text: // sigslot.h: Signal/Slot classes
  //
  // Written by Sarah Thompson (sarah@telergy.com) 2002.
  //
  // License: Public domain. You are free to use this code however you like, with
  // the proviso that the author takes on no responsibility or liability for any
  // use.
  //
  // QUICK DOCUMENTATION
  //
  //        (see also the full documentation at http://sigslot.sourceforge.net/)
  //
  //    #define switches
  //      SIGSLOT_PURE_ISO:
  //        Define this to force ISO C++ compliance. This also disables all of
  //        the thread safety support on platforms where it is available.
  //
  //      SIGSLOT_USE_POSIX_THREADS:
  //        Force use of Posix threads when using a C++ compiler other than gcc
  //        on a platform that supports Posix threads. (When using gcc, this is
  //        the default - use SIGSLOT_PURE_ISO to disable this if necessary)
  //
  //      SIGSLOT_DEFAULT_MT_POLICY:
  //        Where thread support is enabled, this defaults to
  //        multi_threaded_global. Otherwise, the default is single_threaded.
  //        #define this yourself to override the default. In pure ISO mode,
  //        anything other than single_threaded will cause a compiler error.
  //
  //    PLATFORM NOTES
  //
  //      Win32:
  //        On Win32, the WEBRTC_WIN symbol must be #defined. Most mainstream
  //        compilers do this by default, but you may need to define it yourself
  //        if your build environment is less standard. This causes the Win32
  //        thread support to be compiled in and used automatically.
  //
  //      Unix/Linux/BSD, etc.:
  //        If you're using gcc, it is assumed that you have Posix threads
  //        available, so they are used automatically. You can override this (as
  //        under Windows) with the SIGSLOT_PURE_ISO switch. If you're using
  //        something other than gcc but still want to use Posix threads, you
  //        need to #define SIGSLOT_USE_POSIX_THREADS.
  //
  //      ISO C++:
  //        If none of the supported platforms are detected, or if
  //        SIGSLOT_PURE_ISO is defined, all multithreading support is turned
  //        off, along with any code that might cause a pure ISO C++ environment
  //        to complain. Before you ask, gcc -ansi -pedantic won't compile this
  //        library, but gcc -ansi is fine. Pedantic mode seems to throw a lot of
  //        errors that aren't really there. If you feel like investigating this,
  //        please contact the author.
  //
  //
  //    THREADING MODES
  //
  //      single_threaded:
  //        Your program is assumed to be single threaded from the point of view
  //        of signal/slot usage (i.e. all objects using signals and slots are
  //        created and destroyed from a single thread). Behaviour if objects are
  //        destroyed concurrently is undefined (i.e. you'll get the occasional
  //        segmentation fault/memory exception).
  //
  //      multi_threaded_global:
  //        Your program is assumed to be multi threaded. Objects using signals
  //        and slots can be safely created and destroyed from any thread, even
  //        when connections exist. In multi_threaded_global mode, this is
  //        achieved by a single global mutex (actually a critical section on
  //        Windows because they are faster). This option uses less OS resources,
  //        but results in more opportunities for contention, possibly resulting
  //        in more context switches than are strictly necessary.
  //
  //      multi_threaded_local:
  //        Behaviour in this mode is essentially the same as
  //        multi_threaded_global, except that each signal, and each object that
  //        inherits has_slots, all have their own mutex/critical section. In
  //        practice, this means that mutex collisions (and hence context
  //        switches) only happen if they are absolutely essential. However, on
  //        some platforms, creating a lot of mutexes can slow down the whole OS,
  //        so use this option with care.
  //
  //    USING THE LIBRARY
  //
  //      See the full documentation at http://sigslot.sourceforge.net/
  //
  // Libjingle specific:
  //
  // This file has been modified such that has_slots and signalx do not have to be
  // using the same threading requirements. E.g. it is possible to connect a
  // has_slots<single_threaded> and signal0<multi_threaded_local> or
  // has_slots<multi_threaded_local> and signal0<single_threaded>.
  // If has_slots is single threaded the user must ensure that it is not trying
  // to connect or disconnect to signalx concurrently or data race may occur.
  // If signalx is single threaded the user must ensure that disconnect, connect
  // or signal is not happening concurrently or data race may occur.
  
