in2029 programming in c - exam paper 2008

THE CITY UNIVERSITYLONDON

No: 000

BSc Degree in Computer Science

Object-Oriented Programming in C++ (IN2029)

January 2009

Answer TWO questions ONLY.

External examiner: Internal examiners: Ross PatersonTillman Weyde

1. a) Define a class PhoneEntry containing a name and a phone number, bothrepresented as strings. Include a constructor and appropriate selector memberfunctions. (20%)

b) Write an appropriate overloading of the == operator for your class. (20%)

c) Define a Contacts class containing an arbitrary number of PhoneEntryrecords. Your class should include methods to

i) add a new PhoneEntry.ii) given a name, print all the associated phone numbers, if any.

iii) given a phone number, print all the associated names, if any.

(30%)

d) Explain in detail what happens in the following:

{Contacts p1;// add some entries to p1;Contacts p2;p2 = p1;Contacts p3 = p1;

}

(25%)

e) What can go wrong when using delete in C++? (5%)

of 4

2. a) Explain how initialization differs from assignment (or not) for each of the fol-lowing:

i) variables of primitive type (e.g. int).ii) objects

iii) constantsiv) references

(20%)

b) Suppose a class contains a declaration

virtual int capacity() = 0;

What sort of declaration is this, what does it mean, and what implications doesit have for the use of the class and any derived classes? (15%)

c) Write a function that computes the total of a list of doubles. (15%)

d) Write a generic procedure that takes two iterators, assumed to refer to the samecontainer, and returns the distance between them. You may also assume that theposition of first iterator is not after the position of the second iterator. (20%)

e) Suppose a legacy windowing library provides the following global functions:

int win_create(int width, int height);void win_draw_rectangle(int window_id,

int x, int y, int w, int h);void win_draw_text(int window_id,

int x, int y, char *str);...void win_destroy(int window_id);

where the win create function returns a numeric identifier for the windowused by the other functions. The win destroy function removes the windowfrom the screen and releases any resources that it uses. The other functions areused for drawing on the window (before it is destroyed). The last argument towin draw text is a C-style null-terminated character array.Design an idiomatic C++ interface to the above library. Your design should usethe “resource acquisition is initialization” technique to make it easy for clientsto ensure that windows are destroyed when no longer in use, even if exceptionsoccur in other code. Show by example how your interface should be used, andexplain how it copes with exceptions. (30%)

of 4

3. Suppose a class Person includes the following:

Person(const string &n, int a) ...string get_name() const ...int get_age() const ...

a) Explain the meanings of the different uses of const in the above class. (10%)

b) Define a derived class Customer of Person, which also has an accountbalance of type double. Include a constructor and an appropriate selectormethod. (20%)

c) Given the declarations

Person p("Alice", 32);Customer c("Bob", 27, 236.75);

say whether or not each of the following is legal, and why. For those that arelegal, explain what they do.

i) c = p;

ii) p = c;

iii) Person *ptr = &c;

iv) Customer *cptr = &p;

(25%)

d) Consider the following procedure, which is intended to read a series of customerdescriptions and store them in a vector:

void read_all(vector<Customer *> customers) {string name;int age;double balance;while (cin >> name >> age >> balance) {

Customer cust(name, age, balance);customers.push_back(&cust);

}}

This compiles, but does not work as intended:

i) After the method is called, the vector passed as an argument seems un-changed. Explain what is happening and how to fix it. (10%)

ii) After the above problem is fixed, the program crashes on any attempt toaccess the objects in the vector. Explain what is happening and how to fixit, including any implications for the rest of the program. (15%)

e) What complications arise when a language has multiple inheritance, and howdoes C++ deal with these? (20%)

of 4

Model Answers and Marking Scheme1. a) class PhoneEntry {

string _name;string _phone;

public:PhoneEntry(const string & name, const string & phone) :

_name(name), _phone(phone) {}

const string & name() const { return _name; }

const string & phone() const { return _phone; }};

Marking: members [4], constructor [8] (2 for consts and refs), selectors [8] (2for consts and refs).

b) This could be either a member function

bool operator==(const PhoneEntry & other) const {return _name == other._name && _phone == other._phone;

}

or a global function:

bool operator==(const PhoneEntry & p1,const PhoneEntry & p2) {

return p1.name() == p2.name() &&p1.phone() == p2.phone();

}

Marking: procedure head [10, including 1 each for the consts], body [10].

c) class Contacts {vector<PhoneEntry> entries;

public:void addEntry(const PhoneEntry & entry) {

entries.push_back(entry);}

void print_numbers(const string & n) const {for (int i = 0; i < entries.size(); i++)

if (entries[i].name() == n)cout << entries[i].phone() << endl;

}

void print_names(const string & p) const {for (int i = 0; i < entries.size(); i++)

if (entries[i].phone() == p)

1

cout << entries[i].name() << endl;}

};

No constructor is necessary; the compiler-generated default constructor initial-izes an empty vector of entries. Iterator loops are acceptable in place of index-ing. Any sequence type will do in place of vector, but if they use list, theymust use iterators.Marking: class [3], field [3], add [12], print methods [12].

