C++ Primer Plus

// stkoptr1.cpp -- testing stack of pointers

#include

#include      // for rand(), srand()

#include        // for time()

#include "stcktp1.h"

const int Num = 10;

int main()

{

    std::srand(std::time(0)); // randomize rand()

    std::cout << "Please enter stack size: ";

    int stacksize;

    std::cin >> stacksize;

// create an empty stack with stacksize slots

    Stack st(stacksize);

// in basket

    const char * in[Num] = {

            " 1: Hank Gilgamesh", " 2: Kiki Ishtar",

            " 3: Betty Rocker", " 4: Ian Flagranti",

            " 5: Wolfgang Kibble", " 6: Portia Koop",

            " 7: Joy Almondo", " 8: Xaverie Paprika",

            " 9: Juan Moore", "10: Misha Mache"

            };

 // out basket

    const char * out[Num];

    int processed = 0;

    int nextin = 0;

    while (processed < Num)

    {

        if (st.isempty())

            st.push(in[nextin++]);

        else if (st.isfull())

            st.pop(out[processed++]);

        else if (std::rand() % 2  && nextin < Num)   // 50-50 chance

            st.push(in[nextin++]);

        else

            st.pop(out[processed++]);

    }

    for (int i = 0; i < Num; i++)

        std::cout << out[i] << std::endl;

    std::cout << "Bye\n";

    return 0;

}

Two sample runs of the program in Listing 14.16 follow (note that, thanks to the randomizing feature, the final file ordering can differ quite a bit from one trial to the next, even when the stack size is kept unaltered):

Please enter stack size: 5

 2: Kiki Ishtar

 1: Hank Gilgamesh

 3: Betty Rocker

 5: Wolfgang Kibble

 4: Ian Flagranti

 7: Joy Almondo

 9: Juan Moore

 8: Xaverie Paprika

 6: Portia Koop

10: Misha Mache

Bye

Please enter stack size: 5

 3: Betty Rocker

 5: Wolfgang Kibble

 6: Portia Koop

 4: Ian Flagranti

 8: Xaverie Paprika

 9: Juan Moore

10: Misha Mache

 7: Joy Almondo

 2: Kiki Ishtar

 1: Hank Gilgamesh

Bye

Program Notes

The strings in Listing 14.16 never move. Pushing a string onto the stack really creates a new pointer to an existing string. That is, it creates a pointer whose value is the address of an existing string. And popping a string off the stack copies that address value into the out array.

The program uses const char * as a type because the array of pointers is initialized to a set of string constants.

What effect does the stack destructor have on the strings? None. The class constructor uses new to create an array for holding pointers. The class destructor eliminates that array, not the strings to which the array elements pointed.

An Array Template Example and Non-Type Arguments

Templates are frequently used for container classes because the idea of type parameters matches well with the need to apply a common storage plan to a variety of types. Indeed, the desire to provide reusable code for container classes was the main motivation for introducing templates, so let’s look at another example and explore a few more facets of template design and use. In particular, let’s look at non-type, or expression, arguments and at using an array to handle an inheritance family.

Let’s begin with a simple array template that lets you specify an array size. One technique, which the last version of the Stack template uses, is to use a dynamic array within the class and a constructor argument to provide the number of elements. Another approach is to use a template argument to provide the size for a regular array. That is what the new C++11 array template does. Listing 14.17 shows a more modest version of how this can be done.

Listing 14.17. arraytp.h

//arraytp.h  -- Array Template

#ifndef ARRAYTP_H_

#define ARRAYTP_H_

#include

#include

template

class ArrayTP

{

private:

    T ar[n];

public:

    ArrayTP() {};

    explicit ArrayTP(const T & v);

    virtual T & operator[](int i);

    virtual T operator[](int i) const;

};

template

ArrayTP::ArrayTP(const T & v)

{

    for (int i = 0; i < n; i++)

        ar[i] = v;

}

template

T & ArrayTP::operator[](int i)

{

    if (i < 0 || i >= n)

    {

        std::cerr << "Error in array limits: " << i

            << " is out of range\n";

        std::exit(EXIT_FAILURE);

    }

    return ar[i];

}

template

T ArrayTP::operator[](int i) const

{

    if (i < 0 || i >= n)

    {

        std::cerr << "Error in array limits: " << i

            << " is out of range\n";

        std::exit(EXIT_FAILURE);

    }

    return ar[i];

}

#endif

Note the template heading in Listing 14.17:

template

The keyword class (or, equivalently in this context, typename) identifies T as a type parameter, or type argument. int identifies n as being an int type. This second kind of parameter, one that specifies a particular type instead of acting as a generic name for a type, is called a non-type, or expression, argument. Suppose you have the following declaration:

ArrayTP eggweights;

This causes the compiler to define a class called ArrayTP and to create an eggweights object of that class. When defining the class, the compiler replaces T with double and n with 12.