Class ThreadLocal

tests/gtest/gtest.h:1828–1901

// using Google Test have exited when main() returns.
template <typename T>
class ThreadLocal : public ThreadLocalBase {
 public:
  ThreadLocal() : default_factory_(new DefaultValueHolderFactory()) {}
  explicit ThreadLocal(const T& value)
      : default_factory_(new InstanceValueHolderFactory(value)) {}

  ~ThreadLocal() { ThreadLocalRegistry::OnThreadLocalDestroyed(this); }

  T* pointer() { return GetOrCreateValue(); }
  const T* pointer() const { return GetOrCreateValue(); }
  const T& get() const { return *pointer(); }
  void set(const T& value) { *pointer() = value; }

 private:
  // Holds a value of T.  Can be deleted via its base class without the caller
  // knowing the type of T.
  class ValueHolder : public ThreadLocalValueHolderBase {
   public:
    ValueHolder() : value_() {}
    explicit ValueHolder(const T& value) : value_(value) {}

    T* pointer() { return &value_; }

   private:
    T value_;
    GTEST_DISALLOW_COPY_AND_ASSIGN_(ValueHolder);
  };


  T* GetOrCreateValue() const {
    return static_cast<ValueHolder*>(
        ThreadLocalRegistry::GetValueOnCurrentThread(this))->pointer();
  }

  virtual ThreadLocalValueHolderBase* NewValueForCurrentThread() const {
    return default_factory_->MakeNewHolder();
  }

  class ValueHolderFactory {
   public:
    ValueHolderFactory() {}
    virtual ~ValueHolderFactory() {}
    virtual ValueHolder* MakeNewHolder() const = 0;

   private:
    GTEST_DISALLOW_COPY_AND_ASSIGN_(ValueHolderFactory);
  };

  class DefaultValueHolderFactory : public ValueHolderFactory {
   public:
    DefaultValueHolderFactory() {}
    virtual ValueHolder* MakeNewHolder() const { return new ValueHolder(); }

   private:
    GTEST_DISALLOW_COPY_AND_ASSIGN_(DefaultValueHolderFactory);
  };

  class InstanceValueHolderFactory : public ValueHolderFactory {