llvm/llvm/include/llvm/Support/TrailingObjects.h

//===--- TrailingObjects.h - Variable-length classes ------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
///
/// \file
/// This header defines support for implementing classes that have
/// some trailing object (or arrays of objects) appended to them. The
/// main purpose is to make it obvious where this idiom is being used,
/// and to make the usage more idiomatic and more difficult to get
/// wrong.
///
/// The TrailingObject template abstracts away the reinterpret_cast,
/// pointer arithmetic, and size calculations used for the allocation
/// and access of appended arrays of objects, and takes care that they
/// are all allocated at their required alignment. Additionally, it
/// ensures that the base type is final -- deriving from a class that
/// expects data appended immediately after it is typically not safe.
///
/// Users are expected to derive from this template, and provide
/// numTrailingObjects implementations for each trailing type except
/// the last, e.g. like this sample:
///
/// \code
/// class VarLengthObj : private TrailingObjects<VarLengthObj, int, double> {
///   friend TrailingObjects;
///
///   unsigned NumInts, NumDoubles;
///   size_t numTrailingObjects(OverloadToken<int>) const { return NumInts; }
///  };
/// \endcode
///
/// You can access the appended arrays via 'getTrailingObjects', and
/// determine the size needed for allocation via
/// 'additionalSizeToAlloc' and 'totalSizeToAlloc'.
///
/// All the methods implemented by this class are intended for use
/// by the implementation of the class, not as part of its interface
/// (thus, private inheritance is suggested).
///
//===----------------------------------------------------------------------===//

#ifndef LLVM_SUPPORT_TRAILINGOBJECTS_H
#define LLVM_SUPPORT_TRAILINGOBJECTS_H

#include "llvm/Support/AlignOf.h"
#include "llvm/Support/Alignment.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/type_traits.h"
#include <new>
#include <type_traits>

namespace llvm {

namespace trailing_objects_internal {
/// Helper template to calculate the max alignment requirement for a set of
/// objects.
template <typename First, typename... Rest> class AlignmentCalcHelper { … };

AlignmentCalcHelper<First>;

/// The base class for TrailingObjects* classes.
class TrailingObjectsBase { … };

// Just a little helper for transforming a type pack into the same
// number of a different type. e.g.:
//   ExtractSecondType<Foo..., int>::type
template <typename Ty1, typename Ty2> struct ExtractSecondType { … };

// TrailingObjectsImpl is somewhat complicated, because it is a
// recursively inheriting template, in order to handle the template
// varargs. Each level of inheritance picks off a single trailing type
// then recurses on the rest. The "Align", "BaseTy", and
// "TopTrailingObj" arguments are passed through unchanged through the
// recursion. "PrevTy" is, at each level, the type handled by the
// level right above it.

template <int Align, typename BaseTy, typename TopTrailingObj, typename PrevTy,
          typename... MoreTys>
class TrailingObjectsImpl { … };

TrailingObjectsImpl<Align, BaseTy, TopTrailingObj, PrevTy, NextTy, MoreTys...>;

// The base case of the TrailingObjectsImpl inheritance recursion,
// when there's no more trailing types.
TrailingObjectsImpl<Align, BaseTy, TopTrailingObj, PrevTy>;

} // end namespace trailing_objects_internal

// Finally, the main type defined in this file, the one intended for users...

/// See the file comment for details on the usage of the
/// TrailingObjects type.
template <typename BaseTy, typename... TrailingTys>
class TrailingObjects : private trailing_objects_internal::TrailingObjectsImpl<
                            trailing_objects_internal::AlignmentCalcHelper<
                                TrailingTys...>::Alignment,
                            BaseTy, TrailingObjects<BaseTy, TrailingTys...>,
                            BaseTy, TrailingTys...> { … };

} // end namespace llvm

#endif