UnionBasedStatic.h

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/*
 *  Copyright (c) 2015-present, Facebook, Inc.
 *  All rights reserved.
 *
 *  This source code is licensed under the BSD-style license found in the
 *  LICENSE file in the root directory of this source tree. An additional grant
 *  of patent rights can be found in the PATENTS file in the same directory.
 *
 */
#pragma once

// This pattern is used to create a static whose destructor will never be
// called. This is useful since some proxygen clients call exit() directly in
// multithreaded programs explicitly against recommendations which lead to
// shutdown crashes due to dependent statics being cleaned up while the threads
// are still executing.

// IMPLEMENTATION MACROS
// (Don't use these directly.)
#define DECLARE_UNION_STATIC_UNION_IMPL(type, name)                           \
union name##Union {                                                           \
  type data;                                                                  \
  name##Union() {}                                                            \
  ~name##Union() {}                                                           \
}

#define DECLARE_UNION_STATIC_UNION_ARRAY_IMPL(type, size, name)               \
union name##Union {                                                           \
  type data[size];                                                            \
  name##Union() {}                                                            \
  ~name##Union() {}                                                           \
}

#define DEFINE_UNION_STATIC_UNION_IMPL(type, name, var)                       \
DECLARE_UNION_STATIC_UNION_IMPL(type, name) var;

#define DEFINE_UNION_STATIC_UNION_CONST_IMPL(type, name, var)                 \
DECLARE_UNION_STATIC_UNION_IMPL(type, name) const var;

#define DEFINE_UNION_STATIC_UNION_CONST_ARRAY_IMPL(type, size, name, var)     \
DECLARE_UNION_STATIC_UNION_ARRAY_IMPL(type, size, name) const var;

// The const_casts are only needed if creating a const union but it's a
// no-op otherwise so keep it to avoid creating even more macro helpers.
#define DEFINE_UNION_STATIC_CONSTRUCTOR_IMPL(type, name, var)                 \
__attribute__((__constructor__))                                              \
void init##name##Union() {                                                    \
  new (const_cast<type*>(&var.data)) type();                                  \
}

#define DEFINE_UNION_STATIC_CONSTRUCTOR_ARG_IMPL(type, name, var, ...)        \
__attribute__((__constructor__))                                              \
void init##name##Union() {                                                    \
  new (const_cast<type*>(&var.data)) type(__VA_ARGS__);                       \
}
// END IMPLEMENTATION MACROS

// Use var.data to access the actual member of interest. Zero and argument
// versions are provided. If you need to do custom construction, like using a
// brace-enclosed initializer list, use the NO_INIT variant and then define
// your own __attribute__((__constructor__)) function to do the initialization.
#define DEFINE_UNION_STATIC(type, name, var)                                  \
DEFINE_UNION_STATIC_UNION_IMPL(type, name, var)                               \
DEFINE_UNION_STATIC_CONSTRUCTOR_IMPL(type, name, var)

#define DEFINE_UNION_STATIC_ARGS(type, name, var, ...)                        \
DEFINE_UNION_STATIC_UNION_IMPL(type, name, var)                               \
DEFINE_UNION_STATIC_CONSTRUCTOR_ARG_IMPL(type, name, var, __VA_ARGS__)

#define DEFINE_UNION_STATIC_NO_INIT(type, name, var)                          \
DEFINE_UNION_STATIC_UNION_IMPL(type, name, var)

// Same as the above three except used to create a const union.
#define DEFINE_UNION_STATIC_CONST(type, name, var)                            \
DEFINE_UNION_STATIC_UNION_CONST_IMPL(type, name, var)                         \
DEFINE_UNION_STATIC_CONSTRUCTOR_IMPL(type, name, var)

#define DEFINE_UNION_STATIC_CONST_ARGS(type, name, var, ...)                  \
DEFINE_UNION_STATIC_UNION_CONST_IMPL(type, name, var)                         \
DEFINE_UNION_STATIC_CONSTRUCTOR_ARG_IMPL(type, name, var, __VA_ARGS__)

#define DEFINE_UNION_STATIC_CONST_NO_INIT(type, name, var)                    \
DEFINE_UNION_STATIC_UNION_CONST_IMPL(type, name, var)

#define DEFINE_UNION_STATIC_CONST_ARRAY_NO_INIT(type, size, name, var)        \
DEFINE_UNION_STATIC_UNION_CONST_ARRAY_IMPL(type, size, name, var)

// Use these if you need to extern one of these in a header and then
// define it in a .cpp file. For example:
//
// Header:
// DECLARE_UNION_STATIC(std::string, StdString);
// extern const StdStringUnion kStringConstant;
//
// Definition:
// const IMPLEMENT_DECLARED_UNION_STATIC_ARGS(
//   std::string, StdString, DoesNotMatter, kStringConstant, "hello world");
//
#define DECLARE_UNION_STATIC(type, name)                                      \
DECLARE_UNION_STATIC_UNION_IMPL(type, name)

#define IMPLEMENT_DECLARED_UNION_STATIC(type, unionName, name, var)           \
unionName##Union var;                                                         \
DEFINE_UNION_STATIC_CONSTRUCTOR_ARG_IMPL(type, name, var)

#define IMPLEMENT_DECLARED_UNION_STATIC_ARGS(type, unionName, name, var, ...) \
unionName##Union var;                                                         \
DEFINE_UNION_STATIC_CONSTRUCTOR_ARG_IMPL(type, name, var, __VA_ARGS__)