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101 lines
3.1 KiB
C++
101 lines
3.1 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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/*
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* Provides DebugOnly, a type for variables used only in debug builds (i.e. by
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* assertions).
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*/
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#ifndef mozilla_DebugOnly_h
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#define mozilla_DebugOnly_h
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#include "mozilla/Attributes.h"
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#include <utility>
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namespace mozilla {
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/**
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* DebugOnly contains a value of type T, but only in debug builds. In release
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* builds, it does not contain a value. This helper is intended to be used with
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* MOZ_ASSERT()-style macros, allowing one to write:
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*
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* DebugOnly<bool> check = func();
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* MOZ_ASSERT(check);
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*
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* more concisely than declaring |check| conditional on #ifdef DEBUG.
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*
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* DebugOnly instances can only be coerced to T in debug builds. In release
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* builds they don't have a value, so type coercion is not well defined.
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*
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* NOTE: DebugOnly instances still take up one byte of space, plus padding, even
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* in optimized, non-DEBUG builds (see bug 1253094 comment 37 for more info).
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* For this reason the class is MOZ_STACK_CLASS to prevent consumers using
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* DebugOnly for struct/class members and unwittingly inflating the size of
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* their objects in release builds.
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*/
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template <typename T>
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class MOZ_STACK_CLASS DebugOnly {
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public:
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#ifdef DEBUG
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T value;
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DebugOnly() = default;
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MOZ_IMPLICIT DebugOnly(T&& aOther) : value(std::move(aOther)) {}
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MOZ_IMPLICIT DebugOnly(const T& aOther) : value(aOther) {}
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DebugOnly(const DebugOnly& aOther) : value(aOther.value) {}
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DebugOnly& operator=(const T& aRhs) {
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value = aRhs;
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return *this;
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}
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DebugOnly& operator=(T&& aRhs) {
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value = std::move(aRhs);
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return *this;
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}
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void operator++(int) { value++; }
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void operator--(int) { value--; }
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// Do not define operator+=(), etc. here. These will coerce via the
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// implicit cast and built-in operators. Defining explicit methods here
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// will create ambiguity the compiler can't deal with.
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T* operator&() { return &value; }
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operator T&() { return value; }
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operator const T&() const { return value; }
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T& operator->() { return value; }
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const T& operator->() const { return value; }
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#else
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DebugOnly() = default;
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MOZ_IMPLICIT DebugOnly(const T&) {}
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DebugOnly(const DebugOnly&) {}
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DebugOnly& operator=(const T&) { return *this; }
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MOZ_IMPLICIT DebugOnly(T&&) {}
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DebugOnly& operator=(T&&) { return *this; }
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void operator++(int) {}
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void operator--(int) {}
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DebugOnly& operator+=(const T&) { return *this; }
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DebugOnly& operator-=(const T&) { return *this; }
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DebugOnly& operator&=(const T&) { return *this; }
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DebugOnly& operator|=(const T&) { return *this; }
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DebugOnly& operator^=(const T&) { return *this; }
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#endif
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/*
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* DebugOnly must always have a user-defined destructor or else it will
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* generate "unused variable" warnings, exactly what it's intended
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* to avoid!
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*/
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~DebugOnly() {}
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};
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} // namespace mozilla
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#endif /* mozilla_DebugOnly_h */
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