DevAcademia lesson/C++ Classes

Classes and Objects

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~23 min lesson63 learning blocks11 code examples1 checks/interactions

Lesson outline

Introduction to Classes and Objects

In C++, a class is a user-defined type that groups data and the functions that operate on that data. An object is an instance of a class.

Classes are the foundation of Object-Oriented Programming (OOP) in C++ and provide:

Data encapsulation (bundling data with methods)
Access control (public/private/protected members)
Code reusability through inheritance
Polymorphism (multiple forms for functions/operators)

Class vs Structure

Feature	Class	Structure
Default member access	private	public
Default base-class access (when inheriting)	private	public
Typical use	Types with behavior and invariants	Plain data aggregates
Methods	Supported	Supported
Inheritance support	Supported	Supported

Creating Objects

Objects can be created in several ways:

On the stack (automatic storage)
On the heap (dynamic allocation; prefer smart pointers)
As members of other classes
As static or global objects

Accessing Members

Use the dot operator (.) for objects and the arrow operator (->) for pointers.

Access depends on access specifiers:

public: Accessible from anywhere
private: Accessible only inside the class (default for classes)
protected: Accessible within the class and derived classes

Member Functions

Member functions can be defined:

Inside the class (implicitly inline)
Outside the class using the scope resolution operator (::)

Special functions include constructors, destructors, copy/move operations, operator overloads, and conversions.

Constructors and Destructors

Constructors initialize objects, and destructors clean up resources.

Types of constructors:

Default
Parameterized
Copy
Move (C++11+)
Delegating (C++11+)

The 'this' Pointer

Each non-static member function has a hidden this pointer referring to the object.

Common uses:

Resolve name conflicts
Return *this for method chaining
Pass the current object to other functions

Static Members

Static members belong to the class, not an instance.

Only one copy exists regardless of objects
Can be accessed without creating an object
Since C++17, inline static data members can be defined inside the class

Const Objects and Methods

const objects cannot be modified. const member functions guarantee not to modify the object state.

Const objects can only call const member functions
Const methods cannot modify members (except mutable ones)
Improves API clarity but does not ensure thread safety

Friend Functions and Classes

A friend function or class can access private and protected members of another class.

Use cases:

Operator overloading (e.g., I/O operators)
Close cooperation between two types
Utility functions needing internal access

Practical Example: Library System

Example demonstrating encapsulation, methods, and object interaction:

Best Practices

Practice	Description	Example
RAII	Manage resources with object lifetime	Use constructors/destructors or smart pointers
Minimal Interface	Expose only necessary operations	Keep members private; use accessors
Single Responsibility	One cohesive purpose per class	Separate logic from I/O or storage
Const Correctness	Use const where methods don't change state	double getValue() const;
Prefer Composition	Favor composition over inheritance when possible	class Car { Engine engine; }

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Feature

Class

Structure

Default member access

private

public

Default base-class access (when inheriting)

private

public

Typical use

Types with behavior and invariants

Plain data aggregates

Methods

Supported

Inheritance support

Supported

Practice

Description

Example

RAII

Manage resources with object lifetime

Use constructors/destructors or smart pointers

Minimal Interface

Expose only necessary operations

Keep members private; use accessors

Single Responsibility

One cohesive purpose per class

Separate logic from I/O or storage

Const Correctness

Use const where methods don't change state

double getValue() const;

Prefer Composition

Favor composition over inheritance when possible

class Car { Engine engine; }