comparison osx/include/kj/common.h @ 49:3ab5a40c4e3b

Add Capnp and KJ builds for OSX

+// Copyright (c) 2013-2014 Sandstorm Development Group, Inc. and contributors
+// Licensed under the MIT License:
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+// Header that should be #included by everyone.
+//
+// This defines very simple utilities that are widely applicable.
+
+#ifndef KJ_COMMON_H_
+#define KJ_COMMON_H_
+
+#if defined(__GNUC__) && !KJ_HEADER_WARNINGS
+#pragma GCC system_header
+#endif
+
+#ifndef KJ_NO_COMPILER_CHECK
+#if __cplusplus < 201103L && !__CDT_PARSER__ && !_MSC_VER
+  #error "This code requires C++11. Either your compiler does not support it or it is not enabled."
+  #ifdef __GNUC__
+    // Compiler claims compatibility with GCC, so presumably supports -std.
+    #error "Pass -std=c++11 on the compiler command line to enable C++11."
+  #endif
+#endif
+
+#ifdef __GNUC__
+  #if __clang__
+    #if __clang_major__ < 3 || (__clang_major__ == 3 && __clang_minor__ < 2)
+      #warning "This library requires at least Clang 3.2."
+    #elif defined(__apple_build_version__) && __apple_build_version__ <= 4250028
+      #warning "This library requires at least Clang 3.2.  XCode 4.6's Clang, which claims to be "\
+               "version 4.2 (wat?), is actually built from some random SVN revision between 3.1 "\
+               "and 3.2.  Unfortunately, it is insufficient for compiling this library.  You can "\
+               "download the real Clang 3.2 (or newer) from the Clang web site.  Step-by-step "\
+               "instructions can be found in Cap'n Proto's documentation: "\
+               "http://kentonv.github.io/capnproto/install.html#clang_32_on_mac_osx"
+    #elif __cplusplus >= 201103L && !__has_include(<initializer_list>)
+      #warning "Your compiler supports C++11 but your C++ standard library does not.  If your "\
+               "system has libc++ installed (as should be the case on e.g. Mac OSX), try adding "\
+               "-stdlib=libc++ to your CXXFLAGS."
+    #endif
+  #else
+    #if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 7)
+      #warning "This library requires at least GCC 4.7."
+    #endif
+  #endif
+#elif defined(_MSC_VER)
+  #if _MSC_VER < 1900
+    #error "You need Visual Studio 2015 or better to compile this code."
+  #elif !CAPNP_LITE
+    // TODO(cleanup): This is KJ, but we're talking about Cap'n Proto.
+    #error "As of this writing, Cap'n Proto only supports Visual C++ in 'lite mode'; please #define CAPNP_LITE"
+  #endif
+#else
+  #warning "I don't recognize your compiler.  As of this writing, Clang and GCC are the only "\
+           "known compilers with enough C++11 support for this library.  "\
+           "#define KJ_NO_COMPILER_CHECK to make this warning go away."
+#endif
+#endif
+
+#include <stddef.h>
+#include <initializer_list>
+
+#if __linux__ && __cplusplus > 201200L
+// Hack around stdlib bug with C++14 that exists on some Linux systems.
+// Apparently in this mode the C library decides not to define gets() but the C++ library still
+// tries to import it into the std namespace. This bug has been fixed at the source but is still
+// widely present in the wild e.g. on Ubuntu 14.04.
+#undef _GLIBCXX_HAVE_GETS
+#endif
+
+#if defined(_MSC_VER)
+#include <intrin.h>  // __popcnt
+#endif
+
+// =======================================================================================
+
+namespace kj {
+
+typedef unsigned int uint;
+typedef unsigned char byte;
+
+// =======================================================================================
+// Common macros, especially for common yet compiler-specific features.
+
+// Detect whether RTTI and exceptions are enabled, assuming they are unless we have specific
+// evidence to the contrary.  Clients can always define KJ_NO_RTTI or KJ_NO_EXCEPTIONS explicitly
+// to override these checks.
+#ifdef __GNUC__
+  #if !defined(KJ_NO_RTTI) && !__GXX_RTTI
+    #define KJ_NO_RTTI 1
+  #endif
+  #if !defined(KJ_NO_EXCEPTIONS) && !__EXCEPTIONS
+    #define KJ_NO_EXCEPTIONS 1
+  #endif
+#elif defined(_MSC_VER)
+  #if !defined(KJ_NO_RTTI) && !defined(_CPPRTTI)
+    #define KJ_NO_RTTI 1
+  #endif
+  #if !defined(KJ_NO_EXCEPTIONS) && !defined(_CPPUNWIND)
+    #define KJ_NO_EXCEPTIONS 1
+  #endif
+#endif
+
+#if !defined(KJ_DEBUG) && !defined(KJ_NDEBUG)
+// Heuristically decide whether to enable debug mode.  If DEBUG or NDEBUG is defined, use that.
+// Otherwise, fall back to checking whether optimization is enabled.
+#if defined(DEBUG) || defined(_DEBUG)
+#define KJ_DEBUG
+#elif defined(NDEBUG)
+#define KJ_NDEBUG
+#elif __OPTIMIZE__
+#define KJ_NDEBUG
+#else
+#define KJ_DEBUG
+#endif
+#endif
+
+#define KJ_DISALLOW_COPY(classname) \
+  classname(const classname&) = delete; \
+  classname& operator=(const classname&) = delete
+// Deletes the implicit copy constructor and assignment operator.
+
+#ifdef __GNUC__
+#define KJ_LIKELY(condition) __builtin_expect(condition, true)
+#define KJ_UNLIKELY(condition) __builtin_expect(condition, false)
+// Branch prediction macros.  Evaluates to the condition given, but also tells the compiler that we
+// expect the condition to be true/false enough of the time that it's worth hard-coding branch
+// prediction.
+#else
+#define KJ_LIKELY(condition) (condition)
+#define KJ_UNLIKELY(condition) (condition)
+#endif
+
+#if defined(KJ_DEBUG) || __NO_INLINE__
+#define KJ_ALWAYS_INLINE(prototype) inline prototype
+// Don't force inline in debug mode.
+#else
+#if defined(_MSC_VER)
+#define KJ_ALWAYS_INLINE(prototype) __forceinline prototype
+#else
+#define KJ_ALWAYS_INLINE(prototype) inline prototype __attribute__((always_inline))
+#endif
+// Force a function to always be inlined.  Apply only to the prototype, not to the definition.
+#endif
+
+#if defined(_MSC_VER)
+#define KJ_NOINLINE __declspec(noinline)
+#else
+#define KJ_NOINLINE __attribute__((noinline))
+#endif
+
+#if defined(_MSC_VER)
+#define KJ_NORETURN(prototype) __declspec(noreturn) prototype
+#define KJ_UNUSED
+#define KJ_WARN_UNUSED_RESULT
+// TODO(msvc): KJ_WARN_UNUSED_RESULT can use _Check_return_ on MSVC, but it's a prefix, so
+//   wrapping the whole prototype is needed. http://msdn.microsoft.com/en-us/library/jj159529.aspx
+//   Similarly, KJ_UNUSED could use __pragma(warning(suppress:...)), but again that's a prefix.
