1 files changed, 92 insertions, 0 deletions
diff --git a/mfbt/DebugOnly.h b/mfbt/DebugOnly.h
new file mode 100644
index 000000000..a1a669dba
--- /dev/null
+++ b/mfbt/DebugOnly.h
@@ -0,0 +1,92 @@
+/* -*- 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"
+
+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() { }
+  MOZ_IMPLICIT DebugOnly(const T& aOther) : value(aOther) { }
+  DebugOnly(const DebugOnly& aOther) : value(aOther.value) { }
+  DebugOnly& operator=(const T& aRhs) {
+    value = 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; }
+
+#else
+  DebugOnly() { }
+  MOZ_IMPLICIT DebugOnly(const T&) { }
+  DebugOnly(const DebugOnly&) { }
+  DebugOnly& operator=(const 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 destructor or else it will
+   * generate "unused variable" warnings, exactly what it's intended
+   * to avoid!
+   */
+  ~DebugOnly() {}
+};
+
+} // namespace mozilla
+
+#endif /* mozilla_DebugOnly_h */