Structures and User-Defined Types: struct, union, enum, typedef and Introduction to Namespaces

Need for user-defined types

Basic data types (int, float, char, etc.) store single values. But real-world entities (student, book, employee) have multiple attributes.

So we use user-defined types to group related data together and make programs:

• more organised,
• easier to maintain,
• closer to real-world modelling.

Structure (struct): meaning and features

A structure is a user-defined data type that groups variables of different data types under one name.

Key features:

• members are stored in memory (generally) in separate locations
• each member has its own storage
• we access members using the dot operator .

Declaring and using a structure

General steps:

1. define structure type
2. create structure variable
3. access members

Concept example:

• struct Student { int roll; char grade; };
• Student s; s.roll = 10;

In C++, you can write Student s; directly (no need for struct Student s; like in C).

Array of structures and nested structures (intro)

Array of structures

Useful when storing multiple objects of same type. Example idea: store 50 students.

Nested structure

A structure can contain another structure. Example: Student contains Address.

These ideas are important for data management and later OOP.

Structure vs class (basic idea)

In C++:

• struct and class are very similar.
• main difference: default access
  • struct members are public by default
  • class members are private by default

This is a common 2–3 mark question.

Union (union): meaning and memory sharing

A union is a user-defined type where all members share the same memory location.

Key points:

• only one member can store a valid value at a time
• memory size equals the size of the largest member

Unions are used when we want to save memory and we know only one field is used at a time.

Structure vs union (difference)

• In a structure, each member has separate memory.
• In a union, members share the same memory.

So changing one union member may overwrite the others.

Enumeration (enum): meaning and use

An enum is a user-defined type consisting of a set of named integer constants.

Example idea:

• enum Day {Mon, Tue, Wed, Thu, Fri, Sat, Sun};

Uses:

• improves readability (use names instead of numbers)
• restricts values to a fixed set

typedef: creating alias names

typedef is used to create an alias (alternate name) for a data type.

Examples:

• typedef unsigned long ulong;
• typedef struct Student Student; (common in C, less needed in C++)

Benefit:

• simplifies complex type names
• improves readability

Namespaces: why they are needed

As programs grow, different libraries may define functions/variables with the same name.

A namespace is used to avoid naming conflicts by grouping names.

Example: std::cout belongs to std namespace.

Using namespaces (std, using, scope resolution)

Ways:

• use scope resolution: std::cout, std::cin
• use a directive: using namespace std; (convenient but may cause conflicts in big projects)
• use selective using: using std::cout;

Important operator:

• :: is the scope resolution operator.

Common mistakes (exam + coding)

• Writing “union stores all members simultaneously” (wrong).
• Forgetting that struct vs class default access differs.
• Mixing up . and -> (arrow is for pointer to structure/class).
• Using using namespace std; everywhere in large code (can cause name conflicts).

Mini tables/flowcharts for exam answers

Structure vs Union

struct vs class (default)

Namespaces usage

Name conflict? → Put names in namespace → Access using :: → Avoid clashes

Quick Recap (1-minute revision)

• struct groups different data types as one unit.
• union shares memory (only one member meaningful at a time).
• enum provides named constants.
• typedef creates alias names.
• namespace avoids name conflicts; std:: uses scope resolution operator ::.