+#else
+#define KJ_NORETURN(prototype) prototype __attribute__((noreturn))
+#define KJ_UNUSED __attribute__((unused))
+#define KJ_WARN_UNUSED_RESULT __attribute__((warn_unused_result))
+#endif
+
+#if __clang__
+#define KJ_UNUSED_MEMBER __attribute__((unused))
+// Inhibits "unused" warning for member variables.  Only Clang produces such a warning, while GCC
+// complains if the attribute is set on members.
+#else
+#define KJ_UNUSED_MEMBER
+#endif
+
+#if __clang__
+#define KJ_DEPRECATED(reason) \
+    __attribute__((deprecated(reason)))
+#define KJ_UNAVAILABLE(reason) \
+    __attribute__((unavailable(reason)))
+#elif __GNUC__
+#define KJ_DEPRECATED(reason) \
+    __attribute__((deprecated))
+#define KJ_UNAVAILABLE(reason)
+#else
+#define KJ_DEPRECATED(reason)
+#define KJ_UNAVAILABLE(reason)
+// TODO(msvc): Again, here, MSVC prefers a prefix, __declspec(deprecated).
+#endif
+
+namespace _ {  // private
+
+KJ_NORETURN(void inlineRequireFailure(
+    const char* file, int line, const char* expectation, const char* macroArgs,
+    const char* message = nullptr));
+
+KJ_NORETURN(void unreachable());
+
+}  // namespace _ (private)
+
+#ifdef KJ_DEBUG
+#if _MSC_VER
+#define KJ_IREQUIRE(condition, ...) \
+    if (KJ_LIKELY(condition)); else ::kj::_::inlineRequireFailure( \
+        __FILE__, __LINE__, #condition, "" #__VA_ARGS__, __VA_ARGS__)
+// Version of KJ_DREQUIRE() which is safe to use in headers that are #included by users.  Used to
+// check preconditions inside inline methods.  KJ_IREQUIRE is particularly useful in that
+// it will be enabled depending on whether the application is compiled in debug mode rather than
+// whether libkj is.
+#else
+#define KJ_IREQUIRE(condition, ...) \
+    if (KJ_LIKELY(condition)); else ::kj::_::inlineRequireFailure( \
+        __FILE__, __LINE__, #condition, #__VA_ARGS__, ##__VA_ARGS__)
+// Version of KJ_DREQUIRE() which is safe to use in headers that are #included by users.  Used to
+// check preconditions inside inline methods.  KJ_IREQUIRE is particularly useful in that
+// it will be enabled depending on whether the application is compiled in debug mode rather than
+// whether libkj is.
+#endif
+#else
+#define KJ_IREQUIRE(condition, ...)
+#endif
+
+#define KJ_IASSERT KJ_IREQUIRE
+
+#define KJ_UNREACHABLE ::kj::_::unreachable();
+// Put this on code paths that cannot be reached to suppress compiler warnings about missing
+// returns.
+
+#if __clang__
+#define KJ_CLANG_KNOWS_THIS_IS_UNREACHABLE_BUT_GCC_DOESNT
+#else
+#define KJ_CLANG_KNOWS_THIS_IS_UNREACHABLE_BUT_GCC_DOESNT KJ_UNREACHABLE
+#endif
+
+// #define KJ_STACK_ARRAY(type, name, size, minStack, maxStack)
+//
+// Allocate an array, preferably on the stack, unless it is too big.  On GCC this will use
+// variable-sized arrays.  For other compilers we could just use a fixed-size array.  `minStack`
+// is the stack array size to use if variable-width arrays are not supported.  `maxStack` is the
+// maximum stack array size if variable-width arrays *are* supported.
+#if __GNUC__ && !__clang__
+#define KJ_STACK_ARRAY(type, name, size, minStack, maxStack) \
+  size_t name##_size = (size); \
+  bool name##_isOnStack = name##_size <= (maxStack); \
+  type name##_stack[name##_isOnStack ? size : 0]; \
+  ::kj::Array<type> name##_heap = name##_isOnStack ? \
+      nullptr : kj::heapArray<type>(name##_size); \
+  ::kj::ArrayPtr<type> name = name##_isOnStack ? \
+      kj::arrayPtr(name##_stack, name##_size) : name##_heap
+#else
+#define KJ_STACK_ARRAY(type, name, size, minStack, maxStack) \
+  size_t name##_size = (size); \
+  bool name##_isOnStack = name##_size <= (minStack); \
+  type name##_stack[minStack]; \
+  ::kj::Array<type> name##_heap = name##_isOnStack ? \
+      nullptr : kj::heapArray<type>(name##_size); \
+  ::kj::ArrayPtr<type> name = name##_isOnStack ? \
+      kj::arrayPtr(name##_stack, name##_size) : name##_heap
+#endif
+
+#define KJ_CONCAT_(x, y) x##y
+#define KJ_CONCAT(x, y) KJ_CONCAT_(x, y)
+#define KJ_UNIQUE_NAME(prefix) KJ_CONCAT(prefix, __LINE__)
+// Create a unique identifier name.  We use concatenate __LINE__ rather than __COUNTER__ so that
+// the name can be used multiple times in the same macro.
+
+#if _MSC_VER
+
+#define KJ_CONSTEXPR(...) __VA_ARGS__
+// Use in cases where MSVC barfs on constexpr. A replacement keyword (e.g. "const") can be
+// provided, or just leave blank to remove the keyword entirely.
+//
+// TODO(msvc): Remove this hack once MSVC fully supports constexpr.
+
+#ifndef __restrict__
+#define __restrict__ __restrict
+// TODO(msvc): Would it be better to define a KJ_RESTRICT macro?
+#endif
+
+#pragma warning(disable: 4521 4522)
+// This warning complains when there are two copy constructors, one for a const reference and
+// one for a non-const reference. It is often quite necessary to do this in wrapper templates,
+// therefore this warning is dumb and we disable it.
+
+#pragma warning(disable: 4458)
+// Warns when a parameter name shadows a class member. Unfortunately my code does this a lot,
+// since I don't use a special name format for members.
+
+#else  // _MSC_VER
+#define KJ_CONSTEXPR(...) constexpr
+#endif
+
+// =======================================================================================
+// Template metaprogramming helpers.
+
+template <typename T> struct NoInfer_ { typedef T Type; };
+template <typename T> using NoInfer = typename NoInfer_<T>::Type;
+// Use NoInfer<T>::Type in place of T for a template function parameter to prevent inference of
+// the type based on the parameter value.
