Creating JSON arrays in Boost using Property Trees

I'm trying to create a JSON array using boost property trees.

The documentation says: "JSON arrays are mapped to nodes. Each element is a child node with an empty name."

So I'd like to create a property tree with empty names, then call write_json(...) to get the array out. However, the documentation doesn't tell me how to create unnamed child nodes. I tried ptree.add_child("", value), but this yields:

Assertion `!p.empty() && "Empty path not allowed for put_child."' failed

The documentation doesn't seem to address this point, at least not in any way I can figure out. Can anyone help?

Simple Array:

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

ptree pt;
ptree children;
ptree child1, child2, child3;

child1.put("", 1);
child2.put("", 2);
child3.put("", 3);

children.push_back(std::make_pair("", child1));
children.push_back(std::make_pair("", child2));
children.push_back(std::make_pair("", child3));

pt.add_child("MyArray", children);

write_json("test1.json", pt);

results in:

{
    "MyArray":
    [
        "1",
        "2",
        "3"
    ]
}

Array over Objects:

ptree pt;
ptree children;
ptree child1, child2, child3;


child1.put("childkeyA", 1);
child1.put("childkeyB", 2);

child2.put("childkeyA", 3);
child2.put("childkeyB", 4);

child3.put("childkeyA", 5);
child3.put("childkeyB", 6);

children.push_back(std::make_pair("", child1));
children.push_back(std::make_pair("", child2));
children.push_back(std::make_pair("", child3));

pt.put("testkey", "testvalue");
pt.add_child("MyArray", children);

write_json("test2.json", pt);

results in:

{
    "testkey": "testvalue",
    "MyArray":
    [
        {
            "childkeyA": "1",
            "childkeyB": "2"
        },
        {
            "childkeyA": "3",
            "childkeyB": "4"
        },
        {
            "childkeyA": "5",
            "childkeyB": "6"
        }
    ]
}

hope this helps

What you need to do is this piece of fun. This is from memory, but something like this works for me.

boost::property_tree::ptree root;
boost::property_tree::ptree child1;
boost::property_tree::ptree child2;

// .. fill in children here with what you want
// ...

ptree.push_back( std::make_pair("", child1 ) );
ptree.push_back( std::make_pair("", child2 ) );

But watch out there's several bugs in the json parsing and writing. Several of which I've submitted bug reports for - with no response :(

EDIT: to address concern about it serializing incorrectly as {"":"","":""}

This only happens when the array is the root element. The boost ptree writer treats all root elements as objects - never arrays or values. This is caused by the following line in boost/propert_tree/detail/json_parser_writer.hpp

else if (indent > 0 && pt.count(Str()) == pt.size())

Getting rid of the "indent > 0 &&" will allow it to write arrays correctly.

If you don't like how much space is produced you can use the patch I've provided here

When starting to use Property Tree to represent a JSON structure I encountered similar problems which I did not resolve. Also note that from the documentation, the property tree does not fully support type information:

> 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.

After learning this, I switched to the more complete JSON implementation JSON Spirit. This library uses Boost Spirit for the JSON grammar implementation and fully supports JSON including arrays.

I suggest you use an alternative C++ JSON implementation.

In my case I wanted to add an array to a more or less arbitrary location, so, like Michael's answer, create a child tree and populate it with array elements:

using boost::property_tree::ptree;

ptree targetTree;
ptree arrayChild;
ptree arrayElement;

//add array elements as desired, loop, whatever, for example
for(int i = 0; i < 3; i++)
{
  arrayElement.put_value(i);
  arrayChild.push_back(std::make_pair("",arrayElement))
}

When the child has been populated, use the put_child() or add_child() function to add the entire child tree to the target tree, like this...

targetTree.put_child(ptree::path_type("target.path.to.array"),arrayChild)

the put_child function takes a path and a tree for an argument and will "graft" arrayChild into targetTree

As of boost 1.60.0, problem persists.

Offering a Python 3 workaround (Gist), which can be syscalled just after boost::property_tree::write_json.

#!/usr/bin/env python3


def lex_leaf(lf: str):
    if lf.isdecimal():
        return int(lf)
    elif lf in ['True', 'true']:
        return True
    elif lf in ['False', 'false']:
        return False
    else:
        try:
            return float(lf)
        except ValueError:
            return lf

def lex_tree(j):
    tj = type(j)
    if tj == dict:
        for k, v in j.items():
            j[k] = lex_tree(v)
    elif tj == list:
        j = [lex_tree(l) for l in j]
    elif tj == str:
        j = lex_leaf(j)
    else:
        j = lex_leaf(j)
    return j


def lex_file(fn: str):
    import json
    with open(fn, "r") as fp:
        ji = json.load(fp)
    jo = lex_tree(ji)
    with open(fn, 'w') as fp:
        json.dump(jo, fp)


if __name__ == '__main__':
    import sys
    lex_file(sys.argv[1])

If you want JSON in C++, there's no need for Boost. With this library you can get JSON as a first class data type that behaves like an STL container.

// Create JSON on the fly.
json j2 = {
  {"pi", 3.141},
  {"happy", true},
  {"name", "Niels"},
  {"nothing", nullptr},
  {"answer", {
    {"everything", 42}
  }},
  {"list", {1, 0, 2}},
  {"object", {
    {"currency", "USD"},
    {"value", 42.99}
  }}
};

// Or treat is as an STL container; create an array using push_back
json j;
j.push_back("foo");
j.push_back(1);
j.push_back(true);

// also use emplace_back
j.emplace_back(1.78);

// iterate the array
for (json::iterator it = j.begin(); it != j.end(); ++it) {
  std::cout << *it << '\n';
}

Confused with the official document and the above answers. Below is what I understand.

Property Tree consists of nodes.
Each node is like below

 struct ptree
    {
       map<key_name,value>          data;
       vector<pair<key_name,ptree>> children; 
    };

To put 'value' into data with 'put'
To put 'node' into children with 'push_back'\

// Write 
	bt::ptree root;
	bt::ptree active;
	bt::ptree requested;
	bt::ptree n1, n2, n3;
		
	n1.put("name", "Mark");
	n1.put("age", 20);
	n1.put("job", "aaa");

	n2.put("name", "Rosie");
	n2.put("age", "19");
	n2.put("job", "bbb");

	n3.put("name", "sunwoo");
	n3.put("age", "10");
	n3.put("job", "ccc");

	active.push_back   ({ "",l1 });
	active.push_back   ({ "",l2 });
	requested.push_back({ "",l3 });	
	root.push_back     ({"active", active});
	root.push_back     ({"requested", requested});

	bt::write_json("E:\\1.json", root);


	// READ
	bt::ptree root2;
	bt::ptree active2;
	bt::ptree requested2;
	bt::ptree r1, r2, r3;

	bt::read_json("E:\\1.json", root2);

    // loop children
	for (auto& [k,n] : root.get_child("active"))
	{		
		cout << n.get<string>("name", "unknown");
		cout << n.get<int>   ("age" , 11);
		cout << n.get<string>("job" , "man");
		cout << endl << flush;
	}