mfbt/DebugOnly.h

   1 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* This Source Code Form is subject to the terms of the Mozilla Public
   3  * License, v. 2.0. If a copy of the MPL was not distributed with this
   4  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   5
   6 /*
   7  * Provides DebugOnly, a type for variables used only in debug builds (i.e. by
   8  * assertions).
   9  */
  10
  11 #ifndef mozilla_DebugOnly_h_
  12 #define mozilla_DebugOnly_h_
  13
  14 namespace mozilla {
  15
  16 /**
  17  * DebugOnly contains a value of type T, but only in debug builds.  In release
  18  * builds, it does not contain a value.  This helper is intended to be used with
  19  * MOZ_ASSERT()-style macros, allowing one to write:
  20  *
  21  *   DebugOnly<bool> check = func();
  22  *   MOZ_ASSERT(check);
  23  *
  24  * more concisely than declaring |check| conditional on #ifdef DEBUG, but also
  25  * without allocating storage space for |check| in release builds.
  26  *
  27  * DebugOnly instances can only be coerced to T in debug builds.  In release
  28  * builds they don't have a value, so type coercion is not well defined.
  29  */
  30 template<typename T>
  31 class DebugOnly
  32 {
  33   public:
  34 #ifdef DEBUG
  35     T value;
  36
  37     DebugOnly() { }
  38     DebugOnly(const T& other) : value(other) { }
  39     DebugOnly(const DebugOnly& other) : value(other.value) { }
  40     DebugOnly& operator=(const T& rhs) {
  41       value = rhs;
  42       return *this;
  43     }
  44     void operator++(int) {
  45       value++;
  46     }
  47     void operator--(int) {
  48       value--;
  49     }
  50
  51     T* operator&() { return &value; }
  52
  53     operator T&() { return value; }
  54     operator const T&() const { return value; }
  55
  56     T& operator->() { return value; }
  57
  58 #else
  59     DebugOnly() { }
  60     DebugOnly(const T&) { }
  61     DebugOnly(const DebugOnly&) { }
  62     DebugOnly& operator=(const T&) { return *this; }
  63     void operator++(int) { }
  64     void operator--(int) { }
  65 #endif
  66
  67     /*
  68      * DebugOnly must always have a destructor or else it will
  69      * generate "unused variable" warnings, exactly what it's intended
  70      * to avoid!
  71      */
  72     ~DebugOnly() {}
  73 };
  74
  75 }
  76
  77 #endif  /* mozilla_DebugOnly_h_ */