text: /**
   *   Copyright 2011-2015 Quickstep Technologies LLC.
   *   Copyright 2015 Pivotal Software, Inc.
   *
   *   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 QUICKSTEP_QUERY_OPTIMIZER_EXPRESSIONS_EXPRESSION_HPP_
  #define QUICKSTEP_QUERY_OPTIMIZER_EXPRESSIONS_EXPRESSION_HPP_
  
  #include <memory>
  #include <map>
  #include <vector>
  
  #include "query_optimizer/OptimizerTree.hpp"
  #include "query_optimizer/expressions/ExpressionType.hpp"
  #include "types/Type.hpp"
  #include "utility/Macros.hpp"
  
  #include "glog/logging.h"
  
  namespace quickstep {
  namespace optimizer {
  namespace expressions {
  
  /** \addtogroup OptimizerExpressions
   *  @{
   */
  
  class AttributeReference;
  class Expression;
  
  typedef std::shared_ptr<const Expression> ExpressionPtr;
  
  /**
   * @brief Base class for all expressions in the query optimizer.
   */
  class Expression : public OptimizerTree<Expression> {
   public:
    /**
     * @brief Destructor.
     */
    ~Expression() override {}
  
    /**
     * @brief Returns the expression type.
     *
     * @return ExpressionType of this expression.
     */
    virtual ExpressionType getExpressionType() const = 0;
  
    /**
     * @brief Returns the type of the output.
     *
     * @return The type of the output.
     */
    virtual const Type& getValueType() const = 0;
  
    /**
     * @brief Returns AttributeReferences referenced by this expression.
     *
     * @return A vector of AttributeReference referenced by this expression.
     */
    virtual std::vector<std::shared_ptr<const AttributeReference>> getReferencedAttributes() const = 0;
  
    /**
     * @return True if this expression is constant.
     */
    virtual bool isConstant() const = 0;
  
   protected:
    Expression() {}
  
   private:
    DISALLOW_COPY_AND_ASSIGN(Expression);
  };
  
  /** @} */
  
  }  // namespace expressions
  }  // namespace optimizer
  }  // namespace quickstep
  
  #endif /* QUICKSTEP_QUERY_OPTIMIZER_EXPRESSIONS_EXPRESSION_HPP_ */
  