I am getting the below error when reading the value from the JSON ptree using Boost C

Unhandled exception at 0x7682B502 in JSONSampleApp.exe: Microsoft C   exception : 
boost::wrapexcept<boost::property_tree::ptree_bad_path> at memory location 0x00DFEB38.

Below is the program, Could someone please help me what i am missing here.

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

using namespace std;

using boost::property_tree::ptree;

int main()
{
    const char* f_strSetting = "{\"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("Student"))
    {
        std::string l_strColor;
        std::string l_strPattern;
        l_strColor = v.second.get <std::string>("Name");
        l_strPattern = v.second.get <std::string>("Course");
    }
    return 0;
}

CodePudding user response：

There is a shape mismatch between your code and your data:

• The data is a plain nested dictionary: Student.name is "John".
• The code expects to see an array under the Student key, so it tries to fetch Student.0.name, Student.1.name, ... for every subitem of Student.

Either fix the code:

// Drop the BOOST_FOREACH
auto & l_Student = pt1.get_child("Student");
l_strColor = l_Student.get<std::string>("Name");

or fix the data:

// Note the extra []
const char * f_strSetting = R"({"Student": [{"Name":"John","Course":"C  "}]})";

CodePudding user response：

Firstly, may I suggest modernizing and thus simplifying your code, while avoiding using directives:

#include <boost/property_tree/json_parser.hpp>
#include <string>
using boost::property_tree::ptree;

int main() {
    ptree pt;
    {
        std::istringstream l_issJson( R"({"Student": {"Name":"John","Course":"C  "}})");
        read_json(l_issJson, pt);
    }

    for(auto& [k,v] : pt.get_child("Student")) {
        auto name   = v.get<std::string>("Name");
        auto course = v.get<std::string>("Course");
    }
}

Secondly, you're selecting the wrong levels - as the other answer points out.:

#include <boost/property_tree/json_parser.hpp>
#include <iostream>
#include <string>
using boost::property_tree::ptree;

int main() {
    ptree pt;
    {
        std::istringstream l_issJson( R"({"Student": {"Name":"John","Course":"C  "}})");
        read_json(l_issJson, pt);
    }

    auto name   = pt.get<std::string>("Student.Name");
    auto course = pt.get<std::string>("Student.Course");

    std::cout << "Name: '" << name << "', Course: '" << course << "'\n";
}

See it Live

But the REAL problem is:

USE A JSON LIBRARY

Boost Property Tree is not a JSON library.

Boost JSON exists:

Live On Coliru

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

int main() {
    auto pt = json::parse(R"({"Student": {"Name":"John","Course":"C  "}})");

    auto& student = pt.at("Student");
    auto  name    = student.at("Name").as_string();
    auto  course  = student.at("Course").as_string();

    std::cout << "Name: " << name << ", Course: " << course << "\n";
}

Prints

Name: "John", Course: "C  "

BONUS

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

#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) {
        return {
            json::value_to<std::string>(v.at("Name")),
            json::value_to<std::string>(v.at("Course")),
        };
    }
};

int main()
{
    auto doc = json::parse(R"({"Student": {"Name":"John","Course":"C  "}})");
    auto s   = value_to<Student>(doc.at("Student"));

    std::cout << "Name: " << s.name << ", Course: " << s.course << "\n";
}
    

See it Live On Coliru