Line data    Source code

       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             : {
      41             : public:
      42             : #ifdef DEBUG
      43             :   T value;
      44             : 
      45       43867 :   DebugOnly() { }
      46     1749648 :   MOZ_IMPLICIT DebugOnly(const T& aOther) : value(aOther) { }
      47           0 :   DebugOnly(const DebugOnly& aOther) : value(aOther.value) { }
      48       97084 :   DebugOnly& operator=(const T& aRhs) {
      49       97084 :     value = aRhs;
      50       97084 :     return *this;
      51             :   }
      52             : 
      53       16698 :   void operator++(int) { value++; }
      54             :   void operator--(int) { value--; }
      55             : 
      56             :   // Do not define operator+=(), etc. here.  These will coerce via the
      57             :   // implicit cast and built-in operators.  Defining explicit methods here
      58             :   // will create ambiguity the compiler can't deal with.
      59             : 
      60        8038 :   T* operator&() { return &value; }
      61             : 
      62     4918169 :   operator T&() { return value; }
      63      790071 :   operator const T&() const { return value; }
      64             : 
      65       38481 :   T& operator->() { return value; }
      66             :   const T& operator->() const { return value; }
      67             : 
      68             : #else
      69             :   DebugOnly() { }
      70             :   MOZ_IMPLICIT DebugOnly(const T&) { }
      71             :   DebugOnly(const DebugOnly&) { }
      72             :   DebugOnly& operator=(const T&) { return *this; }
      73             :   void operator++(int) { }
      74             :   void operator--(int) { }
      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             :   DebugOnly& operator^=(const T&) { return *this; }
      80             : #endif
      81             : 
      82             :   /*
      83             :    * DebugOnly must always have a destructor or else it will
      84             :    * generate "unused variable" warnings, exactly what it's intended
      85             :    * to avoid!
      86             :    */
      87     1797507 :   ~DebugOnly() {}
      88             : };
      89             : 
      90             : } // namespace mozilla
      91             : 
      92             : #endif /* mozilla_DebugOnly_h */