Getting an exception when reading values using BOOST_FOREACH from the JSON array in C++

Question

I am getting the below error when reading the values using BOOST_FOREACH:

Unhandled exception at 0x76FCB502 in JSONSampleApp.exe: Microsoft C++ exception: boost::wrapexcept<boost::property_tree::ptree_bad_path> at memory location 0x00CFEB18.

Could someone help me how to read values from the array with the below JSON format?

#include <string>
#include <iostream>
#include <boost/property_tree/json_parser.hpp>
#include <boost/foreach.hpp>

using namespace std;

using boost::property_tree::ptree;

int main()
{
const char* f_strSetting = R"({"Class": [{"Student": {"Name":"John","Course":"C++"}}]})";

    boost::property_tree::ptree pt1;
    std::istringstream l_issJson(f_strSetting);
    boost::property_tree::read_json(l_issJson, pt1);

    BOOST_FOREACH(boost::property_tree::ptree::value_type & v, pt1.get_child("Class.Student"))
    {
        std::string l_strName;
        std::string l_strCourse;
        l_strName = v.second.get <std::string>("Name");
        l_strCourse = v.second.get <std::string>("Course");

        cout << l_strName << "\n";
        cout << l_strCourse << "\n";
    }

    return 0;
}

Why are you using `BOOST_FOREACH`? Are you stuck using a pre-C++11 standard? I don't know if the issue is related to `BOOST_FOREACH`, though. — Fred Larson, Nov 23 '21 at 13:35
Your issue seems to be an invalid path in the ptree. Check this answer out: https://stackoverflow.com/a/48412544/8607180 — Sasha, Nov 23 '21 at 13:45
@FredLarson, I tried with C++ for loop. There also i am seeing the same issue. — sas, Nov 23 '21 at 13:49
You had the exact same type of question and error yesterday. — sehe, Nov 23 '21 at 14:51

score 1 · Accepted Answer · answered Nov 23 '21 at 15:05

You asked a very similar question yesterday. We told you not to abuse a property tree library to parse JSON. I even anticipated:

For more serious code you might want to use type-mapping

Here's how you'd expand from that answer to parse the entire array into a vector at once:

Live On Coliru

#include <boost/json.hpp>
#include <boost/json/src.hpp> // for header-only
#include <iostream>
#include <string>
namespace json = boost::json;

struct Student {
    std::string name, course;

    friend Student tag_invoke(json::value_to_tag<Student>, json::value const& v) {
        std::cerr << "DEBUG: " << v << "\n";
        auto const& s = v.at("Student");
        return {
            value_to<std::string>(s.at("Name")),
            value_to<std::string>(s.at("Course")),
        };
    }
};

using Class = std::vector<Student>;

int main()
{
    auto doc = json::parse(R"({ "Class": [
            { "Student": { "Name": "John", "Course": "C++" } },
            { "Student": { "Name": "Carla", "Course": "Cobol" } }
        ]
    })");
    auto c = value_to<Class>(doc.at("Class"));

    for (Student const& s : c)
        std::cout << "Name: " << s.name << ", Course: " << s.course << "\n";
}

Printing

Name: John, Course: C++
Name: Carla, Course: Cobol

I even threw in a handy debug line in case you need to help figuring out exactly what you get at some point:

DEBUG: {"Student":{"Name":"John","Course":"C++"}}
DEBUG: {"Student":{"Name":"Carla","Course":"Cobol"}}

In case you prefer manual JSON traversal without the struct/vector: http://coliru.stacked-crooked.com/a/af3ddd1dac30cbd5 — sehe, Nov 23 '21 at 15:19
Hi @sehe, I am using read_json with property_tree. Can you help me how to do the manual JSON traversal using read_json and property_tree ( as it is in my example). — sas, Nov 23 '21 at 16:18
Sure. http://coliru.stacked-crooked.com/a/d444f8aaf6c611f5 (basically the same as it was in my [older answer](https://stackoverflow.com/a/70069015/85371)). Don't do this, though. And read up on the limitations of the "JSON" backend in Boost Property Tree: https://www.boost.org/doc/libs/1_77_0/doc/html/property_tree/parsers.html#property_tree.parsers.json_parser — sehe, Nov 23 '21 at 16:42

Getting an exception when reading values using BOOST_FOREACH from the JSON array in C++

1 Answers1