wpdb无法转换为字符串

你能帮我解决这个问题吗(代码中标记了第12行):

在中转换为字符串 12号线

<?php
function installer(){
    include('installer.php');
    $installer = new Installer;
    $installer -> activate();
}

register_activation_hook( __file__, 'installer' );

function deactivate(){
    include('installer.php');
    $installer = new Installer;
    $installer -> deactivate();    ;
}

register_deactivation_hook( __FILE__, 'deactivate' );

安装程序.php

<?php

class Installer {

    private $wpdb;
    private $table_name;
    private $charset_collate;

    public function __construct() {
        global $wpdb;
        $this->wpdb = $wpdb;
        $this->table_name = $this->$wpdb->prefix . 'ved_currencies'; // Line 12
        $this->charset_collate = $wpdb->get_charset_collate();
    }

    public function activate(){
        $sql = "CREATE TABLE $this->table_name (
        id mediumint(9) NOT NULL AUTO_INCREMENT UNIQUE,    
        date date not null,
        char_code varchar(3) NOT NULL,
        name varchar(40) NOT NULL,
        nominal int(9) NOT NULL,
        value DECIMAL(20,20) NOT NULL,
        PRIMARY KEY  (date, char_code)
        )    $this->charset_collate;";
        if ( ! function_exists( 'maybe_create_table' ) ) {
            require_once(ABSPATH . 'wp-admin/includes/upgrade.php');
        }
        maybe_create_table( $this->table_name, $sql);
    }

    public function deactivate(){
        $sql = "drop table if exists $this->table_name";
        dbDelta($sql);;
    }
}