Using boost property tree to read int array

Question

I have some JSON with a handful of integer array variables, like so:

{"a": [8, 6, 2], "b": [2, 2, 1]}

I would like to use boost property_tree, for instance:

std::stringstream ss;
boost::property_tree::ptree pt;

ss << "{\"a\": [8, 6, 2], \"b\": [2, 2, 1]}";

boost::property_tree::read_json(ss, pt);
std::vector<int> a = pt.get<std::vector<int> >("a");

This doesn't work, nor does any variation on an int pointer that I've tried. How may I read an array from a property tree?

Tempestuous

possible duplicate of Parsing JSON with boost property tree – Jarvisjary

The answer to that question does not cover the specific case of arrays, so I respectfully disagree. Incidentally, the answer to this question is forthcoming. – Tempestuous

But the question itself has code on dealing with arrays, and there's an array in the example data. He extracts the elements using a loop. – Jarvisjary

oh, I see what you mean. The question as asked contains the answer to my question. I feel that this isn't obvious, though. Intuitively, it would make sense to me that arrays of primitives would be parsed directly. A concise answer to my question would read something like "each element in a primitive type array is a special case of a child node" – Tempestuous

Answer 1

JSON support, is spotty with boost property tree.

The property tree dataset is not typed, and does not support arrays as such. Thus, the following JSON / property tree mapping is used:

• JSON objects are mapped to nodes. Each property is a child node.
• JSON arrays are mapped to nodes. Each element is a child node with an empty name. If a node has both named and unnamed child nodes, it cannot be mapped to a JSON representation.
• JSON values are mapped to nodes containing the value. However, all type information is lost; numbers, as well as the literals "null", "true" and "false" are simply mapped to their string form.
• Property tree nodes containing both child nodes and data cannot be mapped.

(from the documentation)

You can iterate the array with a helper function.

#include <boost/property_tree/ptree.hpp>
#include <boost/property_tree/json_parser.hpp>

using boost::property_tree::ptree;

template <typename T>
std::vector<T> as_vector(ptree const& pt, ptree::key_type const& key)
{
    std::vector<T> r;
    for (auto& item : pt.get_child(key))
        r.push_back(item.second.get_value<T>());
    return r;
}

int main()
{
    std::stringstream ss("{\"a\": [8, 6, 2], \"b\": [2, 2, 1]}");

    ptree pt;
    read_json(ss, pt);

    for (auto i : as_vector<int>(pt, "a")) std::cout << i << ' ';
    std::cout << '\n';
    for (auto i : as_vector<int>(pt, "b")) std::cout << i << ' ';
}

See it Live On Coliru. Output:

8 6 2 
2 2 1

Solecism

Beautiful solution. Could anyone give the example code using boost_foreach? - I'm not able to use C++11 in my project. – Attaway

Answer 2

Read your a list as follows:

#include <boost/foreach.hpp>
BOOST_FOREACH(boost::property_tree::ptree::value_type &v, pt.get_child("a.")) {
    cout << v.second.data() << endl;
}

Blizzard

That's elegant with the trailing . in the path. +1 from me – Solecism