mfbt/RangedPtr.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 /*
   8  * Implements a smart pointer asserted to remain within a range specified at
   9  * construction.
  10  */
  11
  12 #ifndef mozilla_RangedPtr_h
  13 #define mozilla_RangedPtr_h
  14
  15 #include "mozilla/ArrayUtils.h"
  16 #include "mozilla/Assertions.h"
  17 #include "mozilla/Attributes.h"
  18
  19 #include <stdint.h>
  20 #include <cstddef>
  21
  22 namespace mozilla {
  23
  24 /*
  25  * RangedPtr is a smart pointer restricted to an address range specified at
  26  * creation.  The pointer (and any smart pointers derived from it) must remain
  27  * within the range [start, end] (inclusive of end to facilitate use as
  28  * sentinels).  Dereferencing or indexing into the pointer (or pointers derived
  29  * from it) must remain within the range [start, end).  All the standard pointer
  30  * operators are defined on it; in debug builds these operations assert that the
  31  * range specified at construction is respected.
  32  *
  33  * In theory passing a smart pointer instance as an argument can be slightly
  34  * slower than passing a T* (due to ABI requirements for passing structs versus
  35  * passing pointers), if the method being called isn't inlined.  If you are in
  36  * extremely performance-critical code, you may want to be careful using this
  37  * smart pointer as an argument type.
  38  *
  39  * RangedPtr<T> intentionally does not implicitly convert to T*.  Use get() to
  40  * explicitly convert to T*.  Keep in mind that the raw pointer of course won't
  41  * implement bounds checking in debug builds.
  42  */
  43 template <typename T>
  44 class RangedPtr {
  45   template <typename U>
  46   friend class RangedPtr;
  47
  48   T* mPtr;
  49
  50 #ifdef DEBUG
  51   T* const mRangeStart;
  52   T* const mRangeEnd;
  53 #endif
  54
  55   void checkSanity() {
  56     MOZ_ASSERT(mRangeStart <= mPtr);
  57     MOZ_ASSERT(mPtr <= mRangeEnd);
  58   }
  59
  60   /* Creates a new pointer for |aPtr|, restricted to this pointer's range. */
  61   RangedPtr<T> create(T* aPtr) const {
  62 #ifdef DEBUG
  63     return RangedPtr<T>(aPtr, mRangeStart, mRangeEnd);
  64 #else
  65     return RangedPtr<T>(aPtr, nullptr, size_t(0));
  66 #endif
  67   }
  68
  69   uintptr_t asUintptr() const { return reinterpret_cast<uintptr_t>(mPtr); }
  70
  71  public:
  72   RangedPtr(T* aPtr, T* aStart, T* aEnd)
  73       : mPtr(aPtr)
  74 #ifdef DEBUG
  75         ,
  76         mRangeStart(aStart),
  77         mRangeEnd(aEnd)
  78 #endif
  79   {
  80     MOZ_ASSERT(mRangeStart <= mRangeEnd);
  81     checkSanity();
  82   }
  83   RangedPtr(T* aPtr, T* aStart, size_t aLength)
  84       : mPtr(aPtr)
  85 #ifdef DEBUG
  86         ,
  87         mRangeStart(aStart),
  88         mRangeEnd(aStart + aLength)
  89 #endif
  90   {
  91     MOZ_ASSERT(aLength <= size_t(-1) / sizeof(T));
  92     MOZ_ASSERT(reinterpret_cast<uintptr_t>(mRangeStart) + aLength * sizeof(T) >=
  93                reinterpret_cast<uintptr_t>(mRangeStart));
  94     checkSanity();
  95   }
  96
  97   /* Equivalent to RangedPtr(aPtr, aPtr, aLength). */
  98   RangedPtr(T* aPtr, size_t aLength)
  99       : mPtr(aPtr)
 100 #ifdef DEBUG
 101         ,
 102         mRangeStart(aPtr),
 103         mRangeEnd(aPtr + aLength)
 104 #endif
 105   {
 106     MOZ_ASSERT(aLength <= size_t(-1) / sizeof(T));
 107     MOZ_ASSERT(reinterpret_cast<uintptr_t>(mRangeStart) + aLength * sizeof(T) >=
 108                reinterpret_cast<uintptr_t>(mRangeStart));
 109     checkSanity();
 110   }
 111
 112   /* Equivalent to RangedPtr(aArr, aArr, N). */
 113   template <size_t N>
 114   explicit RangedPtr(T (&aArr)[N])
 115       : mPtr(aArr)
 116 #ifdef DEBUG
 117         ,
 118         mRangeStart(aArr),
 119         mRangeEnd(aArr + N)
 120 #endif
 121   {
 122     checkSanity();
 123   }
 124
 125   RangedPtr(const RangedPtr& aOther)
 126       : mPtr(aOther.mPtr)
 127 #ifdef DEBUG
 128         ,
 129         mRangeStart(aOther.mRangeStart),
 130         mRangeEnd(aOther.mRangeEnd)
 131 #endif
 132   {
 133     checkSanity();
 134   }
 135
 136   template <typename U>
 137   MOZ_IMPLICIT RangedPtr(const RangedPtr<U>& aOther)
 138       : mPtr(aOther.mPtr)
 139 #ifdef DEBUG
 140         ,
 141         mRangeStart(aOther.mRangeStart),
 142         mRangeEnd(aOther.mRangeEnd)
 143 #endif
 144   {
 145     checkSanity();
 146   }
 147
 148   T* get() const { return mPtr; }
 149
 150   explicit operator bool() const { return mPtr != nullptr; }
 151
 152   void checkIdenticalRange(const RangedPtr<T>& aOther) const {
