Structures

Table of Contents

• Declaration
• Usage
• Functions
• Arrays

Everything in the world has information that goes along with it. Properties. This is where structures are useful. A structure (struct) allows you to associate variables with each other and create multiple instances of that dataset. An instance of a struct is an object - a variable of a user-defined datatype.

Declaration

#ifndef EMPLOYEE_HH
#define EMPLOYEE_HH

#include <string>

//You can store any data type in a struct.
struct Employee {
    std::string name[2];
    int id;
    int age;
    float income;
    bool college;
}; //You can add instance declarations before the semicolon here. Typically it is not useful to use this.

#endif

Usage

You can declare an instance of a struct by using its name. You can also use the dot operator to access elements within the structure.

#include <iostream>
using namespace std;

struct Point { //It is convention to capitalize the first letter in custom datatypes.
    double x;
    double y;
};

int main() {
    Point a; //Declare a Point
    a.x = 3.14; //Assign a value to a member.
    a.y = 1.27;
    cout << a.x << ' ' << a.y; //Access members
    Point b = a; //The assingment operator copies a struct
}

You can also declare an array of structs...

//Assumes the point struct was defined eariler
int main() {
    Point dotToDot[27];
    dotToDot[18].y = 30.3; //assigns y to be 30.3 in the 19th point
}

Functions

Unlike arrays, structs can be passed into functions be either value or reference. Below are some example functions for working with points.

//Using the Point struct again.

//Adds num to both components of Point p passed by reference.
void add(Point& p, double num) {
    p.x += num;
    p.y += num;
}

//Basic vector addition, adds b to a and returns the result (now a).
Point add(Point a, Point b) {
    a.x += b.x;
    a.y += b.y;
    return a;
}

//Takes two doubles and stores them in a point
Point makePoint(double x, double y) {
    Point out;
    out.x = x; //out.x is different than x
    out.y = y;
    return out;
}

Arrays

A struct allows for a very simple means of returning and passing an array by value. The compiler knows the exact amount of memory needed to hold a struct.

Note

This is actually a very poor method of working with arrays. It will almost never be useful to create a struct that only contains an array.

struct Array10 {
    int a[10];
}

//Set the entire array to be num
void set(Array10& a, int num) {
    for (int i = 0; i < 10; i++)
        a.a[i] = num;
}

//Creates an array set to num
Array10 set(int num) {
    for (int i = 0; i < 10; i++)
        a.a[i] = num;
}

Warning

Out of bounds errors still apply here.

Note

If an array is in a struct, it is also copied when the struct is copied.