  #ifndef RTC_BASE_THIRD_PARTY_SIGSLOT_SIGSLOT_H_
  #define RTC_BASE_THIRD_PARTY_SIGSLOT_SIGSLOT_H_
  
  #include <cstring>
  #include <list>
  #include <set>
  
  // On our copy of sigslot.h, we set single threading as default.
  #define SIGSLOT_DEFAULT_MT_POLICY single_threaded
  
  #if defined(SIGSLOT_PURE_ISO) ||                   \
      (!defined(WEBRTC_WIN) && !defined(__GNUG__) && \
       !defined(SIGSLOT_USE_POSIX_THREADS))
  #define _SIGSLOT_SINGLE_THREADED
  #elif defined(WEBRTC_WIN)
  #define _SIGSLOT_HAS_WIN32_THREADS
  #include "windows.h"
  #elif defined(__GNUG__) || defined(SIGSLOT_USE_POSIX_THREADS)
  #define _SIGSLOT_HAS_POSIX_THREADS
  #include <pthread.h>
  #else
  #define _SIGSLOT_SINGLE_THREADED
  #endif
  
  #ifndef SIGSLOT_DEFAULT_MT_POLICY
  #ifdef _SIGSLOT_SINGLE_THREADED
  #define SIGSLOT_DEFAULT_MT_POLICY single_threaded
  #else
  #define SIGSLOT_DEFAULT_MT_POLICY multi_threaded_local
  #endif
  #endif
  
  // TODO: change this namespace to rtc?
  namespace sigslot {
  
  class single_threaded {
   public:
    void lock() {}
    void unlock() {}
  };
  
  #ifdef _SIGSLOT_HAS_WIN32_THREADS
  // The multi threading policies only get compiled in if they are enabled.
  class multi_threaded_global {
   public:
    multi_threaded_global() {
      static bool isinitialised = false;
  
      if (!isinitialised) {
        InitializeCriticalSection(get_critsec());
        isinitialised = true;
      }
    }
  
    void lock() { EnterCriticalSection(get_critsec()); }
  
    void unlock() { LeaveCriticalSection(get_critsec()); }
  
   private:
    CRITICAL_SECTION* get_critsec() {
      static CRITICAL_SECTION g_critsec;
      return &g_critsec;
    }
  };
  
  class multi_threaded_local {
   public:
    multi_threaded_local() { InitializeCriticalSection(&m_critsec); }
  
    multi_threaded_local(const multi_threaded_local&) {
      InitializeCriticalSection(&m_critsec);
    }
  
    ~multi_threaded_local() { DeleteCriticalSection(&m_critsec); }
  
    void lock() { EnterCriticalSection(&m_critsec); }
  
    void unlock() { LeaveCriticalSection(&m_critsec); }
  
   private:
    CRITICAL_SECTION m_critsec;
  };
  #endif  // _SIGSLOT_HAS_WIN32_THREADS
  
  #ifdef _SIGSLOT_HAS_POSIX_THREADS
  // The multi threading policies only get compiled in if they are enabled.
  class multi_threaded_global {
   public:
    void lock() { pthread_mutex_lock(get_mutex()); }
    void unlock() { pthread_mutex_unlock(get_mutex()); }
  
   private:
    static pthread_mutex_t* get_mutex();
  };
  
  class multi_threaded_local {
   public:
    multi_threaded_local() { pthread_mutex_init(&m_mutex, nullptr); }
    multi_threaded_local(const multi_threaded_local&) {
      pthread_mutex_init(&m_mutex, nullptr);
    }
    ~multi_threaded_local() { pthread_mutex_destroy(&m_mutex); }
    void lock() { pthread_mutex_lock(&m_mutex); }
    void unlock() { pthread_mutex_unlock(&m_mutex); }
  
   private:
    pthread_mutex_t m_mutex;
  };
  #endif  // _SIGSLOT_HAS_POSIX_THREADS
  
  template <class mt_policy>
  class lock_block {
   public:
    mt_policy* m_mutex;
  
    lock_block(mt_policy* mtx) : m_mutex(mtx) { m_mutex->lock(); }
  
    ~lock_block() { m_mutex->unlock(); }
  };
  
  class _signal_base_interface;
  
  class has_slots_interface {
   private:
    typedef void (*signal_connect_t)(has_slots_interface* self,
                                     _signal_base_interface* sender);
    typedef void (*signal_disconnect_t)(has_slots_interface* self,
                                        _signal_base_interface* sender);
    typedef void (*disconnect_all_t)(has_slots_interface* self);
  
    const signal_connect_t m_signal_connect;
    const signal_disconnect_t m_signal_disconnect;
    const disconnect_all_t m_disconnect_all;
  
   protected:
    has_slots_interface(signal_connect_t conn,
                        signal_disconnect_t disc,
                        disconnect_all_t disc_all)
        : m_signal_connect(conn),
          m_signal_disconnect(disc),
          m_disconnect_all(disc_all) {}
  
