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1. Class A Class is a C++ building piece that leads to Object-Oriented programming. It's a user-defined data type with its own set of data members and member functions that can be accessed and used by establishing a class instance. A class defines the blueprint for a data type. Example: #include <bits/stdc++.h> using namespace std; class scaler { public: string student_name; void print_name() { cout << "Student name is: " << student_name << endl; } }; int main() { scaler student1, student2; student1.student_name = "Shivam Singla"; student1.print_name(); student2.student_name = "Sachin Singla"; student2.print_name(); return 0; } Try it yourself Output : Student name is: Shivam Singla Student name is: Sachin Singla In the above example, the scaler is the name of the class. We can include the data members, which are the class variables. Similarly, member functions are added to the class; for example, print_name(), here is the member function, and student_name is the data member. Here, the student1 and student2 are the class scaler objects. 2. Structure A structure datatype is a user-defined data type that combines objects of potentially different data types into a single type. Example: struct student { char name[15]; char roll_no[10]; int marks; }; Here, different data types, such as an array of characters and integer data types, are combined to make a new data type according to the user's need. 3. Union Union is similar to Structures as it is also used to combine the different types of data into a single user-defined data type. All members of a union have access to the same memory. In the below-shown example, we can combine the integer data type and the character data type into a single data type called test. In this case, as both the data types, integer, and character have different data sizes, we would take the larger data type as the size of the new user-defined data type test. We can see how changes in num are reflected in var if we adjust num. Example: union test { int num; char var; }; Here, num and var share the same memory. Therefore, if we change any variables, the changes will automatically reflect in another variable. 4. Enumeration In C++, an enumeration (or enum) is a data type that the user creates. It's primarily used to give integral constant names, making the program easier to comprehend and maintain. In enumeration, if we do not provide the integral values explicitly to the strings, then, in that case, the strings automatically start assigning the integral values starting from value 0, the same as the case of 0-based indexing. Example: enum result {pass = 100, fail = 0}; Here, we have given the integer value 100 to be pass and 0 as fail; therefore, if we write, enum result res; res = pass; Then, the value of res would automatically be 100.