mfbt/DebugOnly.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  * Provides DebugOnly, a type for variables used only in debug builds (i.e. by
   9  * assertions).
  10  */
  11
  12 #ifndef mozilla_DebugOnly_h
  13 #define mozilla_DebugOnly_h
  14
  15 #include "mozilla/Attributes.h"
  16
  17 namespace mozilla {
  18
  19 /**
  20  * DebugOnly contains a value of type T, but only in debug builds.  In release
  21  * builds, it does not contain a value.  This helper is intended to be used with
  22  * MOZ_ASSERT()-style macros, allowing one to write:
  23  *
  24  *   DebugOnly<bool> check = func();
  25  *   MOZ_ASSERT(check);
  26  *
  27  * more concisely than declaring |check| conditional on #ifdef DEBUG.
  28  *
  29  * DebugOnly instances can only be coerced to T in debug builds.  In release
  30  * builds they don't have a value, so type coercion is not well defined.
  31  *
  32  * NOTE: DebugOnly instances still take up one byte of space, plus padding, even
  33  * in optimized, non-DEBUG builds (see bug 1253094 comment 37 for more info).
  34  * For this reason the class is MOZ_STACK_CLASS to prevent consumers using
  35  * DebugOnly for struct/class members and unwittingly inflating the size of
  36  * their objects in release builds.
  37  */
  38 template <typename T>
  39 class MOZ_STACK_CLASS DebugOnly {
  40  public:
  41 #ifdef DEBUG
  42   T value;
  43
  44   DebugOnly() = default;
  45   MOZ_IMPLICIT DebugOnly(const T& aOther) : value(aOther) {}
  46   DebugOnly(const DebugOnly& aOther) : value(aOther.value) {}
  47   DebugOnly& operator=(const T& aRhs) {
  48     value = aRhs;
  49     return *this;
  50   }
  51
  52   void operator++(int) { value++; }
  53   void operator--(int) { value--; }
  54
  55   // Do not define operator+=(), etc. here.  These will coerce via the
  56   // implicit cast and built-in operators.  Defining explicit methods here
  57   // will create ambiguity the compiler can't deal with.
  58
  59   T* operator&() { return &value; }
  60
  61   operator T&() { return value; }
  62   operator const T&() const { return value; }
  63
  64   T& operator->() { return value; }
  65   const T& operator->() const { return value; }
  66
  67 #else
  68   DebugOnly() = default;
  69   MOZ_IMPLICIT DebugOnly(const T&) {}
  70   DebugOnly(const DebugOnly&) {}
  71   DebugOnly& operator=(const T&) { return *this; }
  72   void operator++(int) {}
  73   void operator--(int) {}
  74   DebugOnly& operator+=(const T&) { return *this; }
  75   DebugOnly& operator-=(const T&) { return *this; }
  76   DebugOnly& operator&=(const T&) { return *this; }
  77   DebugOnly& operator|=(const T&) { return *this; }
  78   DebugOnly& operator^=(const T&) { return *this; }
  79 #endif
  80
  81   /*
  82    * DebugOnly must always have a user-defined destructor or else it will
  83    * generate "unused variable" warnings, exactly what it's intended
  84    * to avoid!
  85    */
  86   ~DebugOnly() {}
  87 };
  88
  89 }  // namespace mozilla
  90
  91 #endif /* mozilla_DebugOnly_h */