C++ Classes

We already know about structs which bring together several variables that describe a "thing." However, to implement functions, we'd have to pass the struct by value.

In the object oriented way, we prefer to have related functionality (print_rectangle, area for a rectangle class) be part of the object (the struct)

What is a class

• A class definition is like a blueprint defining a type.
• Objects of that type are created from that blueprint
• Once we define a class, we have one blueprint from which we can create 0 or more objects.
• Each of the objects is an instance of the class and has its own copies of all instance variables.

Writing C++ Classes

• Class definition goes in a .h file.
• Functions can be declared and defined inside class {...}.
• Only define member functions inside the class definition if it's very short. This is called "in-lining" the function definition. Otherwise, put a prototype in the class definition and define the member function in a .cpp file.
  • You'll need to qualify the function with the class scope such as Classname::function(){} in the .cpp file.
  • Do not try to initial class fields immediately when they are declared. E.g., double width = 10; because this is allowed for static fields.

const modifier

void print() const {
	...
}

• The use of const as modifier in method header indicates that the function will not modify any member fields.
• const can be used for member fields too.

Examples

In-lining example:

// rectangle.h
#ifndef RECTANGLE_H
#define RECTANGE_H

class Rectangle {
	...
	double area() const {
	// in-lining
	return width*height;
	}
	...
};
#endif //RECTANGLE_H

Defining a function outside class:

// rectangle.h
#ifndef RECTANGLE_H
#define RECTANGLE_H

class Rectangle {
	...
   double area() const;
   ...
}
#endif //RECTANGLE_H

// rectangle.cpp
#include "rectangle.h"

double Rectangle::area() const {
	//define outside class
	return width * height;
}

class2.cpp:

#include <iostream>

class Rectangle {
public:
	double width;
	double height;
	
	void print() const {
		std::cout << "width=" << width << ", height=" << height << std::endl;
	}
	
	double area() const {
		return width*height;
	}
};

int main() {
	Rectangle r = {30.0, 40.0};
	r.print();
	std::cout << "area=" << r.area() << std;;endl;
	return 0;
}

• Notice that the member functions doesn't need to pass in the parameters since they can directly access the fields because they are in the same class.

Protection

Fields and member functions can be public, private, or protected.

• A public field or member function can be accessed freely by any code with access to the class definition (code that includes the .h file).
• A private field or member function can be accessed from other member functions in the class, but not by a user of the class.
  • Base-class members marked private cannot be accessed from member functions defined in the derived class. (They are still there, only derived class member functions can't use them without public or protected accessor or mutator functions) We use public: and private: to divide class definition into sections according to whether members are public or private. private is default.

How to section:

class Rectangle {
public:
	...
	double area() const {
	//definition inside class
	return width * height;
	}
	...
private:
	double width, height;
};

\

• A protected field or member functions can be accessed from member functions of their class and from functions defined in derived classes.
  • Based class members marked public or protected can be accessed from member functions defined in the derived class

this pointer

When another member function has a parameter with the same name as the instance variable it is supposed to initialize, the local variable (parameter) will hide the instance variable. this is a pointer to the instance variable and can be used to clarify.

• this->init always refers to the instance variable init.

We don't use this unless necessary in C++, unlike Java, where it is a good style to always qualify instance members.

Array of Objects

Declaring an array of a class type makes all the objects, calling the default constructor to create each one. Thus, requires the class to have a default constructor.

What if you don't want to have default constructor?

1. List Use list-initialization to initialize each object in the array. MyThing s[3] = {{3}, {2}, {4}}; Every value inside will be the engines for the objects. MyThing only has one member field, so we only provide one value. This is called listing test when we initialize structures by passing in different values.

2. use STL to reserve some element size

int main() {
	// empty vector and reserve 10 element size
	std::vector<MyThing> s;
	s.reserve(10);
	
	// initialization using emplace_back
	for (int i = 0; i < 10; ++i) s.emplace_back(i);
}