d) First line initializes p1 using the default constructor of Contacts [3], whichinitializes an empty vector [3]. Then p2 is initializes using the default construc-tor like p1 above [3]. Then this is overwritten using the assignment operator ofContacts [3], which assigns the member, destroying the old vector [2] andoverwriting it with a copy [2]. The next line initializes p3 using the copy con-structor of Contacts [3], which copies the members across, using the copyconstructor of vector, which duplicates the vector of phone entries [3]. Theclosing brace destroys all three [2] in reverse order [1].

e) Possible answers:

• destroying an object while it is still in use• destroying an object twice• applying delete to an uninitialized pointer• applying delete to a pointer to local or static storage• using the wrong form of delete

Marking: 5 marks for two, 3 for one.

Model Answers: page 2

2. a) i) For variables of primitive type, there is no difference: both initializationand assignment amount to copying the value.

ii) Objects are initialized by constructors. Initialization from another objectof the same type invokes the copy constructor. Assignment is done usingthe assignment operator. When the object holds resources (e.g. storage)that must be freed by a destructor, these are usually defined to do differ-ent things: the copy constructor operates on an uninitialized object, whileassignment is called for an object that already has these resources, whichmust often be freed first.

iii) Constants can (indeed must) be initialized, but cannot be assigned to.iv) Initialization of a reference makes it another name for an existing location.

Subsequent assignment to the reference assigns to that location.

b) This is a pure virtual member function [3], meaning that the member functionhas no definition in this class, but may be defined in derived classes [3]. Thisimplies that the class is abstract and cannot be instantiated [3], though it can beused as a type [2]. Subclasses that define all these member functions may beinstantiated [3], and their instances used where the base class is expected [1].

c) int sum(const list<double> & l) {double total = 0;typedef list<double>::iterator Iter;for (Iter p = l.begin(); p != l.end(); ++p)

total += *p;return total;

}

Marking: procedure head [4], iterator loop [5], adding [4], return [2].

d) template <typename Iterator>int distance(Iterator first, const Iterator & last) {

int count;while (first != last) {

count++;first++;

}return count;

}

or

template <typename Iterator>int distance(Iterator first, const Iterator & last) {

int count;for ( ; first != last; first++)

count++;return count;

}

Marking: procedure head [10], iterator loop [5], managing the count [5].


e) Define a class

class window {int window_id;

public:window(int width, int height) :

window_id(create_window(int width, int height)) {}

˜window() { destroy_window(window_id); }

void draw_rectangle(int x, int y, int w, int h) {win_draw_rectangle(window_id, x, y, w, h);

void draw_text(int x, int y, const string &s) {win_draw_text(window_id, x, y, s.c_str());

}

...};

Marking: class with constructor saving the window id and destructor using it[10], other functions as member functions of the class [5], proper handling ofthe string [5].An example of the intended use [5]:

{window w(400, 300);...w.draw_rectangle(100, 200, 50, 50);w.draw_text(50, 100, "hello");...

}

If control exits the block, whether by exception or by normal completion, the lo-cally allocated window object will be destroyed, invoking its destructor, whichwill destroy the window on the screen [5].


3. a) The parameter n is passed by reference purely for efficiency, so it is markedwith const to indicate that it won’t be changed [5]. The methods annotatedwith const after the arguments do not alter any of the fields of the object. [5]

b) class Customer : public Person {double balance;

public:Customer(const string &n, int a, double b) :

Person(n, a), balance(b) {}

double get_balance() const { return balance; }};

Marking: inheritance [5], private/protected [2] field [3], constructor [3] initial-izing parent [2] and field [2], selector method [3] (including 1 for the const).

c) i) Illegal [3], because Person is not a subtype of Customer [1].ii) Legal [3], because Customer is a subtype of Person [1], but only the

Person part of c is copied (c is sliced) [5].iii) Legal [3], because Customer is a subtype of Person [1], and no objects

are copied. Only the address of c is placed in ptr, though only Personattributes may be accessed through this pointer [4].

iv) Illegal [3], because Person is not a subtype of Customer [1].

d) i) The parameter customers is passed by value, so all updates will be to acopy, not the original [5]. This may be corrected by changing to call-by-reference [5]:

void read_all(vector<Customer *> &customers) {

ii) The vector is filled with pointers to locally allocated objects, which aredestroyed soon after [6]. This may be corrected by dynamically allocatingthe Customer objects [7]:

void read_all(vector<Customer *> &customers) {string name;int age;string balance;while (cin >> name >> age >> balance)

customers.push_back(new Customer(name, age, balance));

}

It will be necessary to delete these Customer objects when they areno longer required [2].

e) • How to refer to a member if two base classes define the same name? [5] InC++, qualify the name with the appropriate base class. [5] (But no way tooverride it.)

• If both base classes are derived from a common base class, should the cor-responding data be shared or replicated? [5] In C++, the former is selectedby specifying virtual inheritance. [5]


in2029 programming in c - exam paper 2008

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