    // Doesn't really need to be virtual, but is for backwards compatibility
    // (it was virtual in a previous version of sigslot).
    virtual ~has_slots_interface() {}
  
   public:
    void signal_connect(_signal_base_interface* sender) {
      m_signal_connect(this, sender);
    }
  
    void signal_disconnect(_signal_base_interface* sender) {
      m_signal_disconnect(this, sender);
    }
  
    void disconnect_all() { m_disconnect_all(this); }
  };
  
  class _signal_base_interface {
   private:
    typedef void (*slot_disconnect_t)(_signal_base_interface* self,
                                      has_slots_interface* pslot);
    typedef void (*slot_duplicate_t)(_signal_base_interface* self,
                                     const has_slots_interface* poldslot,
                                     has_slots_interface* pnewslot);
  
    const slot_disconnect_t m_slot_disconnect;
    const slot_duplicate_t m_slot_duplicate;
  
   protected:
    _signal_base_interface(slot_disconnect_t disc, slot_duplicate_t dupl)
        : m_slot_disconnect(disc), m_slot_duplicate(dupl) {}
  
    ~_signal_base_interface() {}
  
   public:
    void slot_disconnect(has_slots_interface* pslot) {
      m_slot_disconnect(this, pslot);
    }
  
    void slot_duplicate(const has_slots_interface* poldslot,
                        has_slots_interface* pnewslot) {
      m_slot_duplicate(this, poldslot, pnewslot);
    }
  };
  
  class _opaque_connection {
   private:
    typedef void (*emit_t)(const _opaque_connection*);
    template <typename FromT, typename ToT>
    union union_caster {
      FromT from;
      ToT to;
    };
  
    emit_t pemit;
    has_slots_interface* pdest;
    // Pointers to member functions may be up to 16 bytes for virtual classes,
    // so make sure we have enough space to store it.
    unsigned char pmethod[16];
  
   public:
    template <typename DestT, typename... Args>
    _opaque_connection(DestT* pd, void (DestT::*pm)(Args...)) : pdest(pd) {
      typedef void (DestT::*pm_t)(Args...);
      static_assert(sizeof(pm_t) <= sizeof(pmethod),
                    "Size of slot function pointer too large.");
  
      std::memcpy(pmethod, &pm, sizeof(pm_t));
  
      typedef void (*em_t)(const _opaque_connection* self, Args...);
      union_caster<em_t, emit_t> caster2;
      caster2.from = &_opaque_connection::emitter<DestT, Args...>;
      pemit = caster2.to;
    }
  
    has_slots_interface* getdest() const { return pdest; }
  
    _opaque_connection duplicate(has_slots_interface* newtarget) const {
      _opaque_connection res = *this;
      res.pdest = newtarget;
      return res;
    }
  
    // Just calls the stored "emitter" function pointer stored at construction
    // time.
    template <typename... Args>
    void emitx(Args... args) const {
      typedef void (*em_t)(const _opaque_connection*, Args...);
      union_caster<emit_t, em_t> caster;
      caster.from = pemit;
      (caster.to)(this, args...);
    }
  
   private:
    template <typename DestT, typename... Args>
    static void emitter(const _opaque_connection* self, Args... args) {
      typedef void (DestT::*pm_t)(Args...);
      pm_t pm;
      std::memcpy(&pm, self->pmethod, sizeof(pm_t));
      (static_cast<DestT*>(self->pdest)->*(pm))(args...);
    }
  };
  
  template <class mt_policy>
  class _signal_base : public _signal_base_interface, public mt_policy {
   protected:
    typedef std::list<_opaque_connection> connections_list;
  
    _signal_base()
        : _signal_base_interface(&_signal_base::do_slot_disconnect,
                                 &_signal_base::do_slot_duplicate),
          m_current_iterator(m_connected_slots.end()) {}
  
    ~_signal_base() { disconnect_all(); }
  
   private:
    _signal_base& operator=(_signal_base const& that);
  
   public:
    _signal_base(const _signal_base& o)
        : _signal_base_interface(&_signal_base::do_slot_disconnect,
                                 &_signal_base::do_slot_duplicate),
          m_current_iterator(m_connected_slots.end()) {
      lock_block<mt_policy> lock(this);
      for (const auto& connection : o.m_connected_slots) {
        connection.getdest()->signal_connect(this);
        m_connected_slots.push_back(connection);
      }
    }
  
    bool is_empty() {
      lock_block<mt_policy> lock(this);
      return m_connected_slots.empty();
    }
  
    void disconnect_all() {
      lock_block<mt_policy> lock(this);
  
      while (!m_connected_slots.empty()) {
        has_slots_interface* pdest = m_connected_slots.front().getdest();
        m_connected_slots.pop_front();
        pdest->signal_disconnect(static_cast<_signal_base_interface*>(this));
      }
      // If disconnect_all is called while the signal is firing, advance the
      // current slot iterator to the end to avoid an invalidated iterator from
      // being dereferenced.
      m_current_iterator = m_connected_slots.end();
    }
  
