danielaparker
commented
4 years ago Does it have to be an in-place update? If not, you could use a json filter, e.g.
class my_filter : public jsoncons::json_filter
{
using jsoncons::json_filter::string_view_type;
std::string from_;
std::string to_;
public:
my_filter(jsoncons::json_visitor& destination)
: jsoncons::json_filter(destination), from_("id"), to_("ab")
{
}
bool visit_string(const string_view_type& value,
jsoncons::semantic_tag tag,
const jsoncons::ser_context& context,
std::error_code& ec) override
{
if (value == from_)
{
return this->destination().string_value(to_,
tag,
context,
ec);
}
else
{
return this->destination().string_value(value,
tag,
context,
ec);
}
}
};
int main()
{
std::string input = R"({"items": [{"id":1, "name" : "abc", "expiry" : "0420"}, { "id":2,"name" : "id","expiry" : "0720" }] })";
std::string output;
jsoncons::compact_json_string_encoder encoder(output);
my_filter filter(encoder);
jsoncons::json_reader reader(input, filter);
reader.read();
std::cout << output << "\n";
}Output:
{"items":[{"id":1,"name":"abc","expiry":"0420"},{"id":2,"name":"ab","expiry":"0720"}]}Or alternatively, and consistent with using JSONPath to find the item, jsoncons::jsonpath::json_replace.
But if it must be in-place, and this will currently work only if your json text contains no unescaped newlines, you could try this:
class my_in_place_updater : public jsoncons::default_json_visitor
{
char* data_;
std::size_t length_;
std::string from_;
std::string to_;
public:
using jsoncons::default_json_visitor::string_view_type;
my_in_place_updater(char* data, std::size_t length)
: data_(data), length_(length), from_("id"), to_("ab")
{
}
bool visit_string(const string_view_type& value,
jsoncons::semantic_tag,
const jsoncons::ser_context& context,
std::error_code& ec) override
{
// input cannot contain unescaped newlines
if (context.line() != 1)
{
ec = jsoncons::json_errc::source_error;
return false;
}
if (value == from_)
{
std::copy(to_.begin(), to_.end(), data_ + (context.column() - 1));
}
return true;
}
};
int main()
{
std::string input = R"({"items": [{"id":1, "name" : "abc", "expiry" : "0420"}, { "id":2,"name" : "id","expiry" : "0720" }] })";
my_in_place_updater updater(input.data(), input.size());
jsoncons::json_reader reader(input, updater);
reader.read();
std::cout << input << "\n";
}Output:
{"items": [{"id":1, "name" : "abc", "expiry" : "0420"}, { "id":2,"name" : "aa","expiry" : "0720" }] }Using context.column() - 1 to get the current position will work only if the JSON text doesn't have unescaped newlines, so that context.line() == 1. If users thought it useful, we could add a position() member function to the ser_context, which wouldn't have that limitation.
rbroggi
Alexandre-Dhalenne
Dear Daniel,
Thank you once again for the product, my team loves it and we are heavily using it on our project.
We are currently facing a challenge and I would like to ask you for an advice on how to go about it using jsoncons:
We want to avoid finding the item and then replacing in the original string using string find and replace because the found item can be a very small string (e.g. "abc") and replacing seamlessly after finding could lead to several false positives.
Example:
Considering the following JSON string :
{"items":[{"id":1,"name":"abc","expiry":"0420"},{"id":2,"name":"id","expiry":"0720"}]}
and the following JSONPath: $.items[1].name
I would be retrieving the string "id".
I would like to substitute the found string "id" with the string "ab" in the original message without modifying anything else about it generating the following:
{"items":[{"id":1,"name":"abc","expiry":"0420"},{"id":2,"name":"ab","expiry":"0720"}]}
If I was to seamlessly find and replace in the raw string "id" with "ab" it would lead to the following string:
{"items":[{"ab":1,"name":"abc","expiry":"0420"},{"ab":2,"name":"ab","expiry":"0720"}]}
How would you go about this problem? Thank you once again.