 153     MOZ_ASSERT(mRangeStart == aOther.mRangeStart);
 154     MOZ_ASSERT(mRangeEnd == aOther.mRangeEnd);
 155   }
 156
 157   template <typename U>
 158   RangedPtr<U> ReinterpretCast() const {
 159 #ifdef DEBUG
 160     return {reinterpret_cast<U*>(mPtr), reinterpret_cast<U*>(mRangeStart),
 161             reinterpret_cast<U*>(mRangeEnd)};
 162 #else
 163     return {reinterpret_cast<U*>(mPtr), nullptr, nullptr};
 164 #endif
 165   }
 166
 167   /*
 168    * You can only assign one RangedPtr into another if the two pointers have
 169    * the same valid range:
 170    *
 171    *   char arr1[] = "hi";
 172    *   char arr2[] = "bye";
 173    *   RangedPtr<char> p1(arr1, 2);
 174    *   p1 = RangedPtr<char>(arr1 + 1, arr1, arr1 + 2); // works
 175    *   p1 = RangedPtr<char>(arr2, 3);                  // asserts
 176    */
 177   RangedPtr<T>& operator=(const RangedPtr<T>& aOther) {
 178     checkIdenticalRange(aOther);
 179     mPtr = aOther.mPtr;
 180     checkSanity();
 181     return *this;
 182   }
 183
 184   RangedPtr<T> operator+(size_t aInc) const {
 185     MOZ_ASSERT(aInc <= size_t(-1) / sizeof(T));
 186     MOZ_ASSERT(asUintptr() + aInc * sizeof(T) >= asUintptr());
 187     return create(mPtr + aInc);
 188   }
 189
 190   RangedPtr<T> operator-(size_t aDec) const {
 191     MOZ_ASSERT(aDec <= size_t(-1) / sizeof(T));
 192     MOZ_ASSERT(asUintptr() - aDec * sizeof(T) <= asUintptr());
 193     return create(mPtr - aDec);
 194   }
 195
 196   /*
 197    * You can assign a raw pointer into a RangedPtr if the raw pointer is
 198    * within the range specified at creation.
 199    */
 200   template <typename U>
 201   RangedPtr<T>& operator=(U* aPtr) {
 202     *this = create(aPtr);
 203     return *this;
 204   }
 205
 206   template <typename U>
 207   RangedPtr<T>& operator=(const RangedPtr<U>& aPtr) {
 208     MOZ_ASSERT(mRangeStart <= aPtr.mPtr);
 209     MOZ_ASSERT(aPtr.mPtr <= mRangeEnd);
 210     mPtr = aPtr.mPtr;
 211     checkSanity();
 212     return *this;
 213   }
 214
 215   RangedPtr<T>& operator++() { return (*this += 1); }
 216
 217   RangedPtr<T> operator++(int) {
 218     RangedPtr<T> rcp = *this;
 219     ++*this;
 220     return rcp;
 221   }
 222
 223   RangedPtr<T>& operator--() { return (*this -= 1); }
 224
 225   RangedPtr<T> operator--(int) {
 226     RangedPtr<T> rcp = *this;
 227     --*this;
 228     return rcp;
 229   }
 230
 231   RangedPtr<T>& operator+=(size_t aInc) {
 232     *this = *this + aInc;
 233     return *this;
 234   }
 235
 236   RangedPtr<T>& operator-=(size_t aDec) {
 237     *this = *this - aDec;
 238     return *this;
 239   }
 240
 241   T& operator[](ptrdiff_t aIndex) const {
 242     MOZ_ASSERT(size_t(aIndex > 0 ? aIndex : -aIndex) <= size_t(-1) / sizeof(T));
 243     return *create(mPtr + aIndex);
 244   }
 245
 246   T& operator*() const {
 247     MOZ_ASSERT(mPtr >= mRangeStart);
 248     MOZ_ASSERT(mPtr < mRangeEnd);
 249     return *mPtr;
 250   }
 251
 252   T* operator->() const {
 253     MOZ_ASSERT(mPtr >= mRangeStart);
 254     MOZ_ASSERT(mPtr < mRangeEnd);
 255     return mPtr;
 256   }
 257
 258   template <typename U>
 259   bool operator==(const RangedPtr<U>& aOther) const {
 260     return mPtr == aOther.mPtr;
 261   }
 262   template <typename U>
 263   bool operator!=(const RangedPtr<U>& aOther) const {
 264     return !(*this == aOther);
 265   }
 266
 267   template <typename U>
 268   bool operator==(const U* u) const {
 269     return mPtr == u;
 270   }
 271   template <typename U>
 272   bool operator!=(const U* u) const {
 273     return !(*this == u);
 274   }
 275
 276   bool operator==(std::nullptr_t) const { return mPtr == nullptr; }
 277   bool operator!=(std::nullptr_t) const { return mPtr != nullptr; }
 278
 279   template <typename U>
 280   bool operator<(const RangedPtr<U>& aOther) const {
 281     return mPtr < aOther.mPtr;
 282   }
 283   template <typename U>
 284   bool operator<=(const RangedPtr<U>& aOther) const {
 285     return mPtr <= aOther.mPtr;
 286   }
 287
 288   template <typename U>
 289   bool operator>(const RangedPtr<U>& aOther) const {
 290     return mPtr > aOther.mPtr;
 291   }
 292   template <typename U>
 293   bool operator>=(const RangedPtr<U>& aOther) const {
 294     return mPtr >= aOther.mPtr;
 295   }
 296
 297   size_t operator-(const RangedPtr<T>& aOther) const {
 298     MOZ_ASSERT(mPtr >= aOther.mPtr);
 299     return PointerRangeSize(aOther.mPtr, mPtr);
 300   }
 301
 302  private:
 303   RangedPtr() = delete;
 304 };
 305
 306 } /* namespace mozilla */
 307
 308 #endif /* mozilla_RangedPtr_h */