如何在Symfony 4中使用实例正确调用函数

Question

I'm trying to send an email with SMTP in Symfony 4 and I have the following function as per the docs.

public function send_smtp(\Swift_Mailer $mailer)
{
    $message = (new \Swift_Message('Hello Email'))
        ->setFrom('[e-mail]')
        ->setTo('[e-mail]')
        ->setBody('test email content');
    $mailer->send($message);
}

However, I want to call this function from a different one like

$this->send_smtp();

But it complains about 'No parameters are passed' and

$this->send_smtp(\Swift_Mailer);

also gives the error message

Undefined constant 'Swift_Mailer'

What can be the problem and how could it be solved?