消息：未定义的属性：Session :: $ session

Question

我的代码有问题 我正在购物车，添加产品时会话中出现错误

A PHP Error was encountered
Severity: Notice

Message: Undefined property: Carrinho::$Carrinhocompra

Filename: loja/Carrinho.php

Line Number: 31

Backtrace:

File: C:\Users\Dell\Desktop\loja_virtual_V1.4\application\controllers\loja\Carrinho.php
Line: 31
Function: _error_handler

File: C:\Users\Dell\Desktop\loja_virtual_V1.4\index.php
Line: 315
Function: require_once

aqui esta o codigo

minha library 
<?php 
    if ( ! defined('BASEPATH')) exit('No direct script access allowed');


    class Carrinhocompra {

        public function __construct(){

            if (!isset($_SESSION['Carrinho'])) {
                $_SESSION['Carrinho'] = [];
            }
        }

        //  ADICIONAR UM PRODUTO Carrinho
        public function add($id, $qtd){

            if ( isset($_SESSION['Carrinho'][$id]) ) {

                $_SESSION['Carrinho'][$id] = $_SESSION['Carrinho'][$id] + $qtd;
            }
            else {
                $_SESSION['Carrinho'][$id] = $qtd;
            }
        }
}


meu controller

<?php
defined('BASEPATH') OR exit('No direct script access allowed');

class Carrinho extends CI_Controller {

    public function __construct(){
        parent::__construct();
        $this->load->model('Loja/Loja_model');
        $this->load->library('Carrinhocompra');

    }

    public function index(){
        echo '<pre>';
            print_r($_SESSION['Carrinho']);
        exit;

        $query = $this->Loja_model->getDadosLoja();

        $data['titulo'] = 'Carrinho de compras';
        $data['dados_loja'] = $query;
        $data['categorias'] = $this->Loja_model->getCategoriaLoja();
        $data['redes'] = $this->Loja_model->getRedeSociais();
        $data['view'] = 'loja/carrinho/listar';

        $this->load->view('loja/index', $data);
    }

    public function add(){

        $this->Carrinhocompra->add(1, 5);
    }

}