src/core/lib/gpr++/inlined_vector.h

/*
 *
 * Copyright 2017 gRPC authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 */

#ifndef GRPC_CORE_LIB_GPRXX_INLINED_VECTOR_H
#define GRPC_CORE_LIB_GPRXX_INLINED_VECTOR_H

#include <cassert>

#include "src/core/lib/gpr++/memory.h"

namespace grpc_core {

// NOTE: We eventually want to use absl::InlinedVector here.  However,
// there are currently build problems that prevent us from using absl.
// In the interim, we define a custom implementation as a place-holder,
// with the intent to eventually replace this with the absl
// implementation.
//
// This place-holder implementation does not implement the full set of
// functionality from the absl version; it has just the methods that we
// currently happen to need in gRPC.  If additional functionality is
// needed before this gets replaced with the absl version, it can be
// added, with the following proviso:
//
// ANY METHOD ADDED HERE MUST COMPLY WITH THE INTERFACE IN THE absl
// IMPLEMENTATION!
//
// TODO(ctiller, nnoble, roth): Replace this with absl::InlinedVector
// once we integrate absl into the gRPC build system in a usable way.
template <typename T, size_t N>
class InlinedVector {
 public:
  InlinedVector() {}
  ~InlinedVector() {
    for (size_t i = 0; i < size_ && i < N; ++i) {
      T& value = *reinterpret_cast<T*>(inline_ + i);
      value.~T();
    }
    if (size_ > N) {  // Avoid subtracting two signed values.
      for (size_t i = 0; i < size_ - N; ++i) {
        dynamic_[i].~T();
      }
    }
    gpr_free(dynamic_);
  }

  // For now, we do not support copying.
  InlinedVector(const InlinedVector&) = delete;
  InlinedVector& operator=(const InlinedVector&) = delete;

  T& operator[](size_t offset) {
    assert(offset < size_);
    if (offset < N) {
      return *reinterpret_cast<T*>(inline_ + offset);
    } else {
      return dynamic_[offset - N];
    }
  }

  template <typename... Args>
  void emplace_back(Args&&... args) {
    if (size_ < N) {
      new (&inline_[size_]) T(std::forward<Args>(args)...);
    } else {
      if (size_ - N == dynamic_capacity_) {
        size_t new_capacity =
            dynamic_capacity_ == 0 ? 2 : dynamic_capacity_ * 2;
        T* new_dynamic = static_cast<T*>(gpr_malloc(sizeof(T) * new_capacity));
        for (size_t i = 0; i < dynamic_capacity_; ++i) {
          new (&new_dynamic[i]) T(std::move(dynamic_[i]));
          dynamic_[i].~T();
        }
        gpr_free(dynamic_);
        dynamic_ = new_dynamic;
        dynamic_capacity_ = new_capacity;
      }
      new (&dynamic_[size_ - N]) T(std::forward<Args>(args)...);
    }
    ++size_;
  }

  void push_back(const T& value) { emplace_back(value); }

  void push_back(T&& value) { emplace_back(std::move(value)); }

  size_t size() const { return size_; }

 private:
  typename std::aligned_storage<sizeof(T)>::type inline_[N];
  T* dynamic_ = nullptr;
  size_t size_ = 0;
  size_t dynamic_capacity_ = 0;
};

}  // namespace grpc_core

#endif /* GRPC_CORE_LIB_GPRXX_INLINED_VECTOR_H */