Efficiently returning local object allocated with external memory resource in boost::json

Question

As part of a larger C++11 application, I have a function which creates a Boost JSON object and then returns it. One of the parameters to this function is a Boost JSON storage_ptr to an external monotonic memory resource which is used to create the object like object myObject(sp). I want to avoid returning the object as a value since the object could be a large JSON structure, so I've been attempting to return it by reference.

object& getObject(storage_ptr sp)
{
  // Create object with external storage_ptr
  object myObject(sp);

  // Modify the object
  myObject["key"] = "value";

  // Return reference to local object
  return myObject;
}

This works, but then, understandably, I get the following compiler warning:

warning: reference to local variable 'myObject' returned

Is there a way that I can avoid returning the object by value without upsetting the compiler? I'm quite new to the custom memory resource stuff that storage_ptr references, so I'm assuming there's a neater way to do something with that, but I can't seem to find it anywhere.

Answer 1

The easiest solution is just

object getObject(storage_ptr sp)
{
  // Create object with external storage_ptr
  object myObject(sp);

  // Modify the object
  myObject["key"] = "value";

  // Return local object.
  return myObject;
}

You're worried about a C++98 copy, but C++11 introduced move semantics, and Boost JSON supports that.

Or (untested)

object getObject(storage_ptr sp)
{
  return object({{"key", "value"}}, sp);
}