+
+template <typename T> struct RemoveConst_ { typedef T Type; };
+template <typename T> struct RemoveConst_<const T> { typedef T Type; };
+template <typename T> using RemoveConst = typename RemoveConst_<T>::Type;
+
+template <typename> struct IsLvalueReference_ { static constexpr bool value = false; };
+template <typename T> struct IsLvalueReference_<T&> { static constexpr bool value = true; };
+template <typename T>
+inline constexpr bool isLvalueReference() { return IsLvalueReference_<T>::value; }
+
+template <typename T> struct Decay_ { typedef T Type; };
+template <typename T> struct Decay_<T&> { typedef typename Decay_<T>::Type Type; };
+template <typename T> struct Decay_<T&&> { typedef typename Decay_<T>::Type Type; };
+template <typename T> struct Decay_<T[]> { typedef typename Decay_<T*>::Type Type; };
+template <typename T> struct Decay_<const T[]> { typedef typename Decay_<const T*>::Type Type; };
+template <typename T, size_t s> struct Decay_<T[s]> { typedef typename Decay_<T*>::Type Type; };
+template <typename T, size_t s> struct Decay_<const T[s]> { typedef typename Decay_<const T*>::Type Type; };
+template <typename T> struct Decay_<const T> { typedef typename Decay_<T>::Type Type; };
+template <typename T> struct Decay_<volatile T> { typedef typename Decay_<T>::Type Type; };
+template <typename T> using Decay = typename Decay_<T>::Type;
+
+template <bool b> struct EnableIf_;
+template <> struct EnableIf_<true> { typedef void Type; };
+template <bool b> using EnableIf = typename EnableIf_<b>::Type;
+// Use like:
+//
+//     template <typename T, typename = EnableIf<isValid<T>()>
+//     void func(T&& t);
+
+template <typename...> struct VoidSfinae_ { using Type = void; };
+template <typename... Ts> using VoidSfinae = typename VoidSfinae_<Ts...>::Type;
+// Note: VoidSfinae is std::void_t from C++17.
+
+template <typename T>
+T instance() noexcept;
+// Like std::declval, but doesn't transform T into an rvalue reference.  If you want that, specify
+// instance<T&&>().
+
+struct DisallowConstCopy {
+  // Inherit from this, or declare a member variable of this type, to prevent the class from being
+  // copyable from a const reference -- instead, it will only be copyable from non-const references.
+  // This is useful for enforcing transitive constness of contained pointers.
+  //
+  // For example, say you have a type T which contains a pointer.  T has non-const methods which
+  // modify the value at that pointer, but T's const methods are designed to allow reading only.
+  // Unfortunately, if T has a regular copy constructor, someone can simply make a copy of T and
+  // then use it to modify the pointed-to value.  However, if T inherits DisallowConstCopy, then
+  // callers will only be able to copy non-const instances of T.  Ideally, there is some
+  // parallel type ImmutableT which is like a version of T that only has const methods, and can
+  // be copied from a const T.
+  //
+  // Note that due to C++ rules about implicit copy constructors and assignment operators, any
+  // type that contains or inherits from a type that disallows const copies will also automatically
+  // disallow const copies.  Hey, cool, that's exactly what we want.
+
+  DisallowConstCopy() = default;
+  DisallowConstCopy(DisallowConstCopy&) = default;
+  DisallowConstCopy(DisallowConstCopy&&) = default;
+  DisallowConstCopy& operator=(DisallowConstCopy&) = default;
+  DisallowConstCopy& operator=(DisallowConstCopy&&) = default;
+};
+
+template <typename T>
+struct DisallowConstCopyIfNotConst: public DisallowConstCopy {
+  // Inherit from this when implementing a template that contains a pointer to T and which should
+  // enforce transitive constness.  If T is a const type, this has no effect.  Otherwise, it is
+  // an alias for DisallowConstCopy.
+};
+
+template <typename T>
+struct DisallowConstCopyIfNotConst<const T> {};
+
+template <typename T> struct IsConst_ { static constexpr bool value = false; };
+template <typename T> struct IsConst_<const T> { static constexpr bool value = true; };
+template <typename T> constexpr bool isConst() { return IsConst_<T>::value; }
+
+template <typename T> struct EnableIfNotConst_ { typedef T Type; };
+template <typename T> struct EnableIfNotConst_<const T>;
+template <typename T> using EnableIfNotConst = typename EnableIfNotConst_<T>::Type;
+
+template <typename T> struct EnableIfConst_;
+template <typename T> struct EnableIfConst_<const T> { typedef T Type; };
+template <typename T> using EnableIfConst = typename EnableIfConst_<T>::Type;
+
+template <typename T> struct RemoveConstOrDisable_ { struct Type; };
+template <typename T> struct RemoveConstOrDisable_<const T> { typedef T Type; };
+template <typename T> using RemoveConstOrDisable = typename RemoveConstOrDisable_<T>::Type;
+
+template <typename T> struct IsReference_ { static constexpr bool value = false; };
+template <typename T> struct IsReference_<T&> { static constexpr bool value = true; };
+template <typename T> constexpr bool isReference() { return IsReference_<T>::value; }
+
+template <typename From, typename To>
+struct PropagateConst_ { typedef To Type; };
+template <typename From, typename To>
+struct PropagateConst_<const From, To> { typedef const To Type; };
+template <typename From, typename To>
+using PropagateConst = typename PropagateConst_<From, To>::Type;
+
+namespace _ {  // private
+
+template <typename T>
+T refIfLvalue(T&&);
+
+}  // namespace _ (private)
+
+#define KJ_DECLTYPE_REF(exp) decltype(::kj::_::refIfLvalue(exp))
+// Like decltype(exp), but if exp is an lvalue, produces a reference type.
+//
+//     int i;
+//     decltype(i) i1(i);                         // i1 has type int.
+//     KJ_DECLTYPE_REF(i + 1) i2(i + 1);          // i2 has type int.
+//     KJ_DECLTYPE_REF(i) i3(i);                  // i3 has type int&.
+//     KJ_DECLTYPE_REF(kj::mv(i)) i4(kj::mv(i));  // i4 has type int.
+
+template <typename T>
+struct CanConvert_ {
+  static int sfinae(T);
+  static bool sfinae(...);
+};
+
+template <typename T, typename U>
+constexpr bool canConvert() {
+  return sizeof(CanConvert_<U>::sfinae(instance<T>())) == sizeof(int);
+}
+
+#if __clang__
+template <typename T>
+constexpr bool canMemcpy() {
+  // Returns true if T can be copied using memcpy instead of using the copy constructor or
+  // assignment operator.
+
+  // Clang unhelpfully defines __has_trivial_{copy,assign}(T) to be true if the copy constructor /
+  // assign operator are deleted, on the basis that a strict reading of the definition of "trivial"
+  // according to the standard says that deleted functions are in fact trivial.  Meanwhile Clang
+  // provides these admittedly-better intrinsics, but GCC does not.
