Bug 987253 - Add MOZ_EXPLICIT_CONVERSION - r=Waldo

mfbt/Attributes.h

@@ -57,6 +57,9 @@
 #  if __has_extension(cxx_constexpr)
 #    define MOZ_HAVE_CXX11_CONSTEXPR
 #  endif
+#  if __has_extension(cxx_explicit_conversions)
+#    define MOZ_HAVE_EXPLICIT_CONVERSION
+#  endif
 #  if __has_extension(cxx_deleted_functions)
 #    define MOZ_HAVE_CXX11_DELETE
 #  endif
@@ -79,6 +82,9 @@
 #    if MOZ_GCC_VERSION_AT_LEAST(4, 6, 0)
 #      define MOZ_HAVE_CXX11_CONSTEXPR
 #    endif
+#    if MOZ_GCC_VERSION_AT_LEAST(4, 5, 0)
+#      define MOZ_HAVE_EXPLICIT_CONVERSION
+#    endif
 #    define MOZ_HAVE_CXX11_DELETE
 #  else
      /* __final is a non-C++11 GCC synonym for 'final', per GCC r176655. */
@@ -101,6 +107,8 @@
 #  define MOZ_HAVE_CXX11_OVERRIDE
 #  define MOZ_HAVE_NEVER_INLINE          __declspec(noinline)
 #  define MOZ_HAVE_NORETURN              __declspec(noreturn)
+// Staying away from explicit conversion operators in MSVC for now, see
+// http://stackoverflow.com/questions/20498142/visual-studio-2013-explicit-keyword-bug
 #endif
 
 /*
@@ -121,6 +129,30 @@
 #  define MOZ_CONSTEXPR_VAR     const
 #endif
 
+/*
+ * MOZ_EXPLICIT_CONVERSION is a specifier on a type conversion
+ * overloaded operator that declares that a C++11 compiler should restrict
+ * this operator to allow only explicit type conversions, disallowing
+ * implicit conversions.
+ *
+ * Example:
+ *
+ *   template<typename T>
+ *   class Ptr
+ *   {
+ *      T* ptr;
+ *      MOZ_EXPLICIT_CONVERSION operator bool() const {
+ *        return ptr != nullptr;
+ *      }
+ *   };
+ *
+ */
+#ifdef MOZ_HAVE_EXPLICIT_CONVERSION
+#  define MOZ_EXPLICIT_CONVERSION explicit
+#else
+#  define MOZ_EXPLICIT_CONVERSION /* no support */
+#endif
+
 /*
  * MOZ_NEVER_INLINE is a macro which expands to tell the compiler that the
  * method decorated with it must never be inlined, even if the compiler would