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