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9.x — Chapter 9 comprehensive quiz

In this chapter, we explored topics related to operator overloading, as well as overloaded typecasts, and topics related to the copy constructor.

Summary

Operator overloading is a variant of function overloading that lets you overload operators for your classes. When operators are overloaded, the intent of the operators should be kept as close to the original intent of the operators as possible. If the meaning of an operator when applied to a custom class is not clear and intuitive, use a named function instead.

Operators can be overloaded as a normal function, a friend function, or a member function. The following rules of thumb can help you determine which form is best for a given situation:

  • If you’re overloading assignment (=), subscript ([]), function call (()), or member selection (->), do so as a member function.
  • If you’re overloading a unary operator, do so as a member function.
  • If you’re overloading a binary operator that modifies its left operand (e.g. operator+=), do so as a member function if you can.
  • If you’re overloading a binary operator that does not modify its left operand (e.g. operator+), do so as a normal function or friend function.

Typecasts can be overloaded to provide conversion functions, which can be used to explicitly or implicitly convert your class into another type.

A copy constructor is a special type of constructor used to initialize an object from another object of the same type. Copy constructors are used for direct/uniform initialization from an object of the same type, copy initialization (Fraction f = Fraction(5,3)), and when passing or returning a parameter by value.

If you do not supply a copy constructor, the compiler will create one for you. Compiler-provided copy constructors will use memberwise initialization, meaning each member of the copy is initialized from the original member. The copy constructor may be elided for optimization purposes, even if it has side-effects, so do not rely on your copy constructor actually executing.

Constructors are considered converting constructors by default, meaning that the compiler will use them to implicitly convert objects of other types into objects of your class. You can avoid this by using the explicit keyword in front of your constructor. You can also delete functions within your class, including the copy constructor and overloaded assignment operator if desired. This will cause a compiler error if a deleted function would be called.

The assignment operator can be overloaded to allow assignment to your class. If you do not provide an overloaded assignment operator, the compiler will create one for you. Overloaded assignment operators should always include a self-assignment check.

New programmers often mix up when the assignment operator vs copy constructor are used, but it’s fairly straightforward:

  • If a new object has to be created before the copying can occur, the copy constructor is used (note: this includes passing or returning objects by value).
  • If a new object does not have to be created before the copying can occur, the assignment operator is used.

By default, the copy constructor and assignment operators provided by the compiler do a memberwise initialization or assignment, which is a shallow copy. If your class dynamically allocates memory, this will likely lead to problems, as multiple objects will end up pointing to the same allocated memory. In this case, you’ll need to explicitly define these in order to do a deep copy. Even better, avoid doing your own memory management if you can and use classes from the standard library.

Quiz Time

1) Assuming Point is a class and point is an instance of that class, should you use a normal/friend or member function overload for the following operators?

1a) point + point
1b) -point
1c) std::cout << point
1d) point = 5;

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2) Write a class named Average that will keep track of the average of all integers passed to it. Use two members: The first one should be type std::int_least32_t, and used to keep track of the sum of all the numbers you’ve seen so far. The second should be of type std::int_least8_t, and used to keep track of how many numbers you’ve seen so far. You can divide them to find your average.

2a) Write all of the functions necessary for the following program to run:

and produce the result:

4
6
12
6.5
7
7

Hint: Remember that 8 bit integers are usually chars, so std::cout treats them accordingly.

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2b) Does this class need an explicit copy constructor or assignment operator?

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3) Write your own integer array class named IntArray from scratch (do not use std::array or std::vector). Users should pass in the size of the array when it is created, and the array should be dynamically allocated. Use assert statements to guard against bad data. Create any constructors or overloaded operators needed to make the following program operate correctly:

This programs should print:

5 8 2 3 6
5 8 2 3 6

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4) Extra credit: This one is a little more tricky. A floating point number is a number with a decimal where the number of digits after the decimal can be variable. A fixed point number is a number with a fractional component where the number of digits in the fractional portion is fixed.

In this quiz, we’re going to write a class to implement a fixed point number with two fractional digits (e.g. 12.34, 3.00, or 1278.99). Assume that the range of the class should be -32768.99 to 32767.99, that the fractional component should hold any two digits, that we don’t want precision errors, and that we want to conserve space.

