In this article, we have covered the differences between 3 core terms Definition, Declaration and Initialization in C and C++ along with code snippets.

Table of contents:

1. Declaration
2. Definition
3. Initialization
4. Conclusion / Table of Differences

To understand the difference between the two we should first understand each term independently.So,here we go.

1. Declaration

Declaration of a variable is generally a introduction to a new memory allocated to something that we may call with some name.

Properties of declaration -
1.Memory creation occurs at the time of declaration itself.
2.Variables may have garbage values.
3.Variables cannot be used before declaration.

Synatx-

//declare a variable

int x

//declare a function but not defined

void func();

Code-

#include<iostream>
using namespace std;

int main()
{
    int x; // variable declaration

    cout << "Value of x:" << x << endl; // garbage value
    cout << "Address of x: " << &var << endl; // x's assigned address
    cout << "Size of x: " << sizeof(x) <<" bytes";// allocated memory in bytes

    return 0;
}

Output-

Value of x:32766
Address of x: 0x7ffebb87a084
Size of x: 4 bytes

2. Definition

In declaration, user defines the previously declared variable.

Syntax-

//declaration
int x;
float y;

// definition
x = 1;
y = 78;

Code-

#include<iostream>
using namespace std;

int main()
{
    int x; //variable declaration
    cout << "Value of x: " << x << endl; // garbage value
    
    x = 2;  // variable initialization/definition
    cout << "Value of x:" << x << endl;
    
    x = 4 + x;  //data overriding
    cout << "New x value: " << x << endl;
    
    return 0;
}

Output-

Value of x: 0
Value of x: 2
New x value: 6

3. Initialisation

Initialisation is nothing but assigning the value at the time of declaration.

syntax-

int x = 0;

Code-

#include<iostream>
using namespace std;

int main()
{
    // declaration & initialization at same time
    int x = 1;
    float x2 = 1.25;

    cout << "Value of x: " << x << endl; 
    cout << "Value of x2: " << x2 << endl; 

    return 0;
}

Output-

Value of x: 1
Value of x2: 1.25

Conclusion / Table of Differences

From the above explanation we can conclude the following-

1. Declaration is just naming the variable.
2. Definition does not means declaration '+' Initialisation as definition might be without initialisation.
3. Initialisation is assigning valueto the declared variable. (At the time of declaration)

Declaration	Definition	Initialisation
1.Declaration is just naming the variable.	Definition is declarartion without intialisation.	initialisation is declaration with definition at thesame time.
2.Variables may have garbage values	Variables may or may not have garbage values	Variables do not have garbage values
3 Declaration can be done any number of times.	Definition done only once	Initialisation done only once
4.Memory will not be allocated during declaration	Memory will be allocated during definition	Memory will be allocated during initialisation
5.Declaration provides basic attributes of a variable/function.	definition provides details of that variable/function.	Initialisation provides details of that variable/function and value.

With this article at OpenGenus, you must have the complete idea of Definition vs Declaration vs Initialization in C/ C++.