Variable Declaration & Initialization

C++ is a strongly-typed language, all variables must be declared with their type before they can be used, as this instructs the compiler on the memory size to reserve for the variable and how to interpret its value

In C++, the syntax used in declaring a new variable is simple. It is shown below: firstly, you simply write the type followed by the variable name (i.e its identifier). 

Syntax:

char c;     //character variable declaration.
int area;   //integer variable declaration.
float num;  //float variable declaration.

Three valid methods of variable declaration 

• The first method declares a variable of type char with the identifier c. 
• The second method declares a variable of type int with the identifier area. 
• The third method declares a variable of type float with the identifier num.

Note: After you have declared the variables c, area, and num, they can be used within their scope in the remainder of the program. 

Declaring More than one Variable 

In this case, when you want to declare more than one variable of the same type, you will declare them in a single statement by separating their identifiers with commas.

int a, b, c;    //more than one variable declaration.

The syntax above declares three variables (a, b, and c), and they are of the type int and have the same meaning as this below: 

int a;  //integer variable declaration.
int b;  //integer variable declaration.
int c;  //integer variable declaration.

Initialization of variables 

From the example above, when variables are declared, they have an undetermined or garbage value unless they are assigned a value at the beginning. However, it is possible for a variable to have a definite value at the moment it was declared, this process is referred to as the initialization of the variable. The C++ language presents the same ways to initialize variables as the C Language.

Syntax:

type identifier = initial_value;

Example:

int a = 10;  //integer variable declaration & initialization.

Program Example:

//Write a CPP program for declaration & initialization of variable
#include <iostream.h>
int main ()
{
    int sum;    //Variable declaration
    int a = 10; //Variable declaration & initialization
    int b = 5;  //Variable declaration & initialization
    ans = a + b;
    cout << "Addition is:" << ans << endl;
    return 0;
}

Import features to Know

• The compiler needs actual variable declaration at the time of linking of the program, this shows that variable declaration has its meaning at the initial time of compilation.
• It assures the compiler of an existing variable with a given data type and name so that compiler proceeds with further compilation without needing complete detail about the variable. 
• C++ makes it possible to declare a variable multiple times in a program. However, it can be defined only once in a file, a block of code, or a function. 
• It allows you to use the extern keyword for variable declaration at any place. 
• It is useful when you are using multiple files and then you want to define your variable in one of the files that will be available at the time of linking the program.