How to copy a bsoncxx::builder::basic::document to another?

Is there any way to safely copy a bsoncxx document to another. In following code I am not able to do that

class DocClass
{
private:
    bsoncxx::builder::basic::document m_doc;
public:
    bsoncxx::builder::basic::document& copy(bsoncxx::builder::basic::document& obj)
    {
        obj = m_doc; //Not allowed
        //Error C2280   attempting to reference a deleted function
    }
};

There should not be any harm to the object even after copy. Please help.

Thanks, Shibin