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   T* mPtr;
  46
  47 #ifdef DEBUG
  48   T* const mRangeStart;
  49   T* const mRangeEnd;
  50 #endif
  51
  52   void checkSanity() {
  53     MOZ_ASSERT(mRangeStart <= mPtr);
  54     MOZ_ASSERT(mPtr <= mRangeEnd);
  55   }
  56
  57   /* Creates a new pointer for |aPtr|, restricted to this pointer's range. */
  58   RangedPtr<T> create(T* aPtr) const {
  59 #ifdef DEBUG
  60     return RangedPtr<T>(aPtr, mRangeStart, mRangeEnd);
  61 #else
  62     return RangedPtr<T>(aPtr, nullptr, size_t(0));
  63 #endif
  64   }
  65
  66   uintptr_t asUintptr() const { return reinterpret_cast<uintptr_t>(mPtr); }
  67
  68  public:
  69   RangedPtr(T* aPtr, T* aStart, T* aEnd)
  70       : mPtr(aPtr)
  71 #ifdef DEBUG
  72         ,
  73         mRangeStart(aStart),
  74         mRangeEnd(aEnd)
  75 #endif
  76   {
  77     MOZ_ASSERT(mRangeStart <= mRangeEnd);
  78     checkSanity();
  79   }
  80   RangedPtr(T* aPtr, T* aStart, size_t aLength)
  81       : mPtr(aPtr)
  82 #ifdef DEBUG
  83         ,
  84         mRangeStart(aStart),
  85         mRangeEnd(aStart + aLength)
  86 #endif
  87   {
  88     MOZ_ASSERT(aLength <= size_t(-1) / sizeof(T));
  89     MOZ_ASSERT(reinterpret_cast<uintptr_t>(mRangeStart) + aLength * sizeof(T) >=
  90                reinterpret_cast<uintptr_t>(mRangeStart));
  91     checkSanity();
  92   }
  93
  94   /* Equivalent to RangedPtr(aPtr, aPtr, aLength). */
  95   RangedPtr(T* aPtr, size_t aLength)
  96       : mPtr(aPtr)
  97 #ifdef DEBUG
  98         ,
  99         mRangeStart(aPtr),
 100         mRangeEnd(aPtr + aLength)
 101 #endif
 102   {
 103     MOZ_ASSERT(aLength <= size_t(-1) / sizeof(T));
 104     MOZ_ASSERT(reinterpret_cast<uintptr_t>(mRangeStart) + aLength * sizeof(T) >=
 105                reinterpret_cast<uintptr_t>(mRangeStart));
 106     checkSanity();
 107   }
 108
 109   /* Equivalent to RangedPtr(aArr, aArr, N). */
 110   template <size_t N>
 111   explicit RangedPtr(T (&aArr)[N])
 112       : mPtr(aArr)
 113 #ifdef DEBUG
 114         ,
 115         mRangeStart(aArr),
 116         mRangeEnd(aArr + N)
 117 #endif
 118   {
 119     checkSanity();
 120   }
 121
 122   T* get() const { return mPtr; }
 123
 124   explicit operator bool() const { return mPtr != nullptr; }
 125
 126   void checkIdenticalRange(const RangedPtr<T>& aOther) const {
 127     MOZ_ASSERT(mRangeStart == aOther.mRangeStart);
 128     MOZ_ASSERT(mRangeEnd == aOther.mRangeEnd);
 129   }
 130
 131   template <typename U>
 132   RangedPtr<U> ReinterpretCast() const {
 133 #ifdef DEBUG
 134     return {reinterpret_cast<U*>(mPtr), reinterpret_cast<U*>(mRangeStart),
 135             reinterpret_cast<U*>(mRangeEnd)};
 136 #else
 137     return {reinterpret_cast<U*>(mPtr), nullptr, nullptr};
 138 #endif
 139   }
 140
 141   /*
 142    * You can only assign one RangedPtr into another if the two pointers have
 143    * the same valid range:
 144    *
 145    *   char arr1[] = "hi";
 146    *   char arr2[] = "bye";
 147    *   RangedPtr<char> p1(arr1, 2);
 148    *   p1 = RangedPtr<char>(arr1 + 1, arr1, arr1 + 2); // works
 149    *   p1 = RangedPtr<char>(arr2, 3);                  // asserts
 150    */
 151   RangedPtr<T>& operator=(const RangedPtr<T>& aOther) {
 152     checkIdenticalRange(aOther);
 153     mPtr = aOther.mPtr;
 154     checkSanity();
 155     return *this;
 156   }
 157
 158   RangedPtr<T> operator+(size_t aInc) const {
 159     MOZ_ASSERT(aInc <= size_t(-1) / sizeof(T));
 160     MOZ_ASSERT(asUintptr() + aInc * sizeof(T) >= asUintptr());
 161     return create(mPtr + aInc);
 162   }
 163
 164   RangedPtr<T> operator-(size_t aDec) const {
 165     MOZ_ASSERT(aDec <= size_t(-1) / sizeof(T));
 166     MOZ_ASSERT(asUintptr() - aDec * sizeof(T) <= asUintptr());
 167     return create(mPtr - aDec);
 168   }
 169
 170   /*
 171    * You can assign a raw pointer into a RangedPtr if the raw pointer is
 172    * within the range specified at creation.
