update.md

jsoncons::jsonpath::jsonpath_expression::update

template <typename BinaryOp>
void update(reference root, BinaryOp op);                                   (1) (since 0.172.0)

(1) Evaluates the root value against the compiled JSONPath expression and calls a provided callback repeatedly with the results.

Parameters

root	Root JSON value
op	A function object that accepts a path and a reference to a Json value. It must have function call signature equivalent to void fun(const basic_path_node<Json::char_type>& path, Json& val);

The callback receives nodes with duplicates removed, and paths sorted in descending order.

Examples

Update in place

#include <jsoncons/json.hpp>
#include <jsoncons_ext/jsonpath/jsonpath.hpp>

using json = jsoncons::json;
namespace jsonpath = jsoncons::jsonpath;

int main()
{
    auto expr = jsoncons::jsonpath::make_expression<json>("$.books[*]");

    std::ifstream is(/*path_to_books_file*/);
    json doc = json::parse(is);

    auto callback = [](const jsonpath::path_node& /*path*/, json& book)
    {
        if (book.at("category") == "memoir" && !book.contains("price"))
        {
            book.try_emplace("price", 140.0);
        }
    };

    expr.update(doc, callback);
}

Output:

{
    "author": "Phillips, David Atlee",
    "category": "memoir",
    "price": 140.0,
    "title": "The Night Watch"
}