C++ STRUCTURE - Learn2Done

In C++, classes and structs are blueprints that are used to create the instance of a class. Structs are used for lightweight objects such as Rectangle, color, Point, etc.
Unlike class, structs in C++ are value type than reference type.
It is useful if you have data that is not intended to be modified after creation of struct.
C++ Structure is a collection of different data types. It is similar to the class that holds different types of data

SYNTAX OF STRUCTURE

struct structure_name
{
     // member declarations.
}

In the above declaration, a structure is declared by preceding the struct keyword followed by the identifier(structure name). Inside the curly braces, we can declare the member variables of different types.

Consider the following situation

struct Student
{
    char name[20];
     int id;
     int age;
}

In the above case, Student is a structure contains three variables name, id, and age. When the structure is declared, no memory is allocated. When the variable of a structure is created, then the memory is allocated.

HOW TO CREATE THE STRUCTURE

The ‘struct’ keyword is used to create a structure. The general syntax to create a structure is as shown below:

struct structureName{
    member1;
    member2;
    member3;
    .
    .
    .
    memberN;
};

Structures in C++ can contain two types of members:

Data Member: These members are normal C++ variables. We can create a structure with variables of different data types in C++.
Member Functions: These members are normal C++ functions. Along with variables, we can also include functions inside a structure declaration.

EXAMPLE

// Data Members
int roll;
int age;
int marks;
// Member Functions
void printDetails()
{
	cout<<"Roll = "<<roll<<"n";
	cout<<"Age = "<<age<<"n";
	cout<<"Marks = "<<marks;
}

In the above structure, the data members are three integer variables to store roll number, age and marks of any student and the member function is printDetails() which is printing all of the above details of any student.

DECLARING THE VARIABLE IN STRUCTURE

A structure variable can either be declared with structure declaration or as a separate declaration like basic types.

// A variable declaration with structure declaration.
struct Point
{
int x, y;
} p1; // The variable p1 is declared with 'Point'
// A variable declaration like basic data types
struct Point
{
int x, y;
};
int main()
{
struct Point p1; // The variable p1 is declared like a normal variable
}

Note: In C++, the struct keyword is optional before in declaration of a variable.

INTIALIZING THE STRUCTURES IN C++

Structure members cannot be initialized with declaration. For example the following C program fails in compilation.
But is considered correct in C++11 and above.

struct Point
{
int x = 0; // COMPILER ERROR: cannot initialize members here
int y = 0; // COMPILER ERROR: cannot initialize members here
};

The reason for above error is simple, when a datatype is declared, no memory is allocated for it. Memory is allocated only when variables are created.

Structure members can be initialized with declaration in C++. For Example the following C++ program Executes Successfully without throwing any Error.

// In C++ We can Initialize the Variables with Declaration in Structure.
#include <iostream>
using namespace std;
struct Point {
	int x = 0; // It is Considered as Default Arguments and no Error is Raised
	int y = 1;
};
int main()
{
	struct Point p1;
	// Accessing members of point p1
	// No value is Initialized then the default value is considered. ie x=0 and y=1;
	cout << "x = " << p1.x << ", y = " << p1.y<<endl;
	// Initializing the value of y = 20;
	p1.y = 20;
	cout << "x = " << p1.x << ", y = " << p1.y;
	return 0;
}
// This code is contributed by Samyak Jain

OUTPUT

x=0, y=1
 x=0, y=20

Structure members can be initialized using curly braces ‘{}’. For example, following is a valid initialization.

struct Point {
	int x, y;
};
int main()
{
	// A valid initialization. member x gets value 0 and y
	// gets value 1. The order of declaration is followed.
	struct Point p1 = { 0, 1 };
}

HOW TO ACCESS STRUCTURE ELEMENTS ?

Structure members are accessed using dot (.) operator.

#include <iostream>
using namespace std;
struct Point {
	int x, y;
};
int main()
{
	struct Point p1 = { 0, 1 };
	// Accessing members of point p1
	p1.x = 20;
	cout << "x = " << p1.x << ", y = " << p1.y;
	return 0;
}

OUTPUT

x = 20, y = 1

What is an array of structures?

Like other primitive data types, we can create an array of structures.

#include <iostream>
using namespace std;
struct Point {
	int x, y;
};
int main()
{
	// Create an array of structures
	struct Point arr[10];
	// Access array members
	arr[0].x = 10;
	arr[0].y = 20;
	cout << arr[0].x << " " << arr[0].y;
	return 0;
}

OUTPUT

10 20

What is a structure pointer?

Like primitive types, we can have pointer to a structure. If we have a pointer to structure, members are accessed using arrow ( -> ) operator instead of the dot (.) operator.

#include <iostream>
using namespace std;
struct Point {
	int x, y;
};
int main()
{
	struct Point p1 = { 1, 2 };
	// p2 is a pointer to structure p1
	struct Point* p2 = &p1;
	// Accessing structure members using
	// structure pointer
	cout << p2->x << " " << p2->y;
	return 0;
}

OUTPUT

1 2

SYNTAX OF STRUCTURE

HOW TO CREATE THE STRUCTURE

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