 173    */
 174   template <typename U>
 175   RangedPtr<T>& operator=(U* aPtr) {
 176     *this = create(aPtr);
 177     return *this;
 178   }
 179
 180   template <typename U>
 181   RangedPtr<T>& operator=(const RangedPtr<U>& aPtr) {
 182     MOZ_ASSERT(mRangeStart <= aPtr.mPtr);
 183     MOZ_ASSERT(aPtr.mPtr <= mRangeEnd);
 184     mPtr = aPtr.mPtr;
 185     checkSanity();
 186     return *this;
 187   }
 188
 189   RangedPtr<T>& operator++() { return (*this += 1); }
 190
 191   RangedPtr<T> operator++(int) {
 192     RangedPtr<T> rcp = *this;
 193     ++*this;
 194     return rcp;
 195   }
 196
 197   RangedPtr<T>& operator--() { return (*this -= 1); }
 198
 199   RangedPtr<T> operator--(int) {
 200     RangedPtr<T> rcp = *this;
 201     --*this;
 202     return rcp;
 203   }
 204
 205   RangedPtr<T>& operator+=(size_t aInc) {
 206     *this = *this + aInc;
 207     return *this;
 208   }
 209
 210   RangedPtr<T>& operator-=(size_t aDec) {
 211     *this = *this - aDec;
 212     return *this;
 213   }
 214
 215   T& operator[](ptrdiff_t aIndex) const {
 216     MOZ_ASSERT(size_t(aIndex > 0 ? aIndex : -aIndex) <= size_t(-1) / sizeof(T));
 217     return *create(mPtr + aIndex);
 218   }
 219
 220   T& operator*() const {
 221     MOZ_ASSERT(mPtr >= mRangeStart);
 222     MOZ_ASSERT(mPtr < mRangeEnd);
 223     return *mPtr;
 224   }
 225
 226   T* operator->() const {
 227     MOZ_ASSERT(mPtr >= mRangeStart);
 228     MOZ_ASSERT(mPtr < mRangeEnd);
 229     return mPtr;
 230   }
 231
 232   template <typename U>
 233   bool operator==(const RangedPtr<U>& aOther) const {
 234     return mPtr == aOther.mPtr;
 235   }
 236   template <typename U>
 237   bool operator!=(const RangedPtr<U>& aOther) const {
 238     return !(*this == aOther);
 239   }
 240
 241   template <typename U>
 242   bool operator==(const U* u) const {
 243     return mPtr == u;
 244   }
 245   template <typename U>
 246   bool operator!=(const U* u) const {
 247     return !(*this == u);
 248   }
 249
 250   bool operator==(std::nullptr_t) const { return mPtr == nullptr; }
 251   bool operator!=(std::nullptr_t) const { return mPtr != nullptr; }
 252
 253   template <typename U>
 254   bool operator<(const RangedPtr<U>& aOther) const {
 255     return mPtr < aOther.mPtr;
 256   }
 257   template <typename U>
 258   bool operator<=(const RangedPtr<U>& aOther) const {
 259     return mPtr <= aOther.mPtr;
 260   }
 261
 262   template <typename U>
 263   bool operator>(const RangedPtr<U>& aOther) const {
 264     return mPtr > aOther.mPtr;
 265   }
 266   template <typename U>
 267   bool operator>=(const RangedPtr<U>& aOther) const {
 268     return mPtr >= aOther.mPtr;
 269   }
 270
 271   size_t operator-(const RangedPtr<T>& aOther) const {
 272     MOZ_ASSERT(mPtr >= aOther.mPtr);
 273     return PointerRangeSize(aOther.mPtr, mPtr);
 274   }
 275
 276  private:
 277   RangedPtr() = delete;
 278 };
 279
 280 } /* namespace mozilla */
 281
 282 #endif /* mozilla_RangedPtr_h */