// Copyright 2014 The Bazel Authors. All rights reserved.
//
// 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.

package com.google.devtools.common.options;

import com.google.devtools.build.lib.skybridge.SkybridgeInterface;

/**
 * Base class for all options classes. Extend this class, adding public instance fields annotated
 * with {@link Option}. Then you can create instances either programmatically:
 *
 * <pre>
 *   X x = Options.getDefaults(X.class);
 *   x.host = "localhost";
 *   x.port = 80;
 * </pre>
 *
 * or from an array of command-line arguments:
 *
 * <pre>
 *   OptionsParser parser = OptionsParser.builder()
 *       .optionsClasses(X.class)
 *       .build();
 *   parser.parse("--host", "localhost", "--port", "80");
 *   X x = parser.getOptions(X.class);
 * </pre>
 *
 * <p>Subclasses of {@code OptionsBase} <i>must</i> be constructed reflectively, i.e. using not
 * {@code new MyOptions()}, but one of the above methods instead. (Direct construction creates an
 * empty instance, not containing default values. This leads to surprising behavior and often {@code
 * NullPointerExceptions}, etc.)
 */
@SkybridgeInterface
public abstract class OptionsBase {

  /** Subclasses must provide a default (no argument) constructor. */
  protected OptionsBase() {
    // There used to be a validation here that checks the stack trace of this constructor
    // invocation; unfortunately, that makes the options construction about 10x slower. So be
    // careful with how you construct options classes.
  }

  /**
   * Returns the "options class" that this object belongs to. By default it is the object's class.
   * However, for classes annotated with {@code @OptionsClass}, this returns the base class and not
   * the generated implementation class.
   */
  public Class<? extends OptionsBase> getOptionsClass() {
    return getClass();
  }
}