+  return __is_trivially_constructible(T, const T&) && __is_trivially_assignable(T, const T&);
+}
+#else
+template <typename T>
+constexpr bool canMemcpy() {
+  // Returns true if T can be copied using memcpy instead of using the copy constructor or
+  // assignment operator.
+
+  // GCC defines these to mean what we want them to mean.
+  return __has_trivial_copy(T) && __has_trivial_assign(T);
+}
+#endif
+
+// =======================================================================================
+// Equivalents to std::move() and std::forward(), since these are very commonly needed and the
+// std header <utility> pulls in lots of other stuff.
+//
+// We use abbreviated names mv and fwd because these helpers (especially mv) are so commonly used
+// that the cost of typing more letters outweighs the cost of being slightly harder to understand
+// when first encountered.
+
+template<typename T> constexpr T&& mv(T& t) noexcept { return static_cast<T&&>(t); }
+template<typename T> constexpr T&& fwd(NoInfer<T>& t) noexcept { return static_cast<T&&>(t); }
+
+template<typename T> constexpr T cp(T& t) noexcept { return t; }
+template<typename T> constexpr T cp(const T& t) noexcept { return t; }
+// Useful to force a copy, particularly to pass into a function that expects T&&.
+
+template <typename T, typename U, bool takeT> struct MinType_;
+template <typename T, typename U> struct MinType_<T, U, true> { typedef T Type; };
+template <typename T, typename U> struct MinType_<T, U, false> { typedef U Type; };
+
+template <typename T, typename U>
+using MinType = typename MinType_<T, U, sizeof(T) <= sizeof(U)>::Type;
+// Resolves to the smaller of the two input types.
+
+template <typename T, typename U>
+inline constexpr auto min(T&& a, U&& b) -> MinType<Decay<T>, Decay<U>> {
+  return a < b ? MinType<Decay<T>, Decay<U>>(a) : MinType<Decay<T>, Decay<U>>(b);
+}
+
+template <typename T, typename U, bool takeT> struct MaxType_;
+template <typename T, typename U> struct MaxType_<T, U, true> { typedef T Type; };
+template <typename T, typename U> struct MaxType_<T, U, false> { typedef U Type; };
+
+template <typename T, typename U>
+using MaxType = typename MaxType_<T, U, sizeof(T) >= sizeof(U)>::Type;
+// Resolves to the larger of the two input types.
+
+template <typename T, typename U>
+inline constexpr auto max(T&& a, U&& b) -> MaxType<Decay<T>, Decay<U>> {
+  return a > b ? MaxType<Decay<T>, Decay<U>>(a) : MaxType<Decay<T>, Decay<U>>(b);
+}
+
+template <typename T, size_t s>
+inline constexpr size_t size(T (&arr)[s]) { return s; }
+template <typename T>
+inline constexpr size_t size(T&& arr) { return arr.size(); }
+// Returns the size of the parameter, whether the parameter is a regular C array or a container
+// with a `.size()` method.
+
+class MaxValue_ {
+private:
+  template <typename T>
+  inline constexpr T maxSigned() const {
+    return (1ull << (sizeof(T) * 8 - 1)) - 1;
+  }
+  template <typename T>
+  inline constexpr T maxUnsigned() const {
+    return ~static_cast<T>(0u);
+  }
+
+public:
+#define _kJ_HANDLE_TYPE(T) \
+  inline constexpr operator   signed T() const { return MaxValue_::maxSigned  <  signed T>(); } \
+  inline constexpr operator unsigned T() const { return MaxValue_::maxUnsigned<unsigned T>(); }
+  _kJ_HANDLE_TYPE(char)
+  _kJ_HANDLE_TYPE(short)
+  _kJ_HANDLE_TYPE(int)
+  _kJ_HANDLE_TYPE(long)
+  _kJ_HANDLE_TYPE(long long)
+#undef _kJ_HANDLE_TYPE
+
+  inline constexpr operator char() const {
+    // `char` is different from both `signed char` and `unsigned char`, and may be signed or
+    // unsigned on different platforms.  Ugh.
+    return char(-1) < 0 ? MaxValue_::maxSigned<char>()
+                        : MaxValue_::maxUnsigned<char>();
+  }
+};
+
+class MinValue_ {
+private:
+  template <typename T>
+  inline constexpr T minSigned() const {
+    return 1ull << (sizeof(T) * 8 - 1);
+  }
+  template <typename T>
+  inline constexpr T minUnsigned() const {
+    return 0u;
+  }
+
+public:
+#define _kJ_HANDLE_TYPE(T) \
+  inline constexpr operator   signed T() const { return MinValue_::minSigned  <  signed T>(); } \
+  inline constexpr operator unsigned T() const { return MinValue_::minUnsigned<unsigned T>(); }
+  _kJ_HANDLE_TYPE(char)
+  _kJ_HANDLE_TYPE(short)
+  _kJ_HANDLE_TYPE(int)
+  _kJ_HANDLE_TYPE(long)
+  _kJ_HANDLE_TYPE(long long)
+#undef _kJ_HANDLE_TYPE
+
+  inline constexpr operator char() const {
+    // `char` is different from both `signed char` and `unsigned char`, and may be signed or
+    // unsigned on different platforms.  Ugh.
+    return char(-1) < 0 ? MinValue_::minSigned<char>()
+                        : MinValue_::minUnsigned<char>();
+  }
+};
+
+static KJ_CONSTEXPR(const) MaxValue_ maxValue = MaxValue_();
+// A special constant which, when cast to an integer type, takes on the maximum possible value of
+// that type.  This is useful to use as e.g. a parameter to a function because it will be robust
+// in the face of changes to the parameter's type.
+//
+// `char` is not supported, but `signed char` and `unsigned char` are.
+
+static KJ_CONSTEXPR(const) MinValue_ minValue = MinValue_();
+// A special constant which, when cast to an integer type, takes on the minimum possible value
+// of that type.  This is useful to use as e.g. a parameter to a function because it will be robust
+// in the face of changes to the parameter's type.
+//
+// `char` is not supported, but `signed char` and `unsigned char` are.
+
+#if __GNUC__
+inline constexpr float inf() { return __builtin_huge_valf(); }
+inline constexpr float nan() { return __builtin_nanf(""); }
+
+#elif _MSC_VER
+
+// Do what MSVC math.h does
+#pragma warning(push)
+#pragma warning(disable: 4756)  // "overflow in constant arithmetic"
+inline constexpr float inf() { return (float)(1e300 * 1e300); }
+#pragma warning(pop)
+
+float nan();
+// Unfortunatley, inf() * 0.0f produces a NaN with the sign bit set, whereas our preferred
+// canonical NaN should not have the sign bit set. std::numeric_limits<float>::quiet_NaN()
+// returns the correct NaN, but we don't want to #include that here. So, we give up and make
+// this out-of-line on MSVC.
+//
+// TODO(msvc): Can we do better?
+
+#else
+#error "Not sure how to support your compiler."
