Boost property tree getting element from unamed array - c++

Question

I have the following json file
[
{
"a":5855925.424944928,
"b":0,
"c":96,
"d":2096640,
"e":0
}
]
I do this,
boost::property_tree::ptree jsontree;
std::stringstream ss;
ss << "[{\"a\": 5855925.424944928, \"b\": 0, \"c\": 96, \"d\": 2096640, \"e\": 0}]";
boost::property_tree::read_json(ss, jsontree);
// get the children, i.e. the list elements
auto bounds = jsontree.equal_range("");
std::cout << "Size of list : " << std::distance( bounds.first, bounds.second ) << "\n";
But I don't know how to read this from a property tree (eg: get<float>("a"), get<int>("c"))?
Any help would be greatly appreciated.
Thanks

Answer 1

Notice: top-level arrays are not actually supported (you cannot roundtrip that with Boost Property Tree).
The limitations are described in the documentation
Saving this same tree will lose all type information and convert the array to this:
{
"": {
"a": "5855925.424944928",
"b": "0",
"c": "96",
"d": "2096640",
"e": "0"
}
}
Arrays are "objects with empty keys". You already seemed to know that, judging from your sample. So, just use it:
for (auto& object_node : boost::make_iterator_range(jsontree.equal_range(""))) {
ptree const& object = object_node.second;
std::cout << "a: " << object.get<double>("a") << "\n";
std::cout << "b: " << object.get<int>("b") << "\n";
std::cout << "c: " << object.get<int>("c") << "\n";
std::cout << "d: " << object.get<int>("d") << "\n";
std::cout << "e: " << object.get<int>("e") << "\n";
}
Improved Demo
It's often nicer to extract some types/functions:
Live On Coliru
#include <iostream>
#include <boost/property_tree/json_parser.hpp>
using boost::property_tree::ptree;
struct Object {
double a;
int b, c, d, e;
friend std::ostream& operator<<(std::ostream& os, Object const& object) {
return os << "a: " << object.a << "\n"
<< "b: " << object.b << "\n"
<< "c: " << object.c << "\n"
<< "d: " << object.d << "\n"
<< "e: " << object.e << "\n";
}
static Object parse(ptree const& from) {
return {
from.get<double>("a"),
from.get<int>("b"),
from.get<int>("c"),
from.get<int>("d"),
from.get<int>("e"),
};
}
};
int main() {
boost::property_tree::ptree jsontree;
{
std::stringstream ss(R"([{"a": 5855925.424944928, "b": 0, "c": 96, "d": 2096640, "e": 0}])");
boost::property_tree::read_json(ss, jsontree);
}
for (auto& object_node : boost::make_iterator_range(jsontree.equal_range(""))) {
std::cout << Object::parse(object_node.second);
}
}
Prints
a: 5.85593e+06
b: 0
c: 96
d: 2096640
e: 0

Answer 2

You can use BOOST_FOREACH for this. I am attaching some code but they are based on xml_data. But u can use similar to this for getting each value without knowing the key.
BOOST_FOREACH(pt::ptree::value_type &v, tree) {
std::string at = v.first + ATTR_SET;
std::cout << "Extracting attributes from " << at << ":\n";
BOOST_FOREACH(pt::ptree::value_type &v2,v.second.get_child("<xmlattr>")){
std::cout<<""<<v2.first.data()<<" : " <<v2.second.data()<<"\n";
}
std::cout<<"now in book\n";
BOOST_FOREACH(pt::ptree::value_type &v3,tree.get_child("bookstore."+v.first)){
std::cout<<""<<v3.first.data()<<" : " <<v3.second.data()<<"\n";
}
m_modules.insert(v.second.data());
}
here tree is boost::Property_tree::ptree