Scope

Scope of a variable is its lifetime in the program.

The region of the program in which it can be accessed.

Local Scope

A scope can also be viewed as the space between two curly brackets - { }.

Example:

#include <iostream> using namespace std; int main() { int num1 = 5; if (true) { int num2 = 6; cout << num1 << endl; cout << num2 << endl; } cout << num1 << endl; //cout << num2 << endl; return 0; }

Output:

5 6 5

In this example the num1 variable can be accessed anywhere in the main function.

The num2 variable is defined inside the if statement and can be accessed only inside it - between its curly brackets - { }.

If we uncomment line 16, it will result in an exception because num2 is accessed outside of its scope - between line 9 and line 13.

Scopes are not limited only to if statements. It's also valid for loops too.

Let's look at the following example:

#include <iostream> using namespace std; int main() { for (int i = 0; i < 3; i++) { cout << i << endl; int num = 4; cout << num << endl; } // cout << num1 << endl; // cout << i << endl; return 0; }

Output:

0 4 1 4 2 4

In this case, both variables i and num can be accessed inside their scope - the for loop.

If we uncomment line 13 or line 14, an exception will be thrown.

Global Variables

Global variables can be accessed anywhere in any scope of the program.

Let's compare a global and local variable:

#include <iostream> using namespace std; int myGlobalVar = 5; void printGlobalVar() { cout << myGlobalVar << endl; // cout << localVar << endl; } int main() { int localVar = 4; printGlobalVar(); cout << localVar << endl; cout << myGlobalVar << endl; return 0; }

Output:

5 4 5

At line 5 a global variable is defined.

It can be accessed in both printGlobalVar and main functions.

The localVar variable is defined inside the main function and can only be accessed inside it.

If we uncomment line 11, an exception will be thrown because a local variable can only be accessed inside the function it's defined in.