+#endif
+
+inline constexpr bool isNaN(float f) { return f != f; }
+inline constexpr bool isNaN(double f) { return f != f; }
+
+inline int popCount(unsigned int x) {
+#if defined(_MSC_VER)
+  return __popcnt(x);
+  // Note: __popcnt returns unsigned int, but the value is clearly guaranteed to fit into an int
+#else
+  return __builtin_popcount(x);
+#endif
+}
+
+// =======================================================================================
+// Useful fake containers
+
+template <typename T>
+class Range {
+public:
+  inline constexpr Range(const T& begin, const T& end): begin_(begin), end_(end) {}
+
+  class Iterator {
+  public:
+    Iterator() = default;
+    inline Iterator(const T& value): value(value) {}
+
+    inline const T&  operator* () const { return value; }
+    inline const T&  operator[](size_t index) const { return value + index; }
+    inline Iterator& operator++() { ++value; return *this; }
+    inline Iterator  operator++(int) { return Iterator(value++); }
+    inline Iterator& operator--() { --value; return *this; }
+    inline Iterator  operator--(int) { return Iterator(value--); }
+    inline Iterator& operator+=(ptrdiff_t amount) { value += amount; return *this; }
+    inline Iterator& operator-=(ptrdiff_t amount) { value -= amount; return *this; }
+    inline Iterator  operator+ (ptrdiff_t amount) const { return Iterator(value + amount); }
+    inline Iterator  operator- (ptrdiff_t amount) const { return Iterator(value - amount); }
+    inline ptrdiff_t operator- (const Iterator& other) const { return value - other.value; }
+
+    inline bool operator==(const Iterator& other) const { return value == other.value; }
+    inline bool operator!=(const Iterator& other) const { return value != other.value; }
+    inline bool operator<=(const Iterator& other) const { return value <= other.value; }
+    inline bool operator>=(const Iterator& other) const { return value >= other.value; }
+    inline bool operator< (const Iterator& other) const { return value <  other.value; }
+    inline bool operator> (const Iterator& other) const { return value >  other.value; }
+
+  private:
+    T value;
+  };
+
+  inline Iterator begin() const { return Iterator(begin_); }
+  inline Iterator end() const { return Iterator(end_); }
+
+  inline auto size() const -> decltype(instance<T>() - instance<T>()) { return end_ - begin_; }
+
+private:
+  T begin_;
+  T end_;
+};
+
+template <typename T>
+inline constexpr Range<Decay<T>> range(T begin, T end) { return Range<Decay<T>>(begin, end); }
+// Returns a fake iterable container containing all values of T from `begin` (inclusive) to `end`
+// (exclusive).  Example:
+//
+//     // Prints 1, 2, 3, 4, 5, 6, 7, 8, 9.
+//     for (int i: kj::range(1, 10)) { print(i); }
+
+template <typename T>
+inline constexpr Range<size_t> indices(T&& container) {
+  // Shortcut for iterating over the indices of a container:
+  //
+  //     for (size_t i: kj::indices(myArray)) { handle(myArray[i]); }
+
+  return range<size_t>(0, kj::size(container));
+}
+
+template <typename T>
+class Repeat {
+public:
+  inline constexpr Repeat(const T& value, size_t count): value(value), count(count) {}
+
+  class Iterator {
+  public:
+    Iterator() = default;
+    inline Iterator(const T& value, size_t index): value(value), index(index) {}
+
+    inline const T&  operator* () const { return value; }
+    inline const T&  operator[](ptrdiff_t index) const { return value; }
+    inline Iterator& operator++() { ++index; return *this; }
+    inline Iterator  operator++(int) { return Iterator(value, index++); }
+    inline Iterator& operator--() { --index; return *this; }
+    inline Iterator  operator--(int) { return Iterator(value, index--); }
+    inline Iterator& operator+=(ptrdiff_t amount) { index += amount; return *this; }
+    inline Iterator& operator-=(ptrdiff_t amount) { index -= amount; return *this; }
+    inline Iterator  operator+ (ptrdiff_t amount) const { return Iterator(value, index + amount); }
+    inline Iterator  operator- (ptrdiff_t amount) const { return Iterator(value, index - amount); }
+    inline ptrdiff_t operator- (const Iterator& other) const { return index - other.index; }
+
+    inline bool operator==(const Iterator& other) const { return index == other.index; }
+    inline bool operator!=(const Iterator& other) const { return index != other.index; }
+    inline bool operator<=(const Iterator& other) const { return index <= other.index; }
+    inline bool operator>=(const Iterator& other) const { return index >= other.index; }
+    inline bool operator< (const Iterator& other) const { return index <  other.index; }
+    inline bool operator> (const Iterator& other) const { return index >  other.index; }
+
+  private:
+    T value;
+    size_t index;
+  };
+
+  inline Iterator begin() const { return Iterator(value, 0); }
+  inline Iterator end() const { return Iterator(value, count); }
+
+  inline size_t size() const { return count; }
+
+private:
+  T value;
+  size_t count;
+};
+
+template <typename T>
+inline constexpr Repeat<Decay<T>> repeat(T&& value, size_t count) {
+  // Returns a fake iterable which contains `count` repeats of `value`.  Useful for e.g. creating
+  // a bunch of spaces:  `kj::repeat(' ', indent * 2)`
+
+  return Repeat<Decay<T>>(value, count);
+}
+
+// =======================================================================================
+// Manually invoking constructors and destructors
+//
+// ctor(x, ...) and dtor(x) invoke x's constructor or destructor, respectively.
+
+// We want placement new, but we don't want to #include <new>.  operator new cannot be defined in
+// a namespace, and defining it globally conflicts with the definition in <new>.  So we have to
+// define a dummy type and an operator new that uses it.
+
+namespace _ {  // private
+struct PlacementNew {};
+}  // namespace _ (private)
+} // namespace kj
+
+inline void* operator new(size_t, kj::_::PlacementNew, void* __p) noexcept {
+  return __p;
+}
+
+inline void operator delete(void*, kj::_::PlacementNew, void* __p) noexcept {}
+
+namespace kj {
+
+template <typename T, typename... Params>
+inline void ctor(T& location, Params&&... params) {
+  new (_::PlacementNew(), &location) T(kj::fwd<Params>(params)...);
+}
+
+template <typename T>
+inline void dtor(T& location) {
+  location.~T();
+}
+
+// =======================================================================================
+// Maybe
+//
+// Use in cases where you want to indicate that a value may be null.  Using Maybe<T&> instead of T*
+// forces the caller to handle the null case in order to satisfy the compiler, thus reliably
+// preventing null pointer dereferences at runtime.
+//
+// Maybe<T> can be implicitly constructed from T and from nullptr.  Additionally, it can be
+// implicitly constructed from T*, in which case the pointer is checked for nullness at runtime.
