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As there is a lot of questions already on this, I am kind of apprehensive about asking... but

I've looked at many different Questions and nothing from any of them is working for me. I have this code as my attempt but it doesn't work:

#include "boost/property_tree/ptree.hpp" #include "boost/property_tree/json_parser.hpp" using namespace boost::property_tree; ... std::ifstream jsonFile("test_file.json"); if (!jsonFile){ std::cerr << "Error opening file\n"; return -1; } ptree pt; json_parser::read_json(jsonFile, pt); for (auto& array_element : pt) { for (auto& property : array_element.second) { std::cout << property.first << " = " << property.second.get_value<std::string>() << "\n"; } }

Its contents are in the following format:

[{"number": 1234,"string": "hello world"}, {"number": 5678,"string": "foo bar"}, ... etc }]

I can't get it to read out the 1234 and then the hello world. Infact, it just does nothing. How can I read out from my .JSON file?

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  • 4
    Do you have an ssce? Commented Jul 6, 2015 at 17:01
  • Pretty much all of the examples given by @sehe in his many answers on this Commented Jul 7, 2015 at 9:04

1 Answer 1

Reset to default
7

I'm not completely sure what the problem is. It seems to work (once you make the JSON valid):

UPDATE: Boost JSON

Boost 1.75.0 introduced Boost JSON, far superior way to actually deal with Json: Live On Wandbox

#include <boost/json.hpp> #include <boost/json/src.hpp> #include <iostream> #include <iterator> #include <fstream> namespace json = boost::json; struct Rec { int64_t number; std::string string; friend Rec tag_invoke(json::value_to_tag<Rec>, json::value const& v) { auto& o = v.as_object(); return { o.at("number").as_int64(), value_to<std::string>(o.at("string")), }; } friend void tag_invoke(json::value_from_tag, json::value& v, Rec const& rec) { v = json::object{ {"number", rec.number}, {"string", rec.string}, }; } }; int main() { std::ifstream ifs("input.txt"); std::string input(std::istreambuf_iterator<char>(ifs), {}); using Recs = std::vector<Rec>; Recs recs = value_to<std::vector<Rec>>(json::parse(input)); for (auto& [n, s] : recs) { std::cout << "Rec { " << n << ", " << std::quoted(s) << " }\n"; // some frivolous changes: n *= 2; reverse(begin(s), end(s)); } std::cout << "Modified json: " << json::value_from(recs) << "\n"; }

Printing

Rec { 1234, "hello world" } Rec { 5678, "foo bar" } Modified json: [{"number":2468,"string":"dlrow olleh"},{"number":11356,"string":"rab oof"}]

Live On Coliru

#include "boost/property_tree/ptree.hpp" #include "boost/property_tree/json_parser.hpp" int main() { using boost::property_tree::ptree; std::ifstream jsonFile("input.txt"); ptree pt; read_json(jsonFile, pt); for (auto & array_element: pt) { for (auto & property: array_element.second) { std::cout << property.first << " = " << property.second.get_value < std::string > () << "\n"; } } }

With input.txt containing:

[{"number": 1234, "string": "hello world"},{"number": 5678, "string": "foo bar"}]

Prints

number = 1234 string = hello world number = 5678 string = foo bar
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8 Comments

does it have to be in .txt? my file is currently a .json
Erm. That's just the filename
I'm clutching at straws here, I didn't think it would be an issue but I gotta ask
Did you try the example in isolation?
Cheers. Fyi, this is exactly what creating SSCCE's is about. See also Solve your problem by almost asking on SO
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