Classes in C++

COMP282 Lectures 2022-03-08

The nice thing about C++ being compatible with C is that you don’t have to use classes if you don’t want to.

Classes

A basic example of a class looks like so:

#include <string>
class Animal {
	int weight;
	std::string name;
};

Remember to include the semicolon ; at the end of classes.

Access Modifiers

We can use the following access modifiers:

• public - You can access it from anywhere.
• private - You can only access them from within the class or from friends.
• protected - You can only access them from this class, sub-classes or from friends.

The default is private.

The syntax is like so:

#include <string>
class Animal {
public:
	int weight;
	std::string name;
};

Getters & Setters

#include <string>
#include <iostream>
using namespace std;
class Animal {
private:
	int weight;
	string name;
public:
	void setWeight(int value) {
		weight = value;
	}
	int getWeight(void) {
		return weight;
	}
};
int main() {
	Animal example; // Creates the object
	example.setWeight(55);
	cout << example.getWeight();
}

Using Headers

We should put the following in header files:

• Class Definitions
• Constants

This will split our code like so:

Animal.h:

#include <string>
namespace ani {
	class Animal {
	private:
		int weight;
		std::string name;
	public:
		void setWeight(int value);
		int getWeight(void);
	};
}

You should not using namespace ... in header files.

Animal.cpp

#include "Animal.h"
int ani::Animal::getWeight() {
	return weight
}
void ani::Animal::setWeight(int value) {
	weight = value;
}

main.cpp

#include <iostream>
#include "Animal.h"
using namespace ani;
using namespace std;

int main() {
	Animal example;
	cout << example.getWeight();
}

We can declare multiple namespaces (provided they don’t clash).

Constructors & Destructors

Constructors are used to allocate memory and initialise objects, whereas destructors free the memory. We can extend our header files to allow for this functionality:

Animal.h:

#include <string>
namespace ani {
	class Animal {
	private:
		int weight;
		std::string name;
	public:
		void setWeight(int value);
		int getWeight(void);
		Animal(int value);
		~Animal();
	};
}

Animal.cpp

#include "Animal.h"
int ani::Animal::getWeight() {
	return weight
}
void ani::Animal::setWeight(int value) {
	weight = value;
}
ani::Animal::Animal(int value) {
	weight = value;
}
ani::Animal::~Animal() {} // You can skip this as this does the same as the default

Member Initialisation

To save us manually initialising variables in our constructors, we can use the following syntax:

ani::Animal::Animal(int value) : weight(value) {}