+// To read the value of a Maybe<T>, do:
+//
+//    KJ_IF_MAYBE(value, someFuncReturningMaybe()) {
+//      doSomething(*value);
+//    } else {
+//      maybeWasNull();
+//    }
+//
+// KJ_IF_MAYBE's first parameter is a variable name which will be defined within the following
+// block.  The variable will behave like a (guaranteed non-null) pointer to the Maybe's value,
+// though it may or may not actually be a pointer.
+//
+// Note that Maybe<T&> actually just wraps a pointer, whereas Maybe<T> wraps a T and a boolean
+// indicating nullness.
+
+template <typename T>
+class Maybe;
+
+namespace _ {  // private
+
+#if _MSC_VER
+  // TODO(msvc): MSVC barfs on noexcept(instance<T&>().~T()) where T = kj::Exception and
+  // kj::_::Void. It and every other factorization I've tried produces:
+  //   error C2325: 'kj::Blah' unexpected type to the right of '.~': expected 'void'
+#define MSVC_NOEXCEPT_DTOR_WORKAROUND(T) __is_nothrow_destructible(T)
+#else
+#define MSVC_NOEXCEPT_DTOR_WORKAROUND(T) noexcept(instance<T&>().~T())
+#endif
+
+template <typename T>
+class NullableValue {
+  // Class whose interface behaves much like T*, but actually contains an instance of T and a
+  // boolean flag indicating nullness.
+
+public:
+  inline NullableValue(NullableValue&& other) noexcept(noexcept(T(instance<T&&>())))
+      : isSet(other.isSet) {
+    if (isSet) {
+      ctor(value, kj::mv(other.value));
+    }
+  }
+  inline NullableValue(const NullableValue& other)
+      : isSet(other.isSet) {
+    if (isSet) {
+      ctor(value, other.value);
+    }
+  }
+  inline NullableValue(NullableValue& other)
+      : isSet(other.isSet) {
+    if (isSet) {
+      ctor(value, other.value);
+    }
+  }
+  inline ~NullableValue() noexcept(MSVC_NOEXCEPT_DTOR_WORKAROUND(T)) {
+    if (isSet) {
+      dtor(value);
+    }
+  }
+
+  inline T& operator*() & { return value; }
+  inline const T& operator*() const & { return value; }
+  inline T&& operator*() && { return kj::mv(value); }
+  inline const T&& operator*() const && { return kj::mv(value); }
+  inline T* operator->() { return &value; }
+  inline const T* operator->() const { return &value; }
+  inline operator T*() { return isSet ? &value : nullptr; }
+  inline operator const T*() const { return isSet ? &value : nullptr; }
+
+  template <typename... Params>
+  inline T& emplace(Params&&... params) {
+    if (isSet) {
+      isSet = false;
+      dtor(value);
+    }
+    ctor(value, kj::fwd<Params>(params)...);
+    isSet = true;
+    return value;
+  }
+
+private:  // internal interface used by friends only
+  inline NullableValue() noexcept: isSet(false) {}
+  inline NullableValue(T&& t) noexcept(noexcept(T(instance<T&&>())))
+      : isSet(true) {
+    ctor(value, kj::mv(t));
+  }
+  inline NullableValue(T& t)
+      : isSet(true) {
+    ctor(value, t);
+  }
+  inline NullableValue(const T& t)
+      : isSet(true) {
+    ctor(value, t);
+  }
+  inline NullableValue(const T* t)
+      : isSet(t != nullptr) {
+    if (isSet) ctor(value, *t);
+  }
+  template <typename U>
+  inline NullableValue(NullableValue<U>&& other) noexcept(noexcept(T(instance<U&&>())))
+      : isSet(other.isSet) {
+    if (isSet) {
+      ctor(value, kj::mv(other.value));
+    }
+  }
+  template <typename U>
+  inline NullableValue(const NullableValue<U>& other)
+      : isSet(other.isSet) {
+    if (isSet) {
+      ctor(value, other.value);
+    }
+  }
+  template <typename U>
+  inline NullableValue(const NullableValue<U&>& other)
+      : isSet(other.isSet) {
+    if (isSet) {
+      ctor(value, *other.ptr);
+    }
+  }
+  inline NullableValue(decltype(nullptr)): isSet(false) {}
+
+  inline NullableValue& operator=(NullableValue&& other) {
+    if (&other != this) {
+      // Careful about throwing destructors/constructors here.
+      if (isSet) {
+        isSet = false;
+        dtor(value);
+      }
+      if (other.isSet) {
+        ctor(value, kj::mv(other.value));
+        isSet = true;
+      }
+    }
+    return *this;
+  }
+
+  inline NullableValue& operator=(NullableValue& other) {
+    if (&other != this) {
+      // Careful about throwing destructors/constructors here.
+      if (isSet) {
+        isSet = false;
+        dtor(value);
+      }
+      if (other.isSet) {
+        ctor(value, other.value);
+        isSet = true;
+      }
+    }
+    return *this;
+  }
+
+  inline NullableValue& operator=(const NullableValue& other) {
+    if (&other != this) {
+      // Careful about throwing destructors/constructors here.
+      if (isSet) {
+        isSet = false;
+        dtor(value);
+      }
+      if (other.isSet) {
+        ctor(value, other.value);
+        isSet = true;
+      }
+    }
+    return *this;
+  }
+
+  inline bool operator==(decltype(nullptr)) const { return !isSet; }
+  inline bool operator!=(decltype(nullptr)) const { return isSet; }
+
+private:
+  bool isSet;
+
+#if _MSC_VER
+#pragma warning(push)
+#pragma warning(disable: 4624)
+// Warns that the anonymous union has a deleted destructor when T is non-trivial. This warning
+// seems broken.
+#endif
+
+  union {
+    T value;
+  };
+
+#if _MSC_VER
+#pragma warning(pop)
+#endif
+
+  friend class kj::Maybe<T>;
+  template <typename U>
+  friend NullableValue<U>&& readMaybe(Maybe<U>&& maybe);
+};
+
+template <typename T>
+inline NullableValue<T>&& readMaybe(Maybe<T>&& maybe) { return kj::mv(maybe.ptr); }
+template <typename T>
+inline T* readMaybe(Maybe<T>& maybe) { return maybe.ptr; }
+template <typename T>
+inline const T* readMaybe(const Maybe<T>& maybe) { return maybe.ptr; }
+template <typename T>
+inline T* readMaybe(Maybe<T&>&& maybe) { return maybe.ptr; }
+template <typename T>
+inline T* readMaybe(const Maybe<T&>& maybe) { return maybe.ptr; }
+
+template <typename T>
+inline T* readMaybe(T* ptr) { return ptr; }
+// Allow KJ_IF_MAYBE to work on regular pointers.
+
+}  // namespace _ (private)
+
+#define KJ_IF_MAYBE(name, exp) if (auto name = ::kj::_::readMaybe(exp))
+
+template <typename T>
+class Maybe {
+  // A T, or nullptr.
