Creating JSON arrays in Boost using Property Trees - c++

Question

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?

Answer 1

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"
}
]
}

Answer 2

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

Answer 3

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.

Answer 4

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

Answer 5

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])

Answer 6

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';
}

Answer 7

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;
}