4a) What type of member variable(s) do you think we should use to implement our fixed point number with 2 digits after the decimal? (Make sure you read the answer before proceeding with the next questions)

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4b) Write a class named FixedPoint2 that implements the recommended solution from the previous question. If either (or both) of the non-fractional and fractional part of the number are negative, the number should be treated as negative. Provide the overloaded operators and constructors required for the following program to run:

This program should produce the result:

34.56
-2.08
-2.08
-2.08
-0.05
-0.05

Hint: Although it may seem like more work initially, it’s helpful to store both the non-fractional and fractional parts of the number with the same sign (e.g. both positive if the number is positive, and both negative if the number is negative). This makes doing math much easier later.
Hint: To output your number, first cast it to a double.

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4c) Now add a constructor that takes a double. You can round a number (on the left of the decimal) by using the std::round() function (included in header cmath).

Hint: You can get the non-fractional part of a double by static casting the double to an integer
Hint: To get the fractional part of a double, you’ll first need to zero-out the non-fractional part. Use the integer value to do this.
Hint: You can move a digit from the right of the decimal to the left of the decimal by multiplying by 10. You can move it two digits by multiplying by 100.

The follow program should run:

This program should produce the result

0.01
-0.01
5.01
-5.01

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4d) Overload operator==, operator >>, operator- (unary), and operator+ (binary).

The following program should run:

And produce the output:

true
true
true
true
true
true
true
true
-0.48
0.48
Enter a number: 5.678
You entered: 5.68

Hint: Add your two FixedPoint2 together by leveraging the double cast, adding the results, and converting back to a FixedPoint2.
Hint: For operator>>, use your double constructor to create an anonymous object of type FixedPoint2, and assign it to your FixedPoint2 function parameter

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10.1 -- Object relationships
Index
9.15 -- Shallow vs. deep copying

324 comments to 9.x — Chapter 9 comprehensive quiz

  • srt1104

    Few doubts in the following program:

    It's output on my machine:

    I am unable to understand what is being called when uniform initialization happens on lines 87 and 90.
    Neither of the default constructor nor the copy constructor nor the assignment operator function are called as seen in the output.

    Also, I just looked up all special functions provided by C++ in classes and when I delete IntArray(IntArray&&) constructor, lines 82 and 87 show errors but not line 90 (even when I remove the reference). So which function is being called when you do uniform initialization and is it doing deep copy?

  • Gustaw

    For 4c, is it okay to do it like that?

    • nascardriver

      Prefer initialization over assignment. Assignment is more expensive and not always possible.
      Assigning to `m_decimal` is ok, because it depends on another member variable, but `m_base` should be initialized in the member initializer list.

  • Alek

    Hey, I have a question, in the last lesson you implemented a void function to do the deep copy and then called it within both assignment operator and copy constructor. why is that ? why have you implemented deep copy separately in assignment and copy constructor here ? thanks a bunch.

    another thing, please take a look at code below: I deleted implementation of my deepCopy to see if I will get undefined behavior, but I'm getting the same answer everytime,why is that ?thanks again.

    • Alek

      any Idea guys ? I'm stuck and struggling here :(.
      by the following sentence I mean I get the right output everytime."another thing, please take a look at code below: I deleted implementation of my deepCopy to see if I will get undefined behavior, but I'm getting the same answer everytime".
      I'd appreciate any help thanks!

    • nascardriver

      I won't comment on the `deepCopy` function because that lesson should use copy-swap, but lesson updates are on hold right now. Please read up on the copy-swap idiom yourself.

      You're leaking memory because `m_arr` is never `delete`d. If you fixed the leak by deleting `m_arr` in the destructor, you'd get undefined behavior.

  • CC

    This definition is also not needed:

    You've told the compiler how to cast `FixedPoint2` numbers into doubles, and it'll do that on its own without this definition.

  • CC

    You've already overloaded `operator[]`, so you can index `array` directly here:

    In fact, if the class had a public member function that told you the array's length (which seems natural to include...) then you don't need to implement the overload of `operator<<` as a friend function at all.

  • Yolo

    Two questions for exercise 3.
    1) The destructor seems to be destructing my code, but code works pretty fine without it.
    2) I didn't really need to overload the assignment operator = to make my code work. Why do we have to use it?

    • nascardriver

      1) You're leaking memory which may or may not be cleaned up by your operating system. If you allocate something, you're responsible for freeing it.

      2) `a` and `b` point to the same memory. Modifying one affects the other. Deleting them causes a double free, which causes undefined behavior.

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