Callback support for unbound reference and array arguments.

base/callback_internal.h

Index: base/callback_internal.h
diff --git a/base/callback_internal.h b/base/callback_internal.h
index 4f1d3c346bfbd7ddd5695ed1f6f5d06482050ce5..6b8fdff2cac47aaa6fd60491dd4b2c6879c3cd7d 100644
--- a/base/callback_internal.h
+++ b/base/callback_internal.h
@@ -80,6 +80,52 @@ class CallbackBase {
   InvokeFuncStorage polymorphic_invoke_;
 };
 
+// This is a typetraits object that's used to take an argument type, and
+// extract a suitable type for storing and forwarding arguments.
+//
+// In particular, it strips off references, and converts arrays to
+// pointers for storage; and it avoids accidentally trying to create a
+// "reference of a reference" if the argument is a reference type.
+//
+// This array type becomes an issue for storage because we are passing bound
+// parameters by const reference. In this case, we end up passing an actual
+// array type in the initializer list which C++ does not allow.  This will
+// break passing of C-string literals.
+template <typename T>
+struct ParamTraits {
+  typedef const T& ForwardType;
+  typedef T StorageType;
+};
+
+// The Storage should almost be impossible to trigger unless someone manually
+// specifies type of the bind parameters.  However, in case they do,
+// this will guard against us accidentally storing a reference parameter.
+//
+// The ForwardType should only be used for unbound arguments.
+template <typename T>
+struct ParamTraits<T&> {
+  typedef T& ForwardType;
+  typedef T StorageType;
+};
+
+// Note that for array types, we implicitly add a const in the conversion. This
+// means that it is not possible to bind array arguments to functions that take
+// a non-const pointer. Trying to specialize the template based on a "const
+// T[n]" does not seem to match correctly, so we are stuck with this
+// restriction.
+template <typename T, size_t n>
+struct ParamTraits<T[n]> {
+  typedef const T* ForwardType;
+  typedef const T* StorageType;
+};
+
+// See comment for ParamTraits<T[n]>.
+template <typename T>
+struct ParamTraits<T[]> {
+  typedef const T* ForwardType;
+  typedef const T* StorageType;
+};
+
 }  // namespace internal
 }  // namespace base