carbon-lang/toolchain/base/index_base.h

// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
// Exceptions. See /LICENSE for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

#ifndef CARBON_TOOLCHAIN_BASE_INDEX_BASE_H_
#define CARBON_TOOLCHAIN_BASE_INDEX_BASE_H_

#include <compare>
#include <concepts>

#include "common/ostream.h"

namespace Carbon {

template <typename DataType>
class DataIterator;

// A lightweight handle to an item identified by an opaque ID.
//
// This class is intended to be derived from by classes representing a specific
// kind of ID, whose meaning as an integer is an implementation detail of the
// type that vends the IDs. Typically this will be a vector index.
//
// Classes derived from IdBase are designed to be passed by value, not
// reference or pointer. They are also designed to be small and efficient to
// store in data structures.
struct IdBase : public Printable<IdBase> {
  static constexpr int32_t InvalidIndex = -1;

  IdBase() = delete;
  constexpr explicit IdBase(int index) : index(index) {}

  auto Print(llvm::raw_ostream& output) const -> void {
    if (is_valid()) {
      output << index;
    } else {
      output << "<invalid>";
    }
  }

  constexpr auto is_valid() const -> bool { return index != InvalidIndex; }

  int32_t index;
};

// A lightweight handle to an item that behaves like an index.
//
// Unlike IdBase, classes derived from IndexBase are not completely opaque, and
// provide at least an ordering between indexes that has meaning to an API
// user. Additional semantics may be specified by the derived class.
struct IndexBase : public IdBase {
  using IdBase::IdBase;
};

// Support equality comparison when one operand is a child of `IdBase`
// (including `IndexBase`) and the other operand is either the same type or
// convertible to that type.
template <typename IndexType>
  requires std::derived_from<IndexType, IdBase>
constexpr auto operator==(IndexType lhs, IndexType rhs) -> bool {
  return lhs.index == rhs.index;
}
template <typename IndexType, typename RHSType>
  requires std::derived_from<IndexType, IdBase> &&
           std::convertible_to<RHSType, IndexType>
auto operator==(IndexType lhs, RHSType rhs) -> bool {
  return lhs.index == IndexType(rhs).index;
}

// Relational comparisons are only supported for types derived from `IndexBase`.
template <typename IndexType>
  requires std::derived_from<IndexType, IndexBase>
auto operator<=>(IndexType lhs, IndexType rhs) -> std::strong_ordering {
  return lhs.index <=> rhs.index;
}

}  // namespace Carbon

#endif  // CARBON_TOOLCHAIN_BASE_INDEX_BASE_H_