  #if !defined(NDEBUG)
    bool connected(has_slots_interface* pclass) {
      lock_block<mt_policy> lock(this);
      connections_list::const_iterator it = m_connected_slots.begin();
      connections_list::const_iterator itEnd = m_connected_slots.end();
      while (it != itEnd) {
        if (it->getdest() == pclass)
          return true;
        ++it;
      }
      return false;
    }
  #endif
  
    void disconnect(has_slots_interface* pclass) {
      lock_block<mt_policy> lock(this);
      connections_list::iterator it = m_connected_slots.begin();
      connections_list::iterator itEnd = m_connected_slots.end();
  
      while (it != itEnd) {
        if (it->getdest() == pclass) {
          // If we're currently using this iterator because the signal is firing,
          // advance it to avoid it being invalidated.
          if (m_current_iterator == it) {
            m_current_iterator = m_connected_slots.erase(it);
          } else {
            m_connected_slots.erase(it);
          }
          pclass->signal_disconnect(static_cast<_signal_base_interface*>(this));
          return;
        }
        ++it;
      }
    }
  
   private:
    static void do_slot_disconnect(_signal_base_interface* p,
                                   has_slots_interface* pslot) {
      _signal_base* const self = static_cast<_signal_base*>(p);
      lock_block<mt_policy> lock(self);
      connections_list::iterator it = self->m_connected_slots.begin();
      connections_list::iterator itEnd = self->m_connected_slots.end();
  
      while (it != itEnd) {
        connections_list::iterator itNext = it;
        ++itNext;
  
        if (it->getdest() == pslot) {
          // If we're currently using this iterator because the signal is firing,
          // advance it to avoid it being invalidated.
          if (self->m_current_iterator == it) {
            self->m_current_iterator = self->m_connected_slots.erase(it);
          } else {
            self->m_connected_slots.erase(it);
          }
        }
  
        it = itNext;
      }
    }
  
    static void do_slot_duplicate(_signal_base_interface* p,
                                  const has_slots_interface* oldtarget,
                                  has_slots_interface* newtarget) {
      _signal_base* const self = static_cast<_signal_base*>(p);
      lock_block<mt_policy> lock(self);
      connections_list::iterator it = self->m_connected_slots.begin();
      connections_list::iterator itEnd = self->m_connected_slots.end();
  
      while (it != itEnd) {
        if (it->getdest() == oldtarget) {
          self->m_connected_slots.push_back(it->duplicate(newtarget));
        }
  
        ++it;
      }
    }
  
   protected:
    connections_list m_connected_slots;
  
    // Used to handle a slot being disconnected while a signal is
    // firing (iterating m_connected_slots).
    connections_list::iterator m_current_iterator;
    bool m_erase_current_iterator = false;
  };
  
  template <class mt_policy = SIGSLOT_DEFAULT_MT_POLICY>
  class has_slots : public has_slots_interface, public mt_policy {
   private:
    typedef std::set<_signal_base_interface*> sender_set;
    typedef sender_set::const_iterator const_iterator;
  
   public:
    has_slots()
        : has_slots_interface(&has_slots::do_signal_connect,
                              &has_slots::do_signal_disconnect,
                              &has_slots::do_disconnect_all) {}
  
    has_slots(has_slots const& o)
        : has_slots_interface(&has_slots::do_signal_connect,
                              &has_slots::do_signal_disconnect,
                              &has_slots::do_disconnect_all) {
      lock_block<mt_policy> lock(this);
      for (auto* sender : o.m_senders) {
        sender->slot_duplicate(&o, this);
        m_senders.insert(sender);
      }
    }
  
