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13.x — Chapter 13 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;

Show Solution

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.

Show Solution

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

Show Solution

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

Show Solution

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)

Show Solution

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.

Show Solution

4c) Now add a constructor that takes a double. The follow program should run:

This program should produce the result

0.01
-0.01
5.01
-5.01
107

Recommendation: This one will be a bit tricky. Do this one in three steps. First, solve for the cases where the double parameter is representable directly (cases a & b above). Then, update your code to handle the cases where the double parameter has a rounding error (cases c & d). Lastly, handle the edge case where the decimal rounds up to 100 (case e).

For all cases: Show Hint

For cases a & b: Show Hint

For cases c & d: Show Hint

Show Solution

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

Show Solution


16.1 -- Object relationships
Index
13.16 -- Shallow vs. deep copying

346 comments to 13.x — Chapter 13 comprehensive quiz

  • Sonny Bui

    For question #2, should operator<<() check for "divide by zero"?

    Thank you.

  • hi
    in 4b :check line 17 (change or to are(

  • thx u nascardriver for these lessons
    i have one question i hope u can answer me.
    i change line 17 to return by values not by reference,and when i run it i get result like this :

    why 6.4 , i searched alot and i cant find why,i hope u can answer me.

    • nascardriver

      When you're returning by value

      ie. the `+= 10` doesn't affect `avg`.

      When you return by reference

  • LFJ

    I don't know if it has already been brought up, but the solution in 4c) for the constructor from a double does not always work correctly.

    For instance, if we use it with number 106.9978, the constructor will create a FixedPoint2 with m_base = 106 and m_decimal = 100, which is incorrect and should bw m_base = 107 and m_decimal = 0. I think the solution is to round the double before separating the base from the decimal, for example:

  • Rostyslav

    In question 3, why did not you use a delegating constructor for your copy constructor like so:

    ?

    • nascardriver

      Hi!

      The `IntArray(int)` constructor value-initializes `m_array`, but `IntArray(const IntArray&)` doesn't need that behavior, because it overrides the elements anyway.

  • Marcel

    For question 3 I wrote a private member function to copy the array like this:

    and called it both from the copy ctor and operator=

    My reasoning was that this way I avoid repeating myself and since I initialised the int array with either

    or

    there shouldn't be any harm in calling delete[] on it. Is there a reason I shouldn't do it this way?

  • James

    Greetings! For question 4(d), I had a slightly different answer for overloaded unary operator- and operator==;

    And;

    Are there any advantages/disadvantages to using the above overloads compared to those provided in the solution? Also, I noticed someone below had an issue with their binary operator+ working with anonymous objects (r-values) because they didn't set the function parameters as const. However I am able to get this to work with non-const function parameters and a non-const double typecast overload;

    Are you able to enlighten me on why this worked? Seems counter-intuitive to me.

    James

    • rayT

      converting the FixePpoint2 objects to doubles is just wasted work when you can directly compare the members.

      removing the const from the operator+ would make this example fail:

      because the compiler will require operator+(FixedPoint2&, const FixedPoint2&) to have been defined.

  • yeokaiwei

    For Quiz 3
    Maybe it could be pushed to templates?

    I need a companion Youtube video to explain it.

    https://www.youtube.com/watch?v=TzB5ZeKQIHM

  • yeokaiwei

    3. Quiz 3 Feedback
    Did not know how to start on Quiz 3 beyond the first few steps.

    Just went straight to the solution.

    //This is dynamically allocating a new pointer
        int ptr* { new int{} };

        delete ptr;
        ptr = nullptr;

    Too many errors trying to figure things out.

  • yeokaiwei

    The Quizzes are getting harder to DIY.

    += was not covered explicitly in the earlier Chapter 9 topics.

    It is extremely confusing.

    "When dealing with binary operators that do modify the left operand (e.g. operator+=), the member function version is typically preferred. In these cases, the leftmost operand will always be a class type, and having the object being modified become the one pointed to by *this is natural. Because the rightmost operand becomes an explicit parameter, there’s no confusion over who is getting modified and who is getting evaluated."

    I hope this is acceptable.

    //4
    //6
    //12
    //6.5
    //7
    //7

    //ERROR
    //1.E0898 nonmember operator requires a parameter with class or enum type
    //You need to place a class in parameters for operator+, it doesn't do integers
    //When you overload operator+, are they purely for adding classes?
    //2.E049 no operator "/" matches these operands.
    //3.Assumed we had to overload + and = separately. You actually don't do that. You just overload +=.
    //Why did I think that way? Because x += y is x= x+y.
    //4. friend Average& operator+= (int value);
    //Average operator += (int value)
    //{
    //    return ((value.m_total + value) / value.m_number);//
    //}
    //I tried to make it normal function due to Error 3. Chain mistakes. += should be a member function
    //Else, C8203 and E0345
    //5. C26495 Always intialize a member variable (type.6)
    //This peculiar error does not go away until you {} the member variables and Debug.
    //Building the program again does nothing.
    //6. If you remove the "friend" from the std::ostream, you get E0344 (too many parameters) EO349 (no operator "<<" matches these opearands).
    //Why do you still need to "friend" it when it is already a member function?
    //It's peculiar logic as I assumed "friend" is for normal functions trying to get access to private member variables.

  • yeokaiwei

    1. Feedback
    For Quiz 1, could you add in parenthesis operators as well?

    • yeokaiwei

      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 does not modify its left operand (e.g. operator+), do so as a normal function (preferred) or friend function.
          If you’re overloading a binary operator that modifies its left operand, but you can’t modify the definition of the left operand (e.g. operator<<, which has a left operand of type ostream), do so as a normal function (preferred) or friend function.
          If you’re overloading a binary operator that modifies its left operand (e.g. operator+=), and you can modify the definition of the left operand, do so as a member function. {Seems to be missing this part}

  • Mahmood

    For 4d I am not able to overload operator==
    Apparently overloading it manually makes it so there are a total of 3 overloads:
    - Arithmetic == Arithmetic
    - double = FixedPoint2
    - const FixedPoint2 &fp1, const FixedPoint2 &fp2

    It works without overloading the operator== for me.

    • nascardriver

      Please post your code and error message. No error should occur when you add this operator.

      • Mahmood

        The error is as mentioned above:

        more than one operator "==" matches these operands:
                built-in operator "arithmetic == arithmetic"
                function "operator==(const FixedPoint2 &fp1, const FixedPoint2 &fp2)"
                operand types are: double == FixedPoint2

        hope thats all you need.

        • nascardriver

          Your `operator+` doesn't work with temporaries, because its parameters are non-const references. Temporaries can't bind to non-const references, so `FixedPoint2{ 0.75 } + FixedPoint2{ 1.23 }` can't use your operator. Instead, the 2 `FixedPoint2`s are converted to `double`, then the built-in `operator+(double, double)` is used. Then you try to do `double+FixedPoint2`, which is ambiguous.

  • I feel like I missed something along the line somewhere. In the final program of the review, why does the following work:

    but this doesn't:

    For some reason when I use the braces, the compiler complains about a narrowing conversion, which doesn't make sense. It should be a valid anonymous class object either way.

    • nascardriver

      C++ can only perform integer arithmetic on full integers, not smaller types. `m_base` is `std::int_least16_t`, so the `-` converts it to an `int`.
      Because the `FixedPoint2` constructor wants a `std::int_least16_t` but you're giving it an `int`, the compiler complains, as this conversion could be lossy, which is not allowed in list initialization.

      I've updated the solution to use list initialization and casts. Thanks for pointing it out!

  • 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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