Mastering Object-Oriented Programming (OOPs) for SDE Interviews | Ultimate Resource

 C++ Classes and Objects

C++ is an object-oriented programming language. 

Everything in C++ is associated with classes, objects, attributes, and methods.

Attributes and methods are basically variables and functions that belong to the class. These are often referred to as "class members".

A class is a user-defined data type that we can use in our program, and it works as an object constructor or a "blueprint" for creating objects.

Creating a Class & an Object

To create a class, use the `class` keyword:

In C++, an object is made from a class.

To create an object, specify the class name, followed by the object name.

To access the class attributes, use the dot syntax (.) on the object.

In MyClass obj = MyClass();, MyClass() is an Explicit type conversion in functional notation, so it is a temporary object because it falls under "conversion that creates a prvalue".

#include <bits/stdc++.h>

using namespace std;

class MyClass{ // class keyword and Class Name

    public:  // Access Specifier

        int myNum; // Attribute

        string myString; // Attribute

};

int main(){

    MyClass obj = MyClass();

    obj.myString = "CodingKaro.in";

    cout<<(obj.myString);

    return 0;

}

Output: CodingKaro.in

Class Methods

Methods are function that belongs to the class. They define the behavior/action taken by the object.

There are two ways to define functions that belong to the class:

#include <bits/stdc++.h>

using namespace std;

class MyClass{ // class keyword and Class Name

    public:  // Access Specifier

        int myNum; // Attribute

        string myString; // Attribute

        void myMethod(){ // Method

            cout<<"Welcome to CodingKaro.in"<<endl;

        }

        void myMethod2(); // Method Declaration

};

void MyClass::myMethod2(){

    cout<<"Inside myMethod2"<<endl;

}

int main(){

    MyClass obj = MyClass();

    obj.myString = "CodingKaro.in";

    obj.myMethod2();

    obj.myMethod();

    return 0;

}

Output:   Inside myMethod2
                Welcome to CodingKaro.in

Access Specifiers

Access specifiers define how the members (attributes and methods) of a class can be accessed:

In C++, there are three access specifiers:

public - members are accessible from outside the class

private - members cannot be accessed ( or viewed ) from outside the class.

protected - members cannot be accessible from outside the class, however, they can be accessed in the inherited class.

By default, all members of a class are private if you don't specify an access specifier.

Default Copying

By default, objects can be copied. In particular, a class object can be initialized with a copy of an object of its class. By default, the copy of a class of objects is a copy of each member.

If that default is not the behavior wanted for a class X, then appropriate behavior can be provided using overloading.

Static Members of a C++ Class

We can define a class member static using the static keyword.

When we declare a member of a class as static it means no matter how many objects of the class are created, there is only one copy of the static member.

A static member is shared by all objects of the class.

Memory belongs to object not to class.

We can't put it in the class definition but it can be initialized outside the class as done in the following example by redeclaring the static variable, using the scope resolution operator ::  to identify which class it belongs to.

#include <bits/stdc++.h>

using namespace std;

class MyClass{ // class keyword and Class Name

    public:  // Access Specifier

        static int myNum; // Static Member

        string myString; // Attribute

};

int MyClass::myNum = 0; // initialize static members of class

int main(){

    cout<<MyClass().myNum<<endl;

    MyClass().myNum+=10;

    MyClass obj = MyClass();

    cout<<obj.myNum<<endl;

    return 0;

}

Output:   0

                10

Static Function Members

By declaring a function member as static, you make it independent of any particular object of the class.

A static member function can be called even if no objects of the class exist and the static functions are accessed using only the class name and the scope resolution operator ::.

 A static member function can only access static data members, other static member functions, and any other function from outside the class.

static member functions have a class scope and they do not have access to this pointer of the class.

#include <bits/stdc++.h>

using namespace std;

class MyClass{ // class keyword and Class Name

    int length;

    public:  // Access Specifier

        static int myNum; // Static Member

        static int get(){

            return myNum;

        }

};

int MyClass::myNum=100;

int main(){

    cout<<MyClass().get()<<endl;

    MyClass().myNum+=10;

    MyClass obj = MyClass();

    cout<<obj.get();

    return 0;

}

Output:  100

                110

Passing and Returning objects in C++

In C++ we can pass objects as arguments and also return them from functions the same way we pass and return other variables.

Passing an Object as an argument

To pass an object as an argument we write the object name as the argument while calling the function in the same way we do it for other variables.

#include <bits/stdc++.h>

using namespace std;

class MyClass{ // class keyword and Class Name

    int length;

    public:  // Access Specifier

        static int myNum; // Static Member

        string s;

        string get(MyClass obj){

            return obj.s+" "+s;

        }

};

int MyClass::myNum=100;

int main(){

    MyClass obj = MyClass();

    obj.s="Object 1";

    MyClass obj2 = MyClass();

    obj2.s="Object 2";

    cout<<obj2.get(obj);

    return 0;

}

Output: Object 1 Object 2

Returning Objects in C++

#include <bits/stdc++.h>

using namespace std;

class MyClass{ // class keyword and Class Name

    int length;

    public:  // Access Specifier

        static int myNum; // Static Member

        string s;

        MyClass get(MyClass obj){

            MyClass ex;

            ex.s = "Object Example";

            ex.s= obj.s + " " + s + " " + ex.s;

            return ex;

        }

};

int MyClass::myNum=100;

int main(){

    MyClass obj = MyClass();

    obj.s="Object 1";

    MyClass obj2 = MyClass();

    obj2.s="Object 2";

    cout<<obj2.get(obj).s;

    return 0;

}

Output: Object 1 Object 2 Object Example

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