    ~has_slots() { this->disconnect_all(); }
  
   private:
    has_slots& operator=(has_slots const&);
  
    static void do_signal_connect(has_slots_interface* p,
                                  _signal_base_interface* sender) {
      has_slots* const self = static_cast<has_slots*>(p);
      lock_block<mt_policy> lock(self);
      self->m_senders.insert(sender);
    }
  
    static void do_signal_disconnect(has_slots_interface* p,
                                     _signal_base_interface* sender) {
      has_slots* const self = static_cast<has_slots*>(p);
      lock_block<mt_policy> lock(self);
      self->m_senders.erase(sender);
    }
  
    static void do_disconnect_all(has_slots_interface* p) {
      has_slots* const self = static_cast<has_slots*>(p);
      lock_block<mt_policy> lock(self);
      while (!self->m_senders.empty()) {
        std::set<_signal_base_interface*> senders;
        senders.swap(self->m_senders);
        const_iterator it = senders.begin();
        const_iterator itEnd = senders.end();
  
        while (it != itEnd) {
          _signal_base_interface* s = *it;
          ++it;
          s->slot_disconnect(p);
        }
      }
    }
  
   private:
    sender_set m_senders;
  };
  
  template <class mt_policy, typename... Args>
  class signal_with_thread_policy : public _signal_base<mt_policy> {
   private:
    typedef _signal_base<mt_policy> base;
  
   protected:
    typedef typename base::connections_list connections_list;
  
   public:
    signal_with_thread_policy() {}
  
    template <class desttype>
    void connect(desttype* pclass, void (desttype::*pmemfun)(Args...)) {
      lock_block<mt_policy> lock(this);
      this->m_connected_slots.push_back(_opaque_connection(pclass, pmemfun));
      pclass->signal_connect(static_cast<_signal_base_interface*>(this));
    }
  
    void emitx(Args... args) {
      lock_block<mt_policy> lock(this);
      this->m_current_iterator = this->m_connected_slots.begin();
      while (this->m_current_iterator != this->m_connected_slots.end()) {
        _opaque_connection const& conn = *this->m_current_iterator;
        ++(this->m_current_iterator);
        conn.emitx<Args...>(args...);
      }
    }
  
    void operator()(Args... args) { emitx(args...); }
  };
  
  // Alias with default thread policy. Needed because both default arguments
  // and variadic template arguments must go at the end of the list, so we
  // can't have both at once.
  template <typename... Args>
  using signal = signal_with_thread_policy<SIGSLOT_DEFAULT_MT_POLICY, Args...>;
  
  // The previous verion of sigslot didn't use variadic templates, so you would
  // need to write "sigslot::signal2<Arg1, Arg2>", for example.
  // Now you can just write "sigslot::signal<Arg1, Arg2>", but these aliases
  // exist for backwards compatibility.
  template <typename mt_policy = SIGSLOT_DEFAULT_MT_POLICY>
  using signal0 = signal_with_thread_policy<mt_policy>;
  
  template <typename A1, typename mt_policy = SIGSLOT_DEFAULT_MT_POLICY>
  using signal1 = signal_with_thread_policy<mt_policy, A1>;
  
  template <typename A1,
            typename A2,
            typename mt_policy = SIGSLOT_DEFAULT_MT_POLICY>
  using signal2 = signal_with_thread_policy<mt_policy, A1, A2>;
  
  template <typename A1,
            typename A2,
            typename A3,
            typename mt_policy = SIGSLOT_DEFAULT_MT_POLICY>
  using signal3 = signal_with_thread_policy<mt_policy, A1, A2, A3>;
  
  template <typename A1,
            typename A2,
            typename A3,
            typename A4,
            typename mt_policy = SIGSLOT_DEFAULT_MT_POLICY>
  using signal4 = signal_with_thread_policy<mt_policy, A1, A2, A3, A4>;
  
  template <typename A1,
            typename A2,
            typename A3,
            typename A4,
            typename A5,
            typename mt_policy = SIGSLOT_DEFAULT_MT_POLICY>
  using signal5 = signal_with_thread_policy<mt_policy, A1, A2, A3, A4, A5>;
  
  template <typename A1,
            typename A2,
            typename A3,
            typename A4,
            typename A5,
            typename A6,
            typename mt_policy = SIGSLOT_DEFAULT_MT_POLICY>
  using signal6 = signal_with_thread_policy<mt_policy, A1, A2, A3, A4, A5, A6>;
  
  template <typename A1,
            typename A2,
            typename A3,
            typename A4,
            typename A5,
            typename A6,
            typename A7,
            typename mt_policy = SIGSLOT_DEFAULT_MT_POLICY>
  using signal7 =
      signal_with_thread_policy<mt_policy, A1, A2, A3, A4, A5, A6, A7>;
  
  template <typename A1,
            typename A2,
            typename A3,
            typename A4,
            typename A5,
            typename A6,
            typename A7,
            typename A8,
            typename mt_policy = SIGSLOT_DEFAULT_MT_POLICY>
  using signal8 =
      signal_with_thread_policy<mt_policy, A1, A2, A3, A4, A5, A6, A7, A8>;
  
  }  // namespace sigslot
  
  #endif /* RTC_BASE_THIRD_PARTY_SIGSLOT_SIGSLOT_H_ */
  