Part 4 - Operator overloading, Friend Functions, The Big Three

Question 1

How will the compiler treat ++t when searching for the operator overload?

How is this different to t++?

What is the implication of this?

Answer 1

++t:

t.operator++() or operator++(t)

t++:

t.operator++(0) or operator++(t, 0)

To overload the postfix version we need to write a function with an extra int argument

Question 2

Why should the += operator return a reference to the object that has been assigned to?

Answer 2

so it can be used as part of a larger expression, e.g

(t+=7).printStandard()

or myt = t+=7;

Question 3

Why can the prefix version of operator++ return a reference while the postfix has to return a copy?

Answer 3

Prefix version increments and then returns, so the value before incrementing is not needed by the programmer

Postfix version returns the value before it has been incremented so needs to copy, increment original, return copy

Question 4

In the operator+ function, why would

Time tCopy(*this);

be preferred over

Time tCopy = *this;

Answer 4

Time tCopy = *this; would use the no-arg constructor and overwrite the default values

Time tCopy(*this) instead uses the copy constructor

Question 5

When must a friend function be used in operator overloading?

Answer 5

Where the left hand side operand is not the class itself, e.g. << and the operator needs to access private or protected members of the class

Question 6

friend operator overloads take both parameters as

a) references
b) pointers

Answer 6

a) references

Question 7

Where must the friend declaration be made?

Answer 7

Within the class (.cpp) declaration

Question 8

To have a reference to a const variable, a ___________ must be used

Answer 8

const reference

Question 9

Why is the following code fragment illegal:

const int months = 12;

int &a = months;

a = 13;

What should be done to fix it?

Answer 9

It will result in a const variable being changed

const int &a = months;

a=13;

Question 10

A constant member function is not allowed to return a ________ reference to the object to which it is applied or any of the members of that object

Answer 10

non-constant

Question 11

What 3 things must each class have?

Answer 11

copy constructor

assignment operator

destructor

Question 12

If the programmer fails to provide one of the “big three” functions what will happen?

Answer 12

The compiler will generate a default version of whatever is missing

Question 13

How would the assignment operator for class X be declared?

Answer 13

X& operator=(const X&)

Question 14

How is the destructor named for class X?

Answer 14

~X()

Question 15

How is the copy constructor for class X declared?

Answer 15

X::X(const X&){…}

Question 16

Why is the argument to the copy constructor always a reference?

Answer 16

Because otherwise call by value would be used - which uses the copy constructor to create the copy of the object

Question 17

What will the default copy constructor generated by the compiler do?

Answer 17

Use the copy constructors for any members of the class which are objects of other classes and initialise all other members using simple copying

Question 18

What will the default assignment operator do?

Answer 18

Just perform assignments to all members

Question 19

What will the default destructor do?

Answer 19

Invoke the destructors for any members of the class which are objects of other classes

Question 20

What is the typical circumstance that a non default version of one of the big three is required?

Why?

Answer 20

When the class has a pointer member

The default one will make the pointer point to the same object. Typically you will want it to instead refer to a copy of the object