C ++：如何找出函数的返回类型？

Question

SELECT m.id
    ,owner.id AS msg_owner
    ,owner.name AS msg_owner_name
    ,m.sender_user_id
    ,sender.name AS sender_name
    ,m.recipient_user_id
    ,recipient.name AS recipient_name
    ,m.body AS msg_body
    ,m.created_at
FROM messages m
INNER JOIN users sender
ON sender.id = m.sender_user_id
INNER JOIN users recipient
ON recipient.id = m.recipient_user_id
INNER JOIN users owner
ON owner.id = m.user_id

代码编译，工作，它执行lambda函数的更改，但是假设我想找出foo的确切返回类型。是int，void，bool等吗？我有办法表达它或其他什么吗？