string. Secondly, it is important for the general understanding of the
ideas behind the string method implementations to have a grasp of the basics of a C++ concept
called character traits, i.e., character “characteristics”. For now, we will just show parts of the
definition of basic string:
/* Standard header */
namespace std {
template
class basic_string {
// types:
typedef traits_t traits_type;
typedef typename traits_t::char_type char_type;
typedef unsigned long size_type;
static const size_type npos = -1;
// much more
};
}