C++ - Structures

Overview

Estimated time: 35–50 minutes

Use structs to group related data. Learn aggregate initialization, member functions, and how struct differs from class (only default access).

Learning Objectives

• Define and initialize structs using aggregate and member-init-list approaches.
• Add member functions to structs.
• Understand access defaults and how struct vs class differ.

Prerequisites

• C++ - Naming Conventions & Style

Aggregate initialization

#include <iostream>
struct Point { int x; int y; };
int main(){
  Point p{3,4};
  std::cout << p.x + p.y << "\n";
}

Expected Output: 7

Struct with member functions

#include <iostream>
#include <cmath>
struct Point {
  int x{0}; int y{0};
  double length() const { return std::hypot(x, y); }
  void translate(int dx, int dy){ x += dx; y += dy; }
};
int main(){
  Point p{3,4};
  std::cout << p.length() << "\n";
  p.translate(1,-1);
  std::cout << p.x << "," << p.y << "\n";
}

Expected Output: 5 4,3

Defaulted comparisons (C++20)

#include <compare>
struct Point2 { int x{0}, y{0};
  bool operator==(const Point2&) const = default;
  std::strong_ordering operator<=>(const Point2&) const = default;
};

Access control in struct

struct PublicByDefault { int x{0}; }; // public members by default
struct WithPrivate {
private:
  int x{0};
public:
  int get() const { return x; }
};

Common Pitfalls

• Forgetting const on member functions that don’t modify state (prevents calling on const objects).
• Assuming struct and class are different types—only default access differs: struct members are public by default; class members are private by default.

Checks for Understanding

1. How do struct and class differ in C++?
2. When does aggregate initialization apply?

Exercises

1. Create a struct Rectangle with width/height and an area() const method; print its area.
2. Add a translate(dx, dy) method to Rectangle to move an origin-based rectangle; print new coordinates.