Class Design 101
Slides version: lecture2_slides.html Website version: lecture2.html
- Recap: Hello Class
- Class Definition
- Class Instance
- Class Usage/Member Function
- Example: Triangle
- Example: Vector
Recap: Hello Class
class Point
{
public:
// Data members
int x;
int y;
// Member functions
void init();
void print();
void modify();
};
int main()
{
Point p1;
p1.init();
p1.print();
p1.modify();
p1.print();
return 0;
}
Class Definition
Syntax:
class Point // the class name
{
public: // public data members and member functions
// Note: there is also private: and protected: sections.
// we will discuss them in following lectures
// Data members
// as like as the data in a 'struct', you can have multiple
// variables in different types
int x;
int y;
};
Class Instance
int main()
{
Point p1, p2;
// init 2 instances of Point
p1.x = 0; p1.y = 0;
p2.x = 0; p2.y = 0;
// print 2 instances of Point
cout << "p1: (" << p1.x << ", " << p1.y << ")" << endl;
cout << "p2: (" << p2.x << ", " << p2.y << ")" << endl;
// modify each instance
// p1 and p2 are the same class, but they are different instances
// so they can have different values
p1.x = 1; p1.y = 2;
p2.x = 3; p2.y = 4;
// you can see the change in p1 and p2
cout << "p1: (" << p1.x << ", " << p1.y << ")" << endl;
cout << "p2: (" << p2.x << ", " << p2.y << ")" << endl;
return 0;
}
$ ./a.out
p1: (0, 0)
p2: (0, 0)
p1: (1, 2)
p2: (3, 4)
Class Usage/Member Function
Class as a argument
As like as struct, you can use a class instance as a argument to a function.
void init_point(Point &p)
{
p.x = 0;
p.y = 0;
}
void print_point(const Point &p)
{
cout << "(" << p.x << ", " << p.y << ")";
}
void modify_point(Point &p, const int& arg_x, const int& arg_y)
{
p.x = arg_x;
p.y = arg_y;
}
int main()
{
Point p1, p2;
// init 2 instances of Point
init_point(p1);
init_point(p2);
// print 2 instances of Point
cout << "p1: "; print_point(p1); cout << endl;
cout << "p2: "; print_point(p2); cout << endl;
// modify each instance
modify_point(p1, 1, 2);
modify_point(p2, 3, 4);
// you can see the change in p1 and p2
cout << "p1: "; print_point(p1); cout << endl;
cout << "p2: "; print_point(p2); cout << endl;
return 0;
}
Use class member function
Instead of using extra functions, you can use member functions to manipulate the data.
class Point // the class name
{
public: // public data members and functions
// Note: there is also private: and protected: sections.
// we will discuss them in following lectures
// Data members
// as like as the data in a 'struct', you can have multiple
// variables in different types
int x;
int y;
// Member functions
// different from a 'struct', you can have multiple functions
// in a class that related to the class
void init() { x = 0; y = 0; }
void print() { cout << "(" << x << ", " << y << ")"; }
void modify(const int& arg_x, const int& arg_y) { x = arg_x; y = arg_y; }
};
int main()
{
Point p1, p2;
// init 2 instances of Point
p1.init(); p2.init();
// print 2 instances of Point
p1.print(); p2.print();
// modify each instance
p1.modify(1, 2); p2.modify(3, 4);
// you can see the change in p1 and p2
p1.print(); p2.print();
return 0;
}
Example: Triangle [Source]
#include <iostream>
using namespace std;
class Triangle
{
public:
double x1, y1, x2, y2, x3, y3;
void init();
void print();
void modify(double, double, double, double, double, double);
double area();
};
void Triangle::init()
{
x1 = 0;
y1 = 0;
x2 = 0;
y2 = 0;
x3 = 0;
y3 = 0;
}
void Triangle::print()
{
cout << "Triangle: " << endl;
cout << "(" << x1 << ", " << y1 << ")" << endl;
cout << "(" << x2 << ", " << y2 << ")" << endl;
cout << "(" << x3 << ", " << y3 << ")" << endl;
}
void Triangle::modify(double arg_x1, double arg_y1, double arg_x2,
double arg_y2, double arg_x3, double arg_y3)
{
x1 = arg_x1;
y1 = arg_y1;
x2 = arg_x2;
y2 = arg_y2;
x3 = arg_x3;
y3 = arg_y3;
}
double Triangle::area()
{
return (x1 * y2 + x2 * y3 + x3 * y1 - x1 * y3 - x2 * y1 - x3 * y2) / 2;
}
int main()
{
Triangle t;
t.init();
t.print();
t.modify(1, 1, 2, 2, 0, 3);
t.print();
cout << "Area: " << t.area() << endl;
return 0;
}
Output:
$ ./a.out
Triangle:
(0, 0)
(0, 0)
(0, 0)
Triangle:
(1, 1)
(2, 2)
(0, 3)
Area: 1.5
Example: Vector [Source]
#include <iostream>
using namespace std;
class Vector
{
public:
int x, y, z;
void init();
void print();
void modify(int, int, int);
int dot(const Vector &);
Vector cross(const Vector &);
};
void Vector::init()
{
x = 0;
y = 0;
z = 0;
}
void Vector::print()
{
cout << "Vector: "
<< "(" << x << ", " << y << ", " << z << ")";
}
void Vector::modify(int arg_x, int arg_y, int arg_z)
{
x = arg_x;
y = arg_y;
z = arg_z;
}
int Vector::dot(const Vector &arg_v)
{
return x * arg_v.x + y * arg_v.y + z * arg_v.z;
}
Vector Vector::cross(const Vector &arg_v)
{
Vector v;
v.x = y * arg_v.z - z * arg_v.y;
v.y = z * arg_v.x - x * arg_v.z;
v.z = x * arg_v.y - y * arg_v.x;
return v;
}
int main()
{
Vector v1, v2;
v1.init();
v1.print();
cout << endl;
v1.modify(1, 1, 1);
v1.print();
cout << endl;
cout << endl;
v2.init();
v2.print();
cout << endl;
cout << endl;
v2.modify(2, 3, 4);
cout << "v1: ";
v1.print();
cout << endl;
cout << "v2: ";
v2.print();
cout << endl;
cout << "Dot: " << v1.dot(v2) << endl;
cout << endl;
v2.modify(5, 6, 7);
Vector v3 = v1.cross(v2);
cout << "v1: ";
v1.print();
cout << endl;
cout << "v2: ";
v2.print();
cout << endl;
cout << "Cross: ";
v3.print();
cout << endl;
cout << endl;
return 0;
}
Output:
Vector: (0, 0, 0)
Vector: (1, 1, 1)
Vector: (0, 0, 0)
v1: Vector: (1, 1, 1)
v2: Vector: (2, 3, 4)
Dot: 9
v1: Vector: (1, 1, 1)
v2: Vector: (5, 6, 7)
Cross: Vector: (1, -2, 1)
Pratices
- Pratice 1: 參考
Triangleclass, 如果四邊形、多邊形要如何設計？ - Pratice 2: 參考
Vectorclass, 如果需要紀錄多維度的座標系上、或是轉換 basis 為其他 vector 要如何設計？