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11.x — Chapter 11 comprehensive quiz

Summary

Inheritance allows us to model an is-a relationship between two objects. The object being inherited from is called the parent class, base class, or superclass. The object doing the inheriting is called the child class, derived class, or subclass.

When a derived class inherits from a base class, the derived class acquires all of the members of the base class.

When a derived class is constructed, the base portion of the class is constructed first, and then the derived portion is constructed. In more detail:

  1. Memory for the derived class is set aside (enough for both the base and derived portions).
  2. The appropriate derived class constructor is called.
  3. The base class object is constructed first using the appropriate base class constructor. If no base class constructor is specified, the default constructor will be used.
  4. The initialization list of the derived class initializes members of the derived class.
  5. The body of the derived class constructor executes.
  6. Control is returned to the caller.

Destruction happens in the opposite order, from most-derived to most-base class.

C++ has 3 access specifiers: public, private, and protected. The protected access specifier allows the class the member belongs to, friends, and derived classes to access the protected member, but not the public.

Classes can inherit from another class publicly, privately, or protectedly. Classes almost always inherit publicly.

Here’s a table of all of the access specifier and inheritance types combinations:

Access specifier in base class Access specifier when inherited publicly Access specifier when inherited privately Access specifier when inherited protectedly
Public Public Private Protected
Private Inaccessible Inaccessible Inaccessible
Protected Protected Private Protected

Derived classes can add new functions, change the way functions that exist in the base class work in the derived class, change an inherited member’s access level, or hide functionality.

Multiple inheritance enables a derived class to inherit members from more than one parent. You should avoid multiple inheritance as much as possible.

Quiz time

Question #1


For each of the following programs, determine what they output, or if they would not compile, indicate why. This exercise is meant to be done by inspection, so do not compile these (otherwise the answers are trivial).

a)

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b) Hint: Local variables are destroyed in the opposite order of definition.

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c)

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d)

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e)

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Question #2


a) Write an Apple class and a Banana class that are derived from a common Fruit class. Fruit should have two members: a name, and a color.

The following program should run:

And produce the result:

My apple is red.
My banana is yellow.

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b) Add a new class to the previous program called GrannySmith that inherits from Apple.

The following program should run:

And produce the result:

My apple is red.
My banana is yellow.
My granny smith apple is green.

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Question #3


Challenge time! The following quiz question is more difficult and lengthy. We're going to write a simple game where you fight monsters. The goal of the game is to collect as much gold as you can before you die or get to level 20.

Our program is going to consist of 3 classes: A Creature class, a Player class, and a Monster class. Player and Monster both inherit from Creature.

a) First create the Creature class. Creatures have 5 attributes: A name (std::string), a symbol (a char), an amount of health (int), the amount of damage they do per attack (int), and the amount of gold they are carrying (int). Implement these as class members. Write a full set of getters (a get function for each member). Add three other functions: void reduceHealth(int) reduces the Creature's health by an integer amount. bool isDead() returns true when the Creature's health is 0 or less. void addGold(int) adds gold to the Creature.

The following program should run:

And produce the result:

The orc has 3 health and is carrying 15 gold.

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b) Now we're going to create the Player class. The Player class inherits from Creature. Player has one additional member, the player's level, which starts at 1. The player has a custom name (entered by the user), uses symbol '@', has 10 health, does 1 damage to start, and has no gold. Write a function called levelUp() that increases the player's level and damage by 1. Also write a getter for the level member. Finally, write a function called hasWon() that returns true if the player has reached level 20.

Write a new main() function that asks the user for their name and produces the output as follows:

Enter your name: Alex
Welcome, Alex.
You have 10 health and are carrying 0 gold.

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c) Next up is the Monster class. Monster also inherits from Creature. Monsters have no non-inherited member variables.

First, write an empty Monster class inheriting from Creature, and then add an enum inside the Monster class named Type that contains enumerators for the 3 monsters that we'll have in this game: DRAGON, ORC, and SLIME (you'll also want a max_types enumerator, as that will come in handy in a bit).

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d) Each Monster type will have a different name, symbol, starting health, gold, and damage. Here is a table of stats for each monster Type:

Type Name Symbol Health Damage Gold
dragon dragon D 20 4 100
orc orc o 4 2 25
slime slime s 1 1 10

Next step is to write a Monster constructor, so we can create monsters. The Monster constructor should take a Type enum as a parameter, and then create a Monster with the appropriate stats for that kind of monster.

