friend s4u::Mailbox;
friend s4u::Mailbox* s4u::Engine::mailbox_by_name_or_create(const std::string& name) const;
friend s4u::Mailbox* s4u::Mailbox::by_name(const std::string& name);
- friend mc::CommunicationDeterminismChecker;
static unsigned next_id_; // Next ID to be given
const unsigned id_ = next_id_++;