1.1 --- /dev/null Thu Jan 01 00:00:00 1970 +0000
1.2 +++ b/mfbt/DebugOnly.h Wed Dec 31 06:09:35 2014 +0100
1.3 @@ -0,0 +1,82 @@
1.4 +/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
1.5 +/* vim: set ts=8 sts=2 et sw=2 tw=80: */
1.6 +/* This Source Code Form is subject to the terms of the Mozilla Public
1.7 + * License, v. 2.0. If a copy of the MPL was not distributed with this
1.8 + * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
1.9 +
1.10 +/*
1.11 + * Provides DebugOnly, a type for variables used only in debug builds (i.e. by
1.12 + * assertions).
1.13 + */
1.14 +
1.15 +#ifndef mozilla_DebugOnly_h
1.16 +#define mozilla_DebugOnly_h
1.17 +
1.18 +namespace mozilla {
1.19 +
1.20 +/**
1.21 + * DebugOnly contains a value of type T, but only in debug builds. In release
1.22 + * builds, it does not contain a value. This helper is intended to be used with
1.23 + * MOZ_ASSERT()-style macros, allowing one to write:
1.24 + *
1.25 + * DebugOnly<bool> check = func();
1.26 + * MOZ_ASSERT(check);
1.27 + *
1.28 + * more concisely than declaring |check| conditional on #ifdef DEBUG, but also
1.29 + * without allocating storage space for |check| in release builds.
1.30 + *
1.31 + * DebugOnly instances can only be coerced to T in debug builds. In release
1.32 + * builds they don't have a value, so type coercion is not well defined.
1.33 + *
1.34 + * Note that DebugOnly instances still take up one byte of space, plus padding,
1.35 + * when used as members of structs.
1.36 + */
1.37 +template<typename T>
1.38 +class DebugOnly
1.39 +{
1.40 + public:
1.41 +#ifdef DEBUG
1.42 + T value;
1.43 +
1.44 + DebugOnly() { }
1.45 + DebugOnly(const T& other) : value(other) { }
1.46 + DebugOnly(const DebugOnly& other) : value(other.value) { }
1.47 + DebugOnly& operator=(const T& rhs) {
1.48 + value = rhs;
1.49 + return *this;
1.50 + }
1.51 + void operator++(int) {
1.52 + value++;
1.53 + }
1.54 + void operator--(int) {
1.55 + value--;
1.56 + }
1.57 +
1.58 + T* operator&() { return &value; }
1.59 +
1.60 + operator T&() { return value; }
1.61 + operator const T&() const { return value; }
1.62 +
1.63 + T& operator->() { return value; }
1.64 + const T& operator->() const { return value; }
1.65 +
1.66 +#else
1.67 + DebugOnly() { }
1.68 + DebugOnly(const T&) { }
1.69 + DebugOnly(const DebugOnly&) { }
1.70 + DebugOnly& operator=(const T&) { return *this; }
1.71 + void operator++(int) { }
1.72 + void operator--(int) { }
1.73 +#endif
1.74 +
1.75 + /*
1.76 + * DebugOnly must always have a destructor or else it will
1.77 + * generate "unused variable" warnings, exactly what it's intended
1.78 + * to avoid!
1.79 + */
1.80 + ~DebugOnly() {}
1.81 +};
1.82 +
1.83 +}
1.84 +
1.85 +#endif /* mozilla_DebugOnly_h */
