How to ignore null objects when writing JSON with JsonCpp

Question

According to the Google JSON style guide, it is advisable to remove empty or null values.

When using JsonCpp, how can empty or null values be removed, either from the object structure or when writing to a stream?

I want the following code:

#include <json/json.h>
#include <json/writer.h>

Json::Value json;
json["id"] = 4;
// The "name" property is an empty array.
json["name"] = Json::Value(Json::arrayValue);
Json::FastWriter fw;
std::cout << fw.write(json) << std::endl;

to produce:

{
    "id": 4,
}

Answer 1

You may add a pre-process to remove empty members, something like:

void RemoveNullMember(Json::Value& node)
{
    switch (node.type())
    {
        case Json::ValueType::nullValue: return;
        case Json::ValueType::intValue: return;
        case Json::ValueType::uintValue: return;
        case Json::ValueType::realValue: return;
        case Json::ValueType::stringValue: return;
        case Json::ValueType::booleanValue: return;
        case Json::ValueType::arrayValue:
        {
            for (auto &child : node)
            {
                RemoveNullMember(child);
            }
            return;
        }
        case Json::ValueType::objectValue:
        {
            for (const auto& key : node.getMemberNames())
            {
                auto& child = node[key]
                if (child.empty()) // Possibly restrict to any of
                                   // nullValue, arrayValue, objectValue
                {
                    node.removeMember(key);
                }
                else
                {
                    RemoveNullMember(node[key]);
                }
            }
            return;
        }
    }
}

And so finally:

Json::Value json;
json["id"] = 4;
json["name"] = Json::Value(Json::arrayValue); // The "name" property is an empty array.
RemoveNullMember(json); // Or make a copy before.
Json::FastWriter fw;
std::cout << fw.write(json) << std::endl;