Accessing WP plugin code from a theme

example-plugin.php

<?php
/**
 * Example plugin.
 *
 * @see http://wordpress.stackexchange.com/questions/220286/what-is-the-best-way-to-instantiate-a-class-of-a-plugin-in-your-wordpress-theme/
 *
 * @wordpress-plugin
 * Plugin Name: Example plugin.
 * Description: Accessing a plugin object from a theme.
 * Version:     0.1.0
 * Author:      Alain Schlesser
 * Author URI:  https://www.alainschlesser.com/
 * Text Domain: example-plugin
 * Domain Path: /languages
 * License:     GPL-2.0+
 * License URI: http://www.gnu.org/licenses/gpl-2.0.txt
 */

// If this file is called directly, abort.
if ( ! defined( 'WPINC' ) ) {
	die;
}

// Load Composer autoloader.
if ( file_exists( __DIR__ . '/vendor/autoload.php' ) ) {
	require_once __DIR__ . '/vendor/autoload.php';
}

/**
 * Class ExamplePlugin.
 *
 * This is only a demonstration of how to access a plugin from
 * within a theme using an instance getter.
 *
 * @author Alain Schlesser <alain.schlesser@gmail.com>
 */
class ExamplePlugin {

	/**
	 * Static instance of the plugin.
	 *
	 * @var ExamplePlugin
	 */
	protected static $instance;

	/**
	 * Get a reference to the Plugin instance.
	 */
	public static function get_instance() {
		if ( ! self::$instance ) {
			self::$instance = new self();
		}

		return self::$instance;
	}

	/**
	 * Launch the initialization process.
	 */
	public function run() {
		// Do your normal plugin stuff here.
	}

	/**
	 * Return a string to demonstrate that the plugin works.
	 */
	public function hello_world() {
		return 'Hello World (from a plugin)!';
	}
}

/**
 * This is our function to retrieve an instance to the plugin
 * from the theme.
 *
 * @return ExamplePlugin
 */
function example_plugin_get_instance() {
	return ExamplePlugin::get_instance();
}

// Launch the plugin so that it can do its normal job.
ExamplePlugin::get_instance()->run();

functions.php

<?php
add_filter( 'the_content', 'example_content_filter_static_method' );
/**
 * Fetch the plugin instance through a static method of the plugin.
 */
function example_content_filter_static_method( $content ) {
	$example_plugin = ExamplePlugin::get_instance();
	return $content . '<p style="color: green;">' . $example_plugin->hello_world() . '</p>';
}

add_filter( 'the_content', 'example_content_filter_function' );
/**
 * Fetch the plugin instance through a function of the plugin.
 */
function example_content_filter_function( $content ) {
	$example_plugin = example_plugin_get_instance();
	return $content . '<p style="color: red;">' . $example_plugin->hello_world() . '</p>';
}