+
+  // IF YOU CHANGE THIS CLASS:  Note that there is a specialization of it in memory.h.
+
+public:
+  Maybe(): ptr(nullptr) {}
+  Maybe(T&& t) noexcept(noexcept(T(instance<T&&>()))): ptr(kj::mv(t)) {}
+  Maybe(T& t): ptr(t) {}
+  Maybe(const T& t): ptr(t) {}
+  Maybe(const T* t) noexcept: ptr(t) {}
+  Maybe(Maybe&& other) noexcept(noexcept(T(instance<T&&>()))): ptr(kj::mv(other.ptr)) {}
+  Maybe(const Maybe& other): ptr(other.ptr) {}
+  Maybe(Maybe& other): ptr(other.ptr) {}
+
+  template <typename U>
+  Maybe(Maybe<U>&& other) noexcept(noexcept(T(instance<U&&>()))) {
+    KJ_IF_MAYBE(val, kj::mv(other)) {
+      ptr = *val;
+    }
+  }
+  template <typename U>
+  Maybe(const Maybe<U>& other) {
+    KJ_IF_MAYBE(val, other) {
+      ptr = *val;
+    }
+  }
+
+  Maybe(decltype(nullptr)) noexcept: ptr(nullptr) {}
+
+  template <typename... Params>
+  inline T& emplace(Params&&... params) {
+    // Replace this Maybe's content with a new value constructed by passing the given parametrs to
+    // T's constructor. This can be used to initialize a Maybe without copying or even moving a T.
+    // Returns a reference to the newly-constructed value.
+
+    return ptr.emplace(kj::fwd<Params>(params)...);
+  }
+
+  inline Maybe& operator=(Maybe&& other) { ptr = kj::mv(other.ptr); return *this; }
+  inline Maybe& operator=(Maybe& other) { ptr = other.ptr; return *this; }
+  inline Maybe& operator=(const Maybe& other) { ptr = other.ptr; return *this; }
+
+  inline bool operator==(decltype(nullptr)) const { return ptr == nullptr; }
+  inline bool operator!=(decltype(nullptr)) const { return ptr != nullptr; }
+
+  T& orDefault(T& defaultValue) {
+    if (ptr == nullptr) {
+      return defaultValue;
+    } else {
+      return *ptr;
+    }
+  }
+  const T& orDefault(const T& defaultValue) const {
+    if (ptr == nullptr) {
+      return defaultValue;
+    } else {
+      return *ptr;
+    }
+  }
+
+  template <typename Func>
+  auto map(Func&& f) & -> Maybe<decltype(f(instance<T&>()))> {
+    if (ptr == nullptr) {
+      return nullptr;
+    } else {
+      return f(*ptr);
+    }
+  }
+
+  template <typename Func>
+  auto map(Func&& f) const & -> Maybe<decltype(f(instance<const T&>()))> {
+    if (ptr == nullptr) {
+      return nullptr;
+    } else {
+      return f(*ptr);
+    }
+  }
+
+  template <typename Func>
+  auto map(Func&& f) && -> Maybe<decltype(f(instance<T&&>()))> {
+    if (ptr == nullptr) {
+      return nullptr;
+    } else {
+      return f(kj::mv(*ptr));
+    }
+  }
+
+  template <typename Func>
+  auto map(Func&& f) const && -> Maybe<decltype(f(instance<const T&&>()))> {
+    if (ptr == nullptr) {
+      return nullptr;
+    } else {
+      return f(kj::mv(*ptr));
+    }
+  }
+
+private:
+  _::NullableValue<T> ptr;
+
+  template <typename U>
+  friend class Maybe;
+  template <typename U>
+  friend _::NullableValue<U>&& _::readMaybe(Maybe<U>&& maybe);
+  template <typename U>
+  friend U* _::readMaybe(Maybe<U>& maybe);
+  template <typename U>
+  friend const U* _::readMaybe(const Maybe<U>& maybe);
+};
+
+template <typename T>
+class Maybe<T&>: public DisallowConstCopyIfNotConst<T> {
+public:
+  Maybe() noexcept: ptr(nullptr) {}
+  Maybe(T& t) noexcept: ptr(&t) {}
+  Maybe(T* t) noexcept: ptr(t) {}
+
+  template <typename U>
+  inline Maybe(Maybe<U&>& other) noexcept: ptr(other.ptr) {}
+  template <typename U>
+  inline Maybe(const Maybe<const U&>& other) noexcept: ptr(other.ptr) {}
+  inline Maybe(decltype(nullptr)) noexcept: ptr(nullptr) {}
+
+  inline Maybe& operator=(T& other) noexcept { ptr = &other; return *this; }
+  inline Maybe& operator=(T* other) noexcept { ptr = other; return *this; }
+  template <typename U>
+  inline Maybe& operator=(Maybe<U&>& other) noexcept { ptr = other.ptr; return *this; }
+  template <typename U>
+  inline Maybe& operator=(const Maybe<const U&>& other) noexcept { ptr = other.ptr; return *this; }
+
+  inline bool operator==(decltype(nullptr)) const { return ptr == nullptr; }
+  inline bool operator!=(decltype(nullptr)) const { return ptr != nullptr; }
+
+  T& orDefault(T& defaultValue) {
+    if (ptr == nullptr) {
+      return defaultValue;
+    } else {
+      return *ptr;
+    }
+  }
+  const T& orDefault(const T& defaultValue) const {
+    if (ptr == nullptr) {
+      return defaultValue;
+    } else {
+      return *ptr;
+    }
+  }
+
+  template <typename Func>
+  auto map(Func&& f) -> Maybe<decltype(f(instance<T&>()))> {
+    if (ptr == nullptr) {
+      return nullptr;
+    } else {
+      return f(*ptr);
+    }
+  }
+
+private:
+  T* ptr;
+
+  template <typename U>
+  friend class Maybe;
+  template <typename U>
+  friend U* _::readMaybe(Maybe<U&>&& maybe);
+  template <typename U>
+  friend U* _::readMaybe(const Maybe<U&>& maybe);
+};
+
+// =======================================================================================
+// ArrayPtr
+//
+// So common that we put it in common.h rather than array.h.
+
+template <typename T>
+class ArrayPtr: public DisallowConstCopyIfNotConst<T> {
+  // A pointer to an array.  Includes a size.  Like any pointer, it doesn't own the target data,
+  // and passing by value only copies the pointer, not the target.
+
+public:
+  inline constexpr ArrayPtr(): ptr(nullptr), size_(0) {}
+  inline constexpr ArrayPtr(decltype(nullptr)): ptr(nullptr), size_(0) {}
+  inline constexpr ArrayPtr(T* ptr, size_t size): ptr(ptr), size_(size) {}
+  inline constexpr ArrayPtr(T* begin, T* end): ptr(begin), size_(end - begin) {}
+  inline KJ_CONSTEXPR() ArrayPtr(::std::initializer_list<RemoveConstOrDisable<T>> init)
+      : ptr(init.begin()), size_(init.size()) {}
+
+  template <size_t size>
+  inline constexpr ArrayPtr(T (&native)[size]): ptr(native), size_(size) {}
+  // Construct an ArrayPtr from a native C-style array.
