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 –

Data Types in C++Short NameStorage Type
BooleanboolBoolean or logical values like True/False
CharactercharCharacters values
IntegerintInteger values
Floating PointfloatSingle precision Decimal Values
Double Floating PointdoubleDouble precision Decimal Values
voidvoidValueless entity / Null type
Wide characterwchar_tCharacter data type with larger values

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

Data TypeSizeRange
bool1 byteTrue/False
char1 byte-128 to 127 or 0 to 255
signed char1 byte-128 to 127
unsigned char1 byte0 to 255
short int2 bytes-32,768 to 32,767
signed short int
2 bytes-32,768 to 32,767
unsigned short int2 bytes0 to 65,535
int2 or 4 bytes-32768 to +32768
or -2,147,483,648 to +2,147,483,648
signed int2 or 4 bytes-32768 to +32768
or -2,147,483,648 to +2,147,483,648
unsigned int2 or 4 bytes0 to +65,535
or 0 to +4,294,967,295
long int4 or 8 bytes-2,147,483,648 to +2,147,483,647
or -9,223,372,036,854,775,808 to +9,223,372,036,854,775,807
signed long int4 or 8 bytes0 to 4,294,967,295
or 0 to 18,446,744,073,709,551,615
unsigned long int4 or 8 bytes-2,147,483,648 to +2,147,483,647
or -9,223,372,036,854,775,808 to +9,223,372,036,854,775,807
float4 bytes1.17549e-38 to 3.40282e+38
Double Floating Point8 bytes2.22507e-308 to 1.79769e+308
Long Double Floating Point16 bytes-3.3621e-4932 to +3.3621e-4932
No ValueNot validNot valid
Wide character2 or 41 wide character

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.

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