Creating JSON arrays in Boost using Property Trees

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.