Is there an elegant way to cascade-merge two JSON trees using jsoncpp?

I'm going to assume your settings files are JSON objects.

As seen here, when JSONCpp parses a file, it clears the contents of the root node. This mean that trying to parse a new file on top of the old one won't preserve the old data. However, if you parse both files into separate Json::Value nodes, it's straight forward to recursively copy the values yourself by iterating over the keys in the second object using getMemberNames.

// Recursively copy the values of b into a. Both a and b must be objects.void update(Json::Value& a, Json::Value& b) {    if (!a.isObject() || !b.isObject()) return;    for (const auto& key : b.getMemberNames()) {        if (a[key].isObject()) {            update(a[key], b[key]);        } else {            a[key] = b[key];        }    }}

I know it has been a while. but...

In addition to the correct answer and the commentary, here is a code version for those who use a older g++ version:

void jsonMerge(Json::Value &a, Json::Value &b) {                                                                                                                                                                                             if (!a.isObject() || !b.isObject()) return;                                                                                                                                                                                                  vector<string> member_name = b.getMemberNames();                                                                         string key = "";                                                                                                         for (unsigned i = 0, len = member_name.size(); i < len; i++) {                                                               key = member_name[i];                                                                                                                                                                                                                          if (!a[key].isNull() && a[key].type() == Json::objectValue && b[key].type() == Json::objectValue) {                            jsonMerge(a[key], b[key]);                                                                                               } else {                                                                                                                       a[key] = b[key];                                                                                                         }                                                                                                                      }                                                                                                                        member_name.clear();                                                                                                  }