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<int> ratings(5);       // a vector of 5 ints
int n;
cin >> n;
vector<double> 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;

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 <iostream>
#include <string>
#include <vector>

const int NUM = 5;
int main()
{
    using std::vector;
    using std::string;
    using std::cin;
    using std::cout;
    using std::endl;

    vector<int> ratings(NUM);
    vector<string> titles(NUM);
    cout << "You will do exactly as told. You will enter "
         << NUM << " book titles and your ratings (0-10). ";
    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];
        cin.get();
    }
    cout << "Thank you. You entered the following: "
          << "Rating Book ";
    for (i = 0; i < NUM; i++)
    {
        cout << ratings[i] << " " << titles[i] << endl;
    }

    return 0;
}

Here’s a sample run of the program in Listing 16.7:

You will do exactly as told. You will enter
5 book titles and your ratings (0-10).
Enter title #1: The Cat Who Knew C++
Enter your rating (0-10): 6
Enter title #2: Felonious Felines
Enter your rating (0-10): 4
Enter title #3: Warlords of Wonk
Enter your rating (0-10): 3
Enter title #4: Don't Touch That Metaphor
Enter your rating (0-10): 5
Enter title #5: Panic Oriented Programming
Enter your rating (0-10): 8
Thank you. You entered the following:
Rating  Book
6       The Cat Who Knew C++
4       Felonious Felines
3       Warlords of Wonk
5       Don't Touch That Metaphor
8       Panic Oriented Programming

All this program does is use the vector template as a convenient way to create a dynamically allocated array. The next section shows an example that uses more of the class methods.