There are a number of different ways to implement this (some better, some worse). However in this case, because all of our monster attributes are predefined (not random), we'll use a lookup table. A lookup table is an array that holds all of the predefined attributes. We can use the lookup table to look up the attributes for a given monster as needed.

So how do we implement this lookup table? It's not hard. We just need an array that contains an element for each monster Type. Each array element will contain a Creature that contains all of the predefined attribute values for that Type of Monster. We place this array inside of a static member function of Monster so that we can get a default Creature for a given Monster::Type.

The definition of the lookup table is as follows:

Now we can call this function to lookup any values we need! For example, to get a Dragon's gold, we can call getDefaultCreature(Type::dragon).getGold().

Use this function and delegating constructors to implement your Monster constructor.

The following program should compile:

and print:

A orc (o) was created.

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e) Finally, add a static function to Monster named getRandomMonster(). This function should pick a random number from 0 to max_types-1 and return a monster (by value) with that Type (you'll need to static_cast the int to a Type to pass it to the Monster constructor).

Lesson 5.9 -- Random number generation contains code you can use to pick a random number.

The following main function should run:

The results of this program should be randomized.

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f) We're finally set to write our game logic!

Here are the rules for the game:

The player encounters one randomly generated monster at a time.
For each monster, the player has two choices: (R)un or (F)ight.
If the player decides to Run, they have a 50% chance of escaping.
If the player escapes, they move to the next encounter with no ill effects.
If the player does not escape, the monster gets a free attack, and the player chooses their next action.
If the player chooses to fight, the player attacks first. The monster's health is reduced by the player's damage.
If the monster dies, the player takes any gold the monster is carrying. The player also levels up, increasing their level and damage by 1.
If the monster does not die, the monster attacks the player back. The player's health is reduced by the monster's damage.
The game ends when the player has died (loss) or reached level 20 (win)
If the player dies, the game should tell the player what level they were and how much gold they had.
If the player wins, the game should tell the player they won, and how much gold they had
Here's a sample game session:

Enter your name: Alex
Welcome, Alex
You have encountered a slime (s).
(R)un or (F)ight: f
You hit the slime for 1 damage.
You killed the slime.
You are now level 2.
You found 10 gold.
You have encountered a dragon (D).
(R)un or (F)ight: r
You failed to flee.
The dragon hit you for 4 damage.
(R)un or (F)ight: r
You successfully fled.
You have encountered a orc (o).
(R)un or (F)ight: f
You hit the orc for 2 damage.
The orc hit you for 2 damage.
(R)un or (F)ight: f
You hit the orc for 2 damage.
You killed the orc.
You are now level 3.
You found 25 gold.
You have encountered a dragon (D).
(R)un or (F)ight: r
You failed to flee.
The dragon hit you for 4 damage.
You died at level 3 and with 35 gold.
Too bad you can't take it with you!
Hint: Create 4 functions:

The main() function should handle game setup (creating the Player) and the main game loop.
fightMonster() handles the fight between the Player and a single Monster, including asking the player what they want to do, handling the run or fight cases.
attackMonster() handles the player attacking the monster, including leveling up.
attackPlayer() handles the monster attacking the player.

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g) Extra credit:
Reader Tom didn't sharpen his sword enough to defeat the mighty dragon. Help him by implementing the following potions in different sizes:

Type Effect (Small) Effect (Medium) Effect (Large)
Health +2 Health +2 Health +5 Health
Strength +1 Damage +1 Damage +1 Damage
Poison -1 Health -1 Health -1 Health

Feel free to get creative and add more potions or change their effects!

The player has a 30% chance of finding a potion after every won fight and has the choice between drinking or not drinking it. If the player doesn't drink the potion, it disappears. The player doesn't know what type of potion was found until the player drinks it, at which point the type and size of the potion is revealed and the effect is applied.

In the following example, the player found a poison potion and died from drinking it (Poison was much more damaging in this example)

You have encountered a slime (s).
(R)un or (F)ight: f
You hit the slime for 1 damage.
You killed the slime.
You are now level 2.
You found 10 gold.
You found a mythical potion! Do you want to drink it? [y/n]: y
You drank a Medium potion of Poison
You died at level 2 and with 10 gold.
Too bad you can't take it with you!

