Which modification to login only certain role?

Elaborate plugins exist to restrict page access based on a user role or other settings. However, I guess the following code is as simple as it’s gets for checking a user role.

<?php
global $current_user;
if ( is_page( 'some-page' ) && in_array( 'some-role', $current_user->roles ) ) {
    // Show the page
} else {
    // Howdy, stranger! Nothing to see here.
}
?>

Note: this code could go into your theme’s template files such as index.php or page.php. Be aware that switching themes would remove the user role access restriction. Solution? Move this functionality to a plugin. Update: G. M. provided a plugin version in his answer.