Derive ThreadPool from std::vector

Prefer sub-classing to composition in this case.

No functional change.

src/thread.h

@@ -138,21 +138,16 @@ struct TimerThread : public Thread {
 };
 
 
-/// ThreadPool class handles all the threads related stuff like init, starting,
+/// ThreadPool struct handles all the threads related stuff like init, starting,
 /// parking and, the most important, launching a slave thread at a split point.
 /// All the access to shared thread data is done through this class.
 
-class ThreadPool {
+struct ThreadPool : public std::vector<Thread*> {
 
-public:
   void init(); // No c'tor and d'tor, threads rely on globals that should
   void exit(); // be initialized and valid during the whole thread lifetime.
 
-  Thread& operator[](size_t id) { return *threads[id]; }
-  size_t size() const { return threads.size(); }
-  MainThread* main_thread() { return static_cast<MainThread*>(threads[0]); }
-  TimerThread* timer_thread() { return timer; }
-
+  MainThread* main_thread() { return static_cast<MainThread*>((*this)[0]); }
   void read_uci_options();
   bool slave_available(Thread* master) const;
   void wait_for_think_finished();
@@ -164,9 +159,6 @@ public:
   size_t maxThreadsPerSplitPoint;
   Mutex mutex;
   ConditionVariable sleepCondition;
-
-private:
-  std::vector<Thread*> threads;
   TimerThread* timer;
 };