+
+  inline operator ArrayPtr<const T>() const {
+    return ArrayPtr<const T>(ptr, size_);
+  }
+  inline ArrayPtr<const T> asConst() const {
+    return ArrayPtr<const T>(ptr, size_);
+  }
+
+  inline size_t size() const { return size_; }
+  inline const T& operator[](size_t index) const {
+    KJ_IREQUIRE(index < size_, "Out-of-bounds ArrayPtr access.");
+    return ptr[index];
+  }
+  inline T& operator[](size_t index) {
+    KJ_IREQUIRE(index < size_, "Out-of-bounds ArrayPtr access.");
+    return ptr[index];
+  }
+
+  inline T* begin() { return ptr; }
+  inline T* end() { return ptr + size_; }
+  inline T& front() { return *ptr; }
+  inline T& back() { return *(ptr + size_ - 1); }
+  inline const T* begin() const { return ptr; }
+  inline const T* end() const { return ptr + size_; }
+  inline const T& front() const { return *ptr; }
+  inline const T& back() const { return *(ptr + size_ - 1); }
+
+  inline ArrayPtr<const T> slice(size_t start, size_t end) const {
+    KJ_IREQUIRE(start <= end && end <= size_, "Out-of-bounds ArrayPtr::slice().");
+    return ArrayPtr<const T>(ptr + start, end - start);
+  }
+  inline ArrayPtr slice(size_t start, size_t end) {
+    KJ_IREQUIRE(start <= end && end <= size_, "Out-of-bounds ArrayPtr::slice().");
+    return ArrayPtr(ptr + start, end - start);
+  }
+
+  inline ArrayPtr<PropagateConst<T, byte>> asBytes() const {
+    // Reinterpret the array as a byte array. This is explicitly legal under C++ aliasing
+    // rules.
+    return { reinterpret_cast<PropagateConst<T, byte>*>(ptr), size_ * sizeof(T) };
+  }
+  inline ArrayPtr<PropagateConst<T, char>> asChars() const {
+    // Reinterpret the array as a char array. This is explicitly legal under C++ aliasing
+    // rules.
+    return { reinterpret_cast<PropagateConst<T, char>*>(ptr), size_ * sizeof(T) };
+  }
+
+  inline bool operator==(decltype(nullptr)) const { return size_ == 0; }
+  inline bool operator!=(decltype(nullptr)) const { return size_ != 0; }
+
+  inline bool operator==(const ArrayPtr& other) const {
+    if (size_ != other.size_) return false;
+    for (size_t i = 0; i < size_; i++) {
+      if (ptr[i] != other[i]) return false;
+    }
+    return true;
+  }
+  inline bool operator!=(const ArrayPtr& other) const { return !(*this == other); }
+
+private:
+  T* ptr;
+  size_t size_;
+};
+
+template <typename T>
+inline constexpr ArrayPtr<T> arrayPtr(T* ptr, size_t size) {
+  // Use this function to construct ArrayPtrs without writing out the type name.
+  return ArrayPtr<T>(ptr, size);
+}
+
+template <typename T>
+inline constexpr ArrayPtr<T> arrayPtr(T* begin, T* end) {
+  // Use this function to construct ArrayPtrs without writing out the type name.
+  return ArrayPtr<T>(begin, end);
+}
+
+// =======================================================================================
+// Casts
+
+template <typename To, typename From>
+To implicitCast(From&& from) {
+  // `implicitCast<T>(value)` casts `value` to type `T` only if the conversion is implicit.  Useful
+  // for e.g. resolving ambiguous overloads without sacrificing type-safety.
+  return kj::fwd<From>(from);
+}
+
+template <typename To, typename From>
+Maybe<To&> dynamicDowncastIfAvailable(From& from) {
+  // If RTTI is disabled, always returns nullptr.  Otherwise, works like dynamic_cast.  Useful
+  // in situations where dynamic_cast could allow an optimization, but isn't strictly necessary
+  // for correctness.  It is highly recommended that you try to arrange all your dynamic_casts
+  // this way, as a dynamic_cast that is necessary for correctness implies a flaw in the interface
+  // design.
+
+  // Force a compile error if To is not a subtype of From.  Cross-casting is rare; if it is needed
+  // we should have a separate cast function like dynamicCrosscastIfAvailable().
+  if (false) {
+    kj::implicitCast<From*>(kj::implicitCast<To*>(nullptr));
+  }
+
+#if KJ_NO_RTTI
+  return nullptr;
+#else
+  return dynamic_cast<To*>(&from);
+#endif
+}
+
+template <typename To, typename From>
+To& downcast(From& from) {
+  // Down-cast a value to a sub-type, asserting that the cast is valid.  In opt mode this is a
+  // static_cast, but in debug mode (when RTTI is enabled) a dynamic_cast will be used to verify
+  // that the value really has the requested type.
+
+  // Force a compile error if To is not a subtype of From.
+  if (false) {
+    kj::implicitCast<From*>(kj::implicitCast<To*>(nullptr));
+  }
+
+#if !KJ_NO_RTTI
+  KJ_IREQUIRE(dynamic_cast<To*>(&from) != nullptr, "Value cannot be downcast() to requested type.");
+#endif
+
+  return static_cast<To&>(from);
+}
+
+// =======================================================================================
+// Defer
+
+namespace _ {  // private
+
+template <typename Func>
+class Deferred {
+public:
+  inline Deferred(Func&& func): func(kj::fwd<Func>(func)), canceled(false) {}
+  inline ~Deferred() noexcept(false) { if (!canceled) func(); }
+  KJ_DISALLOW_COPY(Deferred);
+
+  // This move constructor is usually optimized away by the compiler.
+  inline Deferred(Deferred&& other): func(kj::mv(other.func)), canceled(false) {
+    other.canceled = true;
+  }
+private:
+  Func func;
+  bool canceled;
+};
+
+}  // namespace _ (private)
+
+template <typename Func>
+_::Deferred<Func> defer(Func&& func) {
+  // Returns an object which will invoke the given functor in its destructor.  The object is not
+  // copyable but is movable with the semantics you'd expect.  Since the return type is private,
+  // you need to assign to an `auto` variable.
+  //
+  // The KJ_DEFER macro provides slightly more convenient syntax for the common case where you
+  // want some code to run at current scope exit.
+
+  return _::Deferred<Func>(kj::fwd<Func>(func));
+}
+
+#define KJ_DEFER(code) auto KJ_UNIQUE_NAME(_kjDefer) = ::kj::defer([&](){code;})
+// Run the given code when the function exits, whether by return or exception.
+
+}  // namespace kj
+
+#endif  // KJ_COMMON_H_