COMP_SCI 211 (Fall 2022)
Lecture 16:
C++ inheritance +
Building a game
SRUTI BHAGAVATULA
11/10/2022
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Administrivia
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HW 5 due Tuesday
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Reminder that extra time is meant to be used
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Not recommended to delay – haven’t seen much on Piazza
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Need to get started on your projects immediately after
Proposals are due tomorrow!!
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Only seen a 14 proposals so far
Projects meant to be started immediately after HW5
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Wed-Fri of next week (can start earlier once approved)
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Mon-Wed of Thanksgiving week
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All of last week of classes
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Today’s Goals
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Introduce concept of inheritance for C++ classes
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Develop Tic-Tac-Toe game plan and start implementing
from scratch
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Inheritance in C++
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Consider the game Super Mario
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Many characters: Mario, Luigi, Peach, Bowser
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Each has special powers and behavior
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How they might be defined
class Mario {
class Bowser {
private:
Posn<int> position;
double height;
double weight;
bool flower_power;
bool invincible;
int lives_left;
}
private:
Posn<int> position;
double height;
double weight;
bool spitting_fire;
bool defeated_mario;
double poison_gas_left;
}
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What if we took out the common fields?
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We can do this through inheritance!
Parent class (has all common fields)
Child class
(has extra
fields specific
to child)
Child class
(has extra
fields specific
to child)
class Character {
Base class!
public:
Character();
void display();
Parent members are private
• children can’t access them
class Mario: public Character {
private:
bool flower_power;
bool invincible;
int lives_left;
}
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private:
Posn<int> position;
double height;
double weight;
};
class Bowser: public Character {
private:
bool spitting_fire;
bool defeated_mario
double poison_gas_left
}
class Character {
Base class!
public:
Character();
void display();
Change to protected
• children can now access them
• equivalent to private outside of children
class Mario: public Character {
private:
bool flower_power;
bool invincible;
int lives_left;
}
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protected:
Posn<int> position;
double height;
double weight;
};
class Bowser: public Character {
private:
bool spitting_fire;
bool defeated_mario
double poison_gas_left
}
Derived class needs its own unique
constructor
class Mario: public Character {
public:
Mario(bool f, bool i, int l);
private:
bool flower_power;
bool invincible;
int lives_left;
}
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Constructor for our derived class
Mario::Mario(bool f, bool i, int l)
: Character(),
flower_power(f),
invincible(i),
lives(l)
{ }
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Base class constructors are called first in the initializer list
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C++ will automatically call the default constructor if one exists and you don’t
call any
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Class derivation list
class Name : public BaseClass1, public BaseClass2
{ };
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It is possible to inherit from any number of classes
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Can add some difficulties outside the scope of this class (Diamond problem)
public is an access specifier
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Always want to use public
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Private would make everything inherited private
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would mean other things wouldn’t know you had them
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really defeats the whole purpose
Slide adopted from Branden Ghena
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Extending base class functionality
class Mario: public Character {
public:
Mario(bool f, bool i, int l);
void throw();
private:
bool flower_power;
bool invincible;
int lives_left;
}
Extended functionality
Provides features that the
original class does not
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Overriding base class functionality
class Mario: public Character {
public:
Mario(bool f, bool i, int l);
void throw();
void display();
private:
bool flower_power;
bool invincible;
int lives_left;
}
Overridden functionality
Redefines existing
functionality to do something
different
How does a function know which
function to call?
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Character c;
Mario m(true, false, 2);
c.display();
m.display();
Compiler decides based on types at compile type.
Will call the correct function based on the object.
This is static dispatch!
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What if we had a function like this?
void print_char(Character& c) {
c.display();
}
Static dispatch is happening by calling
the Character print based on the type
Character c;
Mario m(true, false, 2);
print_char(c);
print_char(m);
//Function call casts m to base class
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We want dynamic dispatch
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Decide at runtime which implementation is the best fit
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This is dynamic polymorphism!
class Character {
class Mario: public Character {
public:
Character();
virtual void display();
public:
void display() override;
protected:
Posn<int> position;
double height;
double weight;
bool flower_power;
bool invincible;
int lives_left;
}
private:
bool flower_power;
bool invincible;
int lives_left;
}
Making a pure virtual function
(base function must be overridden)
class Character {
public:
virtual void display() const = 0;
}
class Mario: public Character {
public:
void display() const override;
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Planning and building
Tic-Tac-Toe
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What you’ll need
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Downloaded project starter code from Canvas
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A place to take notes where you can take notes to plan out the
program