Structs

• structs are a way to group related data together
• they are useful when you want to create a custom data type that has multiple fields
• structs are defined using the struct keyword

#include <iostream>
using namespace std;
struct Point {
  int x;
  int y;
};

int main() {
  Point p1 = {1, 2}; // method: 1
  Point p2 = {.x = 3, .y = 4}; // method: 2
  cout << p1.x << " " << p1.y << endl;
  cout << p2.x << " " << p2.y << endl;
  return 0;
}

• Both ways of instantiating a struct are correct in C++.

• Point p1 = {1, 2};

  • This is aggregate initialization, which initializes struct members in order.
  • Works in both C and C++.

• Point p2 = {.x = 3, .y = 4};

  • This is designated initialization, which allows initializing specific members explicitly.
  • C++20 and later only (not valid in C++17 or earlier).

If you're using C++17 or older, p2 will cause a compilation error.

---

Functions in Structs

• functions can be defined inside structs
• they can be used to access the fields of the struct
• functions can be defined with the static keyword to make them accessible without creating struct variable.

#include <iostream>
using namespace std;
struct Point
{
    int x;
    int y;

    void init(int x, int y){
        this->x = x;
        this->y = y;
    }
    static int getX(Point p)
    {
        return p.x;
    }

    static double getPi()
    {
        return 3.1415;
    }
};

int main()
{
    Point p;
    p.init(1, 2);
    cout << Point::getX(p) << endl;
    return 0;
}

---

Constructor and Destructor in Structs

• constructors and destructors are special functions that are called when an object is created and destroyed, respectively
• they are defined inside the struct
• Constructors => Same name as the struct
• Destructors => Same name as the struct, but with an extra ~ at the start

#include <iostream>
#include <cmath>
using namespace std;

struct Point
{
    int x;
    int y;
    Point(int x, int y) : x(x), y(y) {
        cout << "constructor called" << endl;
    }
    ~Point() {
        cout << "destructor called" << endl;
    }

    void distanceFromOrigin() {
        cout << "distance from origin: " << sqrt(x * x + y * y) << endl;
    }
};

int main()
{
    Point p(1, 2);
    p.distanceFromOrigin();
    return 0;
}

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Initializer List

• initializer lists are a way to initialize the fields of a struct
• they are defined using the : operator
• they are useful to separate the initialization code from the main constructor body to help developers focus on what special functionality the constructor provides rather than the boilerplate code

struct Point
{
    int x;
    int y;

    // below `: x(x), y(y)` is the initializer list

    Point(int x, int y) : x(x), y(y) {}
};

---

Creating instances of Structs with Struct declarations

struct Point
{
    int x;
    int y;

    // below `: x(x), y(y)` is the initializer list
    Point(int x, int y) : x(x), y(y) {}

}p1(1,2), p2(3,4);
// p1 and p2 are instances of the Point struct