stack_array.h

template <typename T>
class stack_array {
 public:
  __attribute__((always_inline)) stack_array(size_t length)
      : ptr_{new (static_cast<T *>(alloca(sizeof(T) * length))) T[length]},
        length_{length} {}

  ~stack_array() {
    for (size_t i = 0; i < length_; ++i) {
      ptr_[i].~T();
    }
  }

  operator T *() { return ptr_; }
  size_t size() { return length_; }

  using iterator = T *;
  iterator begin() { return ptr_; }
  iterator end() { return ptr_ + length_; }

 private:
  T *ptr_;
  size_t length_;
};

Too bad __attribute__((always_inline)) doesn't apply unless you have -O1 or greater (clang version 4.0.0).