Eliminate JSON entry for empty std::optional

[lowsfer]

This change remove the entry in JSON if what is being serialized/deserialized is of type std::optional, and the value is std::nullopt. When it's not std::nullopt, only the contained value (T) is saved, instead of a flag plus the value.

This is helpful when using JSON as web protocol. People would want to eliminate optional entries when they are empty. This improved human-readability of the JSON protocol and make it easier to interact with other languages.

[lowsfer]

Seems this is exactly what is requested here:
#459

[WSoptics]

It might be a good idea to make this an option of JSONArchive instead of altering its default behavior?

[lowsfer]

@WSoptics Changed it to an option and defaults to false. Also added unit tests.

[fiesh]

I like the change but can't merge :-)

One thing though however, it seems the option is only available for C++17, which seems not really necessary?

You should probably ping @AzothAmmo to potentially have it merged.

[lowsfer]

@fiesh std::optional is only available since C++17. You mean keep the option but leave it with no effect?

@AzothAmmo Can we merge this feature?

[fiesh]

@fiesh std::optional is only available since C++17. You mean keep the option but leave it with no effect?

Sorry, I somehow thought std::optional was C++11 -- you're right of course!

[winding-lines]

Would also love to have this behavior in cereal, @AzothAmmo :)

[lowsfer]

It has been ~1.75 years.
@AzothAmmo Do you have any concerns on this PR?

[akalsi87]

This seems like a good change, and one that makes the output more intuitive and interoperable. Any reason for the delay in merging this in?

[yuziqii]

@lowsfer it seems this change can't support out-of-order json fileds.

[yuziqii]

I made a few changes to support out-of-order JSON fields. It works fine in my case, but I don't know if there are any side effects.

here are my changes:

template <class T> inline  std::enable_if_t<IsOptional<T>::value>
CEREAL_LOAD_FUNCTION_NAME( JSONInputArchive & ar, NameValuePair<T> & t )
{
  if (ar.skippedNullopt())
  {
    // search node name
    if (ar.searchNodeName(t.name))
    {
      ar.setNextName( t.name );
      std::decay_t<decltype(*t.value)> val;
      ar( val );
      t.value = std::move(val);
    }
    else
    {
      t.value = std::nullopt;
    }
  }
  else
  {
    loadImpl(ar, t);
  }
}

const char* searchNodeName(const char* name) const
{
  auto currentIterator = itsIteratorStack.back();
  const auto len = std::strlen( name );
  for (const auto& it : currentIterator)
  {
    const auto currentName = it.name.GetString();
    if( ( std::strncmp( name, currentName, len ) == 0 ) &&
        ( std::strlen( currentName ) == len ) )
    {
    return currentName;
    }
  }
  return nullptr;
}

// Iterator support range for syntax
auto begin()
{
  return itsMemberItBegin;
}

auto end()
{
   return itsMemberItEnd;
}

[lowsfer]

@yuziqii Would you like to create a commit including your changes? You can create an PR into my forked branch lowsfer:eliminate-empty-optional, then I merge it and your commit will be included in this PR.

[yuziqii]

@lowsfer Sure, I'd be happy to submit a PR , and I've already done that. Let me know if there's any problems. here is the PR.