Add advice about Optional function arguments

webrtc/base/optional.h

 /*
  *  Copyright 2015 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 59 matching lines @@
 //   Optional<size_t> so that it can return nothing in case the caller passed
 //   it a null pointer; the function should probably use RTC_[D]CHECK instead,
 //   and return plain size_t.
 //
 // - As a return value for functions that may fail to return a value on all
 //   allowed inputs, but need to tell the caller what went wrong. Returning
 //   Optional<double> when parsing a single number as in the example above
 //   might make sense, but any larger parse job is probably going to need to
 //   tell the caller what the problem was, not just that there was one.
 //
+// - As a non-mutable function argument. When you want to pass a value of a
+//   type T that can fail to be there, const T* is almost always both fastest
+//   and cleanest. (If you're *sure* that the the caller will always already
+//   have an Optional<T>, const Optional<T>& is slightly faster than const T*,
+//   but this is a micro-optimization. In general, stick to const T*.)
+//
 // TODO(kwiberg): Get rid of this class when the standard library has
 // std::optional (and we're allowed to use it).
 template <typename T>
 class Optional final {
  public:
   // Construct an empty Optional.
   Optional() : has_value_(false), empty_('\0') {
     PoisonValue();
   }
 
@@ skipping 190 matching lines @@
     // construct it, and the Optional destructor won't automatically destroy
     // it. Basically, this just allocates a properly sized and aligned block of
     // memory in which we can manually put a T with placement new.
     T value_;
   };
 };
 
 }  // namespace rtc
 
 #endif  // WEBRTC_BASE_OPTIONAL_H_