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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 */