C++ Primer Plus

The STL provides a collection of templates representing containers, iterators, function objects, and algorithms. A container is a unit, like an array, that can hold several values. STL containers are homogeneous; that is, they hold values all of the same kind. Algorithms are recipes for accomplishing particular tasks, such as sorting an array or finding a particular value in a list. Iterators are objects that let you move through a container much as pointers let you move through an array; they are generalizations of pointers. Function objects are objects that act like functions; they can be class objects or function pointers (including function names because a function name acts as a pointer). The STL lets you construct a variety of containers, including arrays, queues, and lists, and it lets you perform a variety of operations, including searching, sorting, and randomizing.

Alex Stepanov and Meng Lee developed STL at Hewlett-Packard Laboratories, releasing the implementation in 1994. The ISO/ANSI C++ committee voted to incorporate it as a part of the C++ Standard. The STL is not an example of object-oriented programming. Instead, it represents a different programming paradigm called generic programming. This makes STL interesting both in terms of what it does and in terms of its approach. There’s too much information about the STL to present in a single chapter, so we’ll look at some representative examples and examine the spirit of the generic programming approach. We’ll begin by looking at a few specific examples. Then, when you have a hands-on appreciation for containers, iterators, and algorithms, we’ll look at the underlying design philosophy and then take an overview of the whole STL. Appendix G, “The STL Methods and Functions,” summarizes the various STL methods and functions.

The vector Template Class

Chapter 4 touched briefly on the vector class. We’ll look more closely at it now. In computing, the term vector corresponds to an array rather than to the mathematical vector discussed in Chapter 11, “Working with Classes.” (Mathematically, an N-dimensional mathematical vector can be represented by a set of N components, so in that aspect, a mathematical vector is like an N-dimensional array. However, a mathematical vector has additional properties, such as inner and outer products, that a computer vector doesn’t necessarily have.) A computing-style vector holds a set of like values that can be accessed randomly. That is, you can use, say, an index to directly access the 10th element of a vector without having to access the preceding 9 elements first. So a vector class would provide operations similar to those of the valarray and ArrayTP classes introduced in Chapter 14 and to those of the array class introduced in Chapter 4. That is, you could create a vector object, assign one vector object to another, and use the [] operator to access vector elements. To make the class generic, you make it a template class. That’s what the STL does, defining a vector template in the vector (formerly vector.h) header file.

To create a vector template object, you use the usual <type> notation to indicate the type to be used. Also the vector template uses dynamic memory allocation, and you can use an initialization argument to indicate how many vector elements you want:

#include vector

using namespace std;

vector ratings(5);       // a vector of 5 ints

int n;

cin >> n;

vector scores(n);     // a vector of n doubles

After you create a vector object, operator overloading for [] makes it possible to use the usual array notation for accessing individual elements:

ratings[0] = 9;

for (int i = 0; i < n; i++)

    cout << scores[i] << endl;

Allocators Again

Like the string class, the various STL container templates take an optional template argument that specifies what allocator object to use to manage memory. For example, the vector template begins like this:

template >

    class vector {...

If you omit a value for this template argument, the container template uses the allocator class by default. This class uses new and delete.

Listing 16.7 uses this class in an undemanding application. This particular program creates two vector objects, one an int specialization and one a string specialization; each has five elements.

Listing 16.7. vect1.cpp

// vect1.cpp -- introducing the vector template

#include

#include

#include

const int NUM = 5;

int main()

{

    using std::vector;

    using std::string;

    using std::cin;

    using std::cout;

    using std::endl;

    vector ratings(NUM);

    vector titles(NUM);

    cout << "You will do exactly as told. You will enter\n"

         << NUM << " book titles and your ratings (0-10).\n";

    int i;

    for (i = 0; i < NUM; i++)

    {

        cout << "Enter title #" << i + 1 << ": ";

        getline(cin,titles[i]);

        cout << "Enter your rating (0-10): ";

        cin >> ratings[i];