Declaring vs Defining variables

[magesh.p]

Declaring vs Defining variables

In C++, there are 2 ways of introducing variables. The first way is called declaration of a variable. The second is called definition of a variable. The following code snippet shows the differences:

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int i; //declaration of i as type int
int i = 5; //definition of variable i and assigning value 5 to it
string s;         //declaration of s as type string
s = "Hello World; //assigning "Hello World" to s
string s2 = "Hello Reader"; //definition of s as string with value "Hello Reader


Note that, when assigning values to variables, you need to make sure that the value is a valid value . This means that, when, for example, you define a variable of type int you have to make sure that the value you assign to it is an integer.

int i;
i = 5; //correct! 5 is integer

int j;
j = "Hello World" //wrong! j is not a string

double d;
d = 3.145; //correct! 3.145 is a double

int k;
k = 1.23456; //wrong! 1.23456 is not an int

If you assing wrong values to variables of another data type, the compiler will complain an error.

Defining a variable means to declare it and assign a value to it.

Declaring a variable means only to introduce it to the compiler.