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### jsoncons::jsonpath::jsonpath_expression::update
```cpp
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
<table>
<tr>
<td>root</td>
<td>Root JSON value</td>
</tr>
<tr>
<td><code>op</code></td>
<td>A function object that accepts a path and a reference to a Json value.
It must have function call signature equivalent to
<br/><br/><code>void fun(const basic_path_node<Json::char_type>& path, Json& val);</code><br/><br/>
</tr>
</table>
The callback receives nodes with duplicates removed, and paths sorted in descending order.
### Examples
#### Update in place
```cpp
#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"
}
```
|
