boost ptree access first element with no path name

问题

I am using boost library to manipulate a JSON string and I would like to access to a first element.

I was wondering if there where some convenient way to access a first element of ptree with no path name.

I do this, but I got no value :

namespace pt = boost::property_tree;
pt::ptree pt2;
string json = "\"ok\"";
istringstream is(json);
try
{
        pt::read_json(is, pt2);
        cout << pt2.get_child("").equal_range("").first->first.data() << endl;
}
catch (std::exception const& e)
{
        cerr << e.what() << endl;
}

Solution:

replace cout << pt2.get_child("").equal_range("").first->first.data() << endl;

by cout << pt2.get_value<std::string>() << endl;

回答1:

Firstly, Property Tree is not a JSON library.

Secondly, the input is not in the subset of JSON supported by the library (e.g.).

Thirdly, since the input results in a tree that has no child nodes, you should use the value of the root node itself.

Lastly, if you had wanted the first node, use ordered_begin()->second:

Live On Coliru

#include <boost/property_tree/json_parser.hpp>
#include <boost/property_tree/xml_parser.hpp>
#include <iostream>

void broken_input() {
    boost::property_tree::ptree pt;
    std::istringstream is("\"ok\"");
    read_json(is, pt);
    std::cout << "Root value is " << pt.get_value<std::string>() << std::endl;
}

void normal_tree() {
    boost::property_tree::ptree pt;
    pt.put("first", "hello");
    pt.put("second", "world");
    pt.put("third", "bye");

    std::cout << pt.ordered_begin()->second.get_value<std::string>() << std::endl;

    write_json(std::cout, pt);
}


int main() {
    try {
        broken_input();
        normal_tree();
    }
    catch (std::exception const& e)
    {
        std::cerr << e.what() << std::endl;
    }
}

Prints

Root value is ok
hello
{
    "first": "hello",
    "second": "world",
    "third": "bye"
}

回答2:

I would like to access to a first element.

It is impossible in general case, since JSON elements are not place-fixed by definition. The current first element can change its place after JSON transformations and a resulting JSON will be the same, although elements are reordered. Thus such API is not provided by BOOST.

来源：https://stackoverflow.com/questions/49462690/boost-ptree-access-first-element-with-no-path-name

标签

c++

boost

boost-propertytree

ptree