Strings in C and C++

http://cs.stmarys.ca/~porter/csc/ref/c_cpp_strings.html

This page summarizes many of the things you may find it    useful to know when working with either C-strings or objects of the C++   string class.

The term string generally means an ordered sequence of    characters, with a first character, a second character, and so on, and in    most programming languages such strings are enclosed in either single or    double quotes. In C++ the enclosing delimiters are double quotes. In this    form the string is referred to as a string literal and we often    use such string literals in output statements when we wish to display    text on the screen for the benefit of our users. For example, the usual    first C++ program displays the string literal "Hello, world!" on the    screen with the following output statement:

cout << "Hello, world!" << endl;

However, without string variables about all we can do with strings is    output string literals to the screen, so we need to expand our ability to    handle string data. When we talk about strings in C++, we must be careful    because the C language, with which C++ is meant to be backward    compatible, had one way of dealing with strings, while C++ has another,    and to further complicate matters there are many non-standard    implementations of C++ strings. These should gradually disappear as    compiler vendors update their products to implement the string component    of the C++ Standard Library.

As a programmer, then, you must distinguish between the following    three things:

1. An "ordinary" array of characters, which is just like any other      array and has no special properties that other arrays do not have.
2. A C-string, which consists of an array of characters terminated by      the null character '\0', and which therefore is different from an      ordinary array of characters. There is a whole library of functions for      dealing with strings represented in this form. Its header file is      <cstring>. In some implementations this library may be      automatically included when you include other libraries such as the     <iostream> library. Note that the null character may      very well not be the very last character in the C-string array, but it      will be the first character beyond the last character of the actual      string data in in that array. For example if you have a C-string      storing "Hello" in a character array of size 10, then the letters of      the word "Hello" will be in positions with indices 0 to 4, there will      be a null character at index 5, and the locations with indices 6 to 9      will contain who-knows-what. In any case, it's the null character at      index 5 that makes this otherwise ordinary character array a      C-string.
3. A C++ string object, which is an instance of a "class" data type      whose actual internal representation you need not know or care about,      as long as you know what you can and can't do with variables (and      constants) having this data type. There is a library of C++ string      functions as well, available by including the <string>      header file.

Both the C-string library functions and the C++ string library    functions are available to C++ programs. But, don't forget that these are    two *different* function libraries, and the functions of the first    library have a different notion of what a string is from the    corresponding notion held by the functions of the second library. There    are two further complicating aspects to this situation: first, though a    function from one of the libraries may have a counterpart in the other    library (i.e., a function in the other library designed to perform the    same operation), the functions may not be used in the same way, and may    not even have the same name; second, because of backward compatibility    many functions from the C++ string library can be expected to work fine    and do the expected thing with C-style strings, but not the other way    around.

The last statement above might seem to suggest we should use C++    strings and forget about C-strings altogether, and it is certainly true    that there is a wider variety of more intuitive operations available for    C++ strings. However, C-strings are more primitive, you may therefore    find them simpler to deal with (provided you remember a few simple rules,    such as the fact that the null character must always terminate such    strings), and certainly if you read other, older programs you will see    lots of C-strings. So, use whichever you find more convenient, but if you    choose C++ strings and occasionally need to mix the two for some reason,    be extra careful. Finally, there are certain situations in which    C-stringsmust be used.

To understand strings, you will have to spend some time studying    sample programs. This study must include the usual prediction of how you    expect a program to behave for given input, followed by a compile, link    and run to test your prediction, as well as subsequent modification and    testing to investigate questions that will arise along the way. In    addition to experimenting with any supplied sample programs, you should    be prepared to make up your own.

In the following examples we attempt to draw the distinction between    the two string representations and their associated operations. The list    is not complete, but we do indicate how to perform many of the more    useful kinds of tasks with each kind of string. The left-hand column    contains examples relevant to C-strings and the right-hand column shows    analogous examples in the context of C++ strings.

C-strings  (#include <cstring>)         C++ strings  (#include <string>)
===============================         ================================
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!         !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Declaring a C-string variable           Declaring a C++ string object
-----------------------------           -----------------------------
char str[10];                           string str;

Initializing a C-string variable        Initializing a C++ string object
--------------------------------        --------------------------------
char str1[11] = "Call home!";           string str1("Call home!");
char str2[] = "Send money!";            string str2 = "Send money!";
char str3[] = {'O', 'K', '\0'};         string str3("OK");
Last line above has same effect as:
char str3[] = "OK";
                                        string str4(10, 'x');
                                        
Assigning to a C-string variable        Assigning to a C++ string object
--------------------------------        --------------------------------
Can't do it, i.e., can't do this:       string str;
char str[10];                           str = "Hello";
str = "Hello!";                         str = otherString;

Concatenating two C-strings             Concatenating two C++ string objects
---------------------------             ------------------------------------
strcat(str1, str2);                     str1 += str2;
strcpy(str, strcat(str1, str2));        str = str1 + str2;

Copying a C-string variable             Copying a C++ string object
---------------------------             ---------------------------
char str[20];                           string str;
strcpy(str, "Hello!");                  str = "Hello";
strcpy(str, otherString);               str = otherString;

Accessing a single character            Accessing a single character
----------------------------            ----------------------------
str[index]                              str[index]
                                        str.at(index)
                                        str(index, count)
                                        
Comparing two C-strings                 Comparing two C++ string objects
-----------------------                 --------------------------------
if (strcmp(str1, str2) < 0)             if (str1 < str2)
    cout << "str1 comes 1st.";              cout << "str1 comes 1st.";
if (strcmp(str1, str2) == 0)            if (str1 == str2)
    cout << "Equal strings.";               cout << "Equal strings.";
if (strcmp(str1, str2) > 0)             if (str1 > str2)
    cout << "str2 comes 1st.";              cout << "str2 comes 1st.";
    
Finding the length of a C-string        Finding the length of a C++ string object
--------------------------------        -----------------------------------------
strlen(str)                             str.length()

Output of a C-string variable           Output of a C++ string object
-----------------------------           -----------------------------
cout << str;                            cout << str;
cout << setw(width) << str;             cout << setw(width) << str;

In what follows, keep in mind that cin ignores white space when    reading a string, whilecin.get(), cin.getline() and     getline() do not. Remember too thatcin.getline() and     getline() consume the delimiter while cin.get() does not.    Finally, cin can be replaced with any open input stream, since file    input withinFile, say, behaves in a manner completely analogous to    the corresponding behavior ofcin. Analogously, in the output    examples given immediately above, cout could be replaced with any    text output stream variable, say outFile. In all cases,   numCh is the maximum number of characters that will be read.

Input of a C-style string variable         Input of a C++ string object
----------------------------------         ----------------------------
cin >> s;                                  cin >> s;
cin.get(s, numCh+1);
cin.get(s, numCh+1,'\n');
cin.get(s, numCh+1,'x');
cin.getline(s, numCh+1);                   getline(cin, s);
cin.getline(s, numCh+1, '\n');
cin.getline(s, numCh+1, 'x');              getline(cin, s, 'x');

A useful naming convention for C-strings is illustrated by examples    like

typedef char String80[81];
typedef char String20[21];

in which the two numbers in each definition differ by 1 to allow for    the null character '\0' to be stored in the array of characters, but to    *not* be considered as part of the string stored there. No analog to this    naming convention is necessary for C++ strings, since for all practical    purposes, each C++ string variable may contain a string value of    virtually unlimited length.