Show Hint

Show Solution


12.1 -- Pointers and references to the base class of derived objects
Index
11.7 -- Multiple inheritance

368 comments to 11.x — Chapter 11 comprehensive quiz

  • giang

    So this is my codes for the game with some energy boost and there're also some differences with the Solution (especially in the playGame() function, the Monster and Potion classes):

    Any suggestions or recommendations? Thanks

    • nascardriver

      - `hasWon`, `getSize` should be `const`
      - Initialize variables with list initialization
      - Don't assign values to enumerators unless you have a good reason to
      - Line 122: Magic number: 1 (The first one)
      - `monsterAttacks::m` should be `const`
      - Line 158, 160: Duplicate comparison
      - Comparing strings is slow and easy to break. Strings are for interaction with humans, not for implementation details.
      - Line 165-202: Duplicate strings
      - I'm stopping the "should be `const`" now. A lot of your variables should be `const`.
      - Don't use `std::endl` unless you need to
      - Line 230, 232
      - Line 298, 299
      - Conditions are booleans. 1 is an integer. Use booleans where booleans are needed.

      You made some weird decisions, but you're getting there :)

  • giang

    Hi,
    Firstly, with the 'string' type but not other types, why do we have to use 'const string&'?

    Secondly, you said 'If you're using std::string in MonsterData, you can't make monsterData constexpr', can I ask Why is there an error with this?

    Thirdly, when I followed your instruction: 'remove constexpr and define the array outside of the class', it worked. Why do we have to do this and make a declaration and then an outside definition for this and Why did it work by doing that?

    When I didn't follow your instruction, the program compiled finely, but when I ran the program, a box appeared and told me that '... .exe has stopped working'. Why?

    Fourthly, Why do we have to 'get rid of first result of rand()'?

    Sorry for a lot of 'whys' but I hope you can clear st out for me a little bit =D Thanks

    • nascardriver

      > with the 'string' type but not other types, why do we have to use 'const string&'?
      Only fundamental types (`int`, `float`, etc.) and types that are fast to copy by design should be passed by copy. All other types should be passed by reference to prevent an unnecessary copy.

      > Why is there an error with [constexpr std::string]?
      (The lesson has been updated, this statement is no longer there but still correct)
      `std::string` uses dynamic memory allocation, which requires a run-time. `constexpr` has to be evaluatable at compile-time, so you can't use dynamic memory allocation and thus can't use `std::string`. These rules have been relaxed in C++20.

      > remove constexpr and define the array outside of the class
      You cannot define `static` members inside the class at their declaration, because if the class appears in multiple files (Through includes), the compiler/linker wouldn't know when to create the variable. `constexpr` variables are `inline` by default, and can therefore be defined multiple times in different files.

      > exe has stopped working
      Please use a debugger and post code and mark the location where the program crashed. If you tell you my car doesn't drive, can you tell me why?

      > Why do we have to 'get rid of first result of rand()'?
      `std::rand` might generate very similar results on the first call. See section "Help! My random number generator always generates the same first number!" in lesson 5.9.

  • Tom

    Hi!
    I have a recomendation about the last quiz.

    Right now, it is nearly impossible to beat the game (you will surely find a couple of dragons and they will destroy you if you don't flee. You can only win if you have an enormous luck and find a lot of slimes and orcs), so I changed it so you get all your health replenished every 5 levels. It made the game way more fair, even if you can still easily die if you don't have good luck with the monsters. I even managed to win a couple times!

    To do this, i added a function member to the Player class:

    and added:

    after you level up.

    Hope you consider this suggestion. Thanks!

    • nascardriver

      Thanks for the suggestion Tom! I've added quiz 3g, which might give you a health boost, if you're brave enough to drink something that fell out of a monster.

  • sv

    i wrote the game
    everything works just fine but at the end when player dies or wins the game, console closes quickly and the messages i wrote for these situations are not shown
    how can i fix that?

  • salah

    Result:

    constructor
    constructor
    -----------------------------
    Here it's clear that 'copy constructor' was not touched, but When I delete it I got this error: "attempting to reference a deleting a deleted function"  so what is the problem ?

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