Difference between Declaration , Definition and Initialization in C++

February 18, 2023

Declaration, Definition and Initialization 

Here, on this page, we will discuss the declaration, definition and initialization of a variable.  A variable is a memory unit that is capable of storing data which can be modified(rewritten) at any point of time in a program. Simply a variable is a name given to a memory location.

Difference between Declaration , Definition and Initialization in C++ img

Declaration , Definition and Initialization in C++

Difference between Declaration , Definition and Initialization in C++

A variable may have the following –

  • Variable Declaration
  • Variable definition (initialization)

In C++, all the variables must be declared before use.

Let us look at both of them in detail –

Variables in C++

Variable Declaration & Definition

Declaration

Variable declaration is the notification to the program/programmer that a particular type of memory may be required and we plan to call that memory with some name.

  • Memory creation (as per specified datatypes) happens at the time of declaration itself.
  • But the variables may have garbage values.
  • Variables can not be used before declaration.

Example

int a,b,c;

C++ Introduction

History of C++

Structure of a C++ Program

String in C++

Program to check armstrong number or not

Variable Definiton/Initialization

In this stage, the user assigns value as per the requirement within the memory bounds i.e garbage value is overridden

Example

//declaration
int a;
float b;

// definition/initialization later
a = 10;
b = 78.9;

Example program to demonstrate variable initialization

Run 
#include <iostream>
using namespace std;

int main()
{
    int var; // variable declaration
    
    cout << "Value:" << var << endl; // garbage value
    cout << "Address of var: " << &var << endl; // a's assigned address
    cout << "Size of var: " << sizeof(var) << " bytes"; // allocated memory in bytes
    
    return 0;
}

Declaration cum initialization 

Variable can be initialized at the time of declaration itself

Example

Run 
#include<iostream>
using namespace std;

int main()
{
    // declaration & initialization at same time
    int var = 10;
    float var2 = 10.25;
    
    cout << "Value var: " << var << endl; 
    cout << "Value: var2: " << var2 << endl; 
    
    return 0;
}
Output
Value var: 10
Value: var2: 10.25

What are data types?

Data types are used by variables to tell what kind of data it can store. Example – character/integer/decimal etc.

There are three types of data types in C++ –

  • Primary
  • Derived
  • User-Defined
Data Types in C++ New

Data Types Modifiers

These are used in conjunction with primitive(built-in) data types to modify the length of data that a particular data type can hold these are –

  • Unsigned
  • Signed
  • Short
  • Long

Primitive Data Types (Built-in)

  • Boolean
  • Character
  • Integer
  • Floating point
  • Double floating point
  • Void
  • Wide character

Below we have given a list of all primitive data types and their usage –

[table id=550 /]

Below we have given data types and their sizes, limits and types with different modifiers.

[table id=551 /]

C++ Code for Sizes

The following code will give you the output of the size of each data type –
Run 
// C++ program to sizes of data types 
#include<iostream>
using namespace std;
 
int main() 
{ 
    cout << "Size of char : " << sizeof(char) << " byte" << endl; 
    cout << "Size of int : " << sizeof(int) << " bytes" << endl; 
    cout << "Size of short int : " << sizeof(short int) << " bytes" << endl; 
    cout << "Size of long int : " << sizeof(long int) << " bytes" << endl; 
    cout << "Size of signed long int : " << sizeof(signed long int) << " bytes" << endl; 
    cout << "Size of unsigned long int : " << sizeof(unsigned long int) << " bytes" << endl; 
    cout << "Size of float : " << sizeof(float) << " bytes" << endl; 
    cout << "Size of double : " << sizeof(double) << " bytes" << endl; 
    cout << "Size of wchar_t : " << sizeof(wchar_t) << " bytes" << endl; 
    
    return 0; 
}

The output of the code depending on the default sizes of data types and modifiers in your compiler will be –

Size of char : 1 byte
Size of int : 4 bytes
Size of short int : 2 bytes
Size of long int : 8 bytes
Size of signed long int : 8 bytes
Size of unsigned long int : 8 bytes
Size of float : 4 bytes
Size of double : 8 bytes
Size of wchar_t : 4 bytes

Understanding Each Datatype

We will look at all different data types in C++ :

Integer (int)

  • Uses to store integer values like : -200, 150, 6812 etc
  • Usual Range – it can store values from -2147483648 to 2147483647
  • Usual Size – 4 bytes(some older compilers may support 2 bytes)
int age = 25;

Float and Double

Float and double are used to store floating-point numbers (decimals and exponentials)

  • The size of float is 4 bytes
  • The size of the double is 8 bytes.
  • Double has two times the precision of float.
Datatype                               Range                            Macro
float                       1.17549e-38 to 3.40282e+38                  FLT_MIN 
float(negative)            -1.17549e-38 to -3.40282e+38                -FLT_MIN
double                      2.22507e-308 to  1.79769e+308               DBL_MIN
double(negative)           -2.22507e-308 to -1.79769e+308              -DBL_MIN

An example of initializing variables would be –

float val1 = 21.25;
double val2 = 1531.24595;
double val3 = 21.34E14    // 45E12 is equal to 21.34 * 10^14
double val4 = 1.23E-12    // 45E12 is equal to 1.23 * 10^-12

Char

  • Its size is 1 byte.
  • Characters in C++ are enclosed inside single quotes ' '.

For example –

char c = 'a';

char val = ‘5’; // 5 stored as ascii character rather than int

Bool

  • Has a size of 1 byte
  • Used to store true/false values
  • Also used in conditional operations

Example –

bool val = false;

wchar_t

  • Wide character wchar_t is similar to the char data type
  • However, its size is 2 bytes instead of 1
  • These are used to represent characters that need more memory than 1 byte due to a large number of unique characters

Example –

wchar_t test = L'א‎' // storing Hebrew character;

Void

  • Void is used with functions and pointers
  • They mean that they do not have any data type of value
  • We will learn more about it later

We cannot declare variables of the void type.

Prime Course Trailer

Related Banners

Get PrepInsta Prime & get Access to all 200+ courses offered by PrepInsta in One Subscription

Get Prime

Get over 200+ course One Subscription

Courses like AI/ML, Cloud Computing, Ethical Hacking, C, C++, Java, Python, DSA (All Languages), Competitive Coding (All Languages), TCS, Infosys, Wipro, Amazon, DBMS, SQL and others

Get Prime
Course List
Course List

Checkout list of all the video courses in PrepInsta Prime Subscription

Checkout list of all the video courses in PrepInsta Prime Subscription

Checkout
Checkout