michael@0: /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
michael@0: /* vim: set ts=8 sts=2 et sw=2 tw=80: */
michael@0: /* This Source Code Form is subject to the terms of the Mozilla Public
michael@0:  * License, v. 2.0. If a copy of the MPL was not distributed with this
michael@0:  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
michael@0: 
michael@0: /*
michael@0:  * Provides DebugOnly, a type for variables used only in debug builds (i.e. by
michael@0:  * assertions).
michael@0:  */
michael@0: 
michael@0: #ifndef mozilla_DebugOnly_h
michael@0: #define mozilla_DebugOnly_h
michael@0: 
michael@0: namespace mozilla {
michael@0: 
michael@0: /**
michael@0:  * DebugOnly contains a value of type T, but only in debug builds.  In release
michael@0:  * builds, it does not contain a value.  This helper is intended to be used with
michael@0:  * MOZ_ASSERT()-style macros, allowing one to write:
michael@0:  *
michael@0:  *   DebugOnly<bool> check = func();
michael@0:  *   MOZ_ASSERT(check);
michael@0:  *
michael@0:  * more concisely than declaring |check| conditional on #ifdef DEBUG, but also
michael@0:  * without allocating storage space for |check| in release builds.
michael@0:  *
michael@0:  * DebugOnly instances can only be coerced to T in debug builds.  In release
michael@0:  * builds they don't have a value, so type coercion is not well defined.
michael@0:  *
michael@0:  * Note that DebugOnly instances still take up one byte of space, plus padding,
michael@0:  * when used as members of structs.
michael@0:  */
michael@0: template<typename T>
michael@0: class DebugOnly
michael@0: {
michael@0:   public:
michael@0: #ifdef DEBUG
michael@0:     T value;
michael@0: 
michael@0:     DebugOnly() { }
michael@0:     DebugOnly(const T& other) : value(other) { }
michael@0:     DebugOnly(const DebugOnly& other) : value(other.value) { }
michael@0:     DebugOnly& operator=(const T& rhs) {
michael@0:       value = rhs;
michael@0:       return *this;
michael@0:     }
michael@0:     void operator++(int) {
michael@0:       value++;
michael@0:     }
michael@0:     void operator--(int) {
michael@0:       value--;
michael@0:     }
michael@0: 
michael@0:     T* operator&() { return &value; }
michael@0: 
michael@0:     operator T&() { return value; }
michael@0:     operator const T&() const { return value; }
michael@0: 
michael@0:     T& operator->() { return value; }
michael@0:     const T& operator->() const { return value; }
michael@0: 
michael@0: #else
michael@0:     DebugOnly() { }
michael@0:     DebugOnly(const T&) { }
michael@0:     DebugOnly(const DebugOnly&) { }
michael@0:     DebugOnly& operator=(const T&) { return *this; }
michael@0:     void operator++(int) { }
michael@0:     void operator--(int) { }
michael@0: #endif
michael@0: 
michael@0:     /*
michael@0:      * DebugOnly must always have a destructor or else it will
michael@0:      * generate "unused variable" warnings, exactly what it's intended
michael@0:      * to avoid!
michael@0:      */
michael@0:     ~DebugOnly() {}
michael@0: };
michael@0: 
michael@0: }
michael@0: 
michael@0: #endif /* mozilla_DebugOnly_h */