lazy.h

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/*
 * Licensed to the Apache Software Foundation (ASF) under one
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 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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.
 */
 
#pragma once
 
 
#include <concepts>
#include <functional>
#include <mutex>
#include <utility>
 
#include "iceberg/result.h"
#include "iceberg/util/macros.h"
 
namespace iceberg {
 
template <auto InitFunc>
class Lazy {
  template <typename R, typename... Args>
  static R ExtractReturnType(R (*)(Args...));  // only declaration, never defined
 
  using T = ResultValueT<decltype(ExtractReturnType(InitFunc))>;
 
 public:
  template <typename... Args>
    requires std::invocable<decltype(InitFunc), Args...>
  Result<std::reference_wrapper<T>> Get(Args&&... args) const {
    std::call_once(
        flag_, [this, &args...]() { value_ = InitFunc(std::forward<Args>(args)...); });
    ICEBERG_RETURN_UNEXPECTED(value_);
    return std::ref(*value_);
  }
 
 private:
  mutable Result<T> value_ = Invalid("Lazy value has not been initialized");
  mutable std::once_flag flag_;
};
 
};  // namespace iceberg