I want to use boost to read data from a JSON string that might look something like:
{
"version": 2,
"data_type": "alpr_results",
"epoch_time": 1534253281728,
"img_width": 1920,
"img_height": 1080,
"processing_time_ms": 271.27728271484,
"uuid": "",
"error": false,
"regions_of_interest": [
{
"x": 0,
"y": 0,
"width": 1920,
"height": 1080
}
],
"results": [
{
"plate": "FRJ7248",
"confidence": 94.583724975586,
"matches_template": 1,
"plate_index": 0,
"region": "us-ny",
"region_confidence": 88,
"processing_time_ms": 94.328330993652,
"requested_topn": 25,
"coordinates": [
{
"x": 1545,
"y": 233
},
{
"x": 1640,
"y": 241
},
{
"x": 1635,
"y": 284
},
{
"x": 1543,
"y": 276
}
],
"vehicle_region": {
"x": 1388,
"y": 0,
"width": 403,
"height": 403
},
"candidates": [
{
"plate": "FRJ7248",
"confidence": 94.583724975586,
"matches_template": 1
},
{
"plate": "FRJ724",
"confidence": 81.629249572754,
"matches_template": 1
},
{
"plate": "FR7248",
"confidence": 81.344184875488,
"matches_template": 0
},
{
"plate": "FJ7248",
"confidence": 81.199394226074,
"matches_template": 0
},
{
"plate": "RJ7248",
"confidence": 81.19913482666,
"matches_template": 0
},
{
"plate": "FRJ7246",
"confidence": 81.167663574219,
"matches_template": 1
},
{
"plate": "FRJ7240",
"confidence": 81.13761138916,
"matches_template": 1
},
{
"plate": "FRJ7243",
"confidence": 81.127967834473,
"matches_template": 1
},
{
"plate": "FRJ7245",
"confidence": 81.124664306641,
"matches_template": 1
},
{
"plate": "FRU7248",
"confidence": 81.089027404785,
"matches_template": 1
},
{
"plate": "FBJ7248",
"confidence": 81.082328796387,
"matches_template": 1
},
{
"plate": "FRD7248",
"confidence": 81.080703735352,
"matches_template": 1
},
{
"plate": "ERJ7248",
"confidence": 81.068466186523,
"matches_template": 1
},
{
"plate": "FR724",
"confidence": 68.389724731445,
"matches_template": 0
},
{
"plate": "FJ724",
"confidence": 68.244926452637,
"matches_template": 0
},
{
"plate": "RJ724",
"confidence": 68.244659423828,
"matches_template": 0
},
{
"plate": "FRU724",
"confidence": 68.134559631348,
"matches_template": 1
},
{
"plate": "FBJ724",
"confidence": 68.127861022949,
"matches_template": 1
},
{
"plate": "FRD724",
"confidence": 68.126243591309,
"matches_template": 1
},
{
"plate": "ERJ724",
"confidence": 68.113998413086,
"matches_template": 1
},
{
"plate": "F7248",
"confidence": 67.959869384766,
"matches_template": 0
},
{
"plate": "R7248",
"confidence": 67.959594726562,
"matches_template": 0
},
{
"plate": "FR7246",
"confidence": 67.92813873291,
"matches_template": 0
},
{
"plate": "FR7240",
"confidence": 67.898078918457,
"matches_template": 0
},
{
"plate": "FR7243",
"confidence": 67.888442993164,
"matches_template": 0
}
]
},
{
"plate": "T701486C",
"confidence": 93.531463623047,
"matches_template": 0,
"plate_index": 1,
"region": "us-ny",
"region_confidence": 84,
"processing_time_ms": 94.328330993652,
"requested_topn": 25,
"coordinates": [
{
"x": 1005,
"y": 407
},
{
"x": 1101,
"y": 400
},
{
"x": 1102,
"y": 454
},
{
"x": 1007,
"y": 462
}
],
"vehicle_region": {
"x": 782,
"y": 69,
"width": 543,
"height": 543
},
"candidates": [
{
"plate": "T701486C",
"confidence": 93.531463623047,
"matches_template": 0
},
{
"plate": "T70T486C",
"confidence": 82.387817382812,
"matches_template": 0
},
{
"plate": "T70486C",
"confidence": 82.198287963867,
"matches_template": 0
},
{
"plate": "T70I486C",
"confidence": 82.151023864746,
"matches_template": 0
},
{
"plate": "701486C",
"confidence": 82.046699523926,
"matches_template": 0
},
{
"plate": "T70146C",
"confidence": 81.921669006348,
"matches_template": 0
},
{
"plate": "7701486C",
"confidence": 81.844429016113,
"matches_template": 0
},
{
"plate": "1701486C",
"confidence": 81.822814941406,
"matches_template": 0
},
{
"plate": "T701466C",
"confidence": 81.808143615723,
"matches_template": 0
},
{
"plate": "70T486C",
"confidence": 70.903053283691,
"matches_template": 0
},
{
"plate": "T70T46C",
"confidence": 70.778022766113,
"matches_template": 0
},
{
"plate": "70486C",
"confidence": 70.713523864746,
"matches_template": 0
},
{
"plate": "770T486C",
"confidence": 70.700782775879,
"matches_template": 0
},
{
"plate": "170T486C",
"confidence": 70.679168701172,
"matches_template": 0
},
{
"plate": "70I486C",
"confidence": 70.666259765625,
"matches_template": 0
},
{
"plate": "T70T466C",
"confidence": 70.664497375488,
"matches_template": 0
},
{
"plate": "T7046C",
"confidence": 70.588493347168,
"matches_template": 0
},
{
"plate": "T70I46C",
"confidence": 70.541229248047,
"matches_template": 0
},
{
"plate": "770486C",
"confidence": 70.511253356934,
"matches_template": 0
},
{
"plate": "170486C",
"confidence": 70.489639282227,
"matches_template": 0
},
{
"plate": "T70466C",
"confidence": 70.474967956543,
"matches_template": 0
},
{
"plate": "770I486C",
"confidence": 70.463996887207,
"matches_template": 0
},
{
"plate": "170I486C",
"confidence": 70.442375183105,
"matches_template": 0
},
{
"plate": "70146C",
"confidence": 70.436904907227,
"matches_template": 0
},
{
"plate": "T70I466C",
"confidence": 70.427703857422,
"matches_template": 0
}
]
}
]
}
All I'm interested in for now is the "plate" in "results", i.e. FRJ7248 and T701486C.
So far, I've got this:
int alpr_json_decode(std::string json)
{
try
{
std::stringstream ss;
ss << json;
boost::property_tree::ptree pt;
boost::property_tree::read_json(ss,pt);
BOOST_FOREACH(boost::property_tree::ptree::value_type &v, pt.get_child("results.plate")) {
assert(v.first.empty());
std::cout << v.second.data() << std::endl;
}
}
catch(std::exception const& e)
{
std::cerr <<e.what() << std::endl;
}
return -1;
}
And I get No such node (results.plate). Why, what am I doing wrong here?
I've also tried .get_child("results") which returns nothing.
results is array of objects, so according to reference
JSON arrays are mapped to nodes. Each element is a child node with an empty name.
your json is translated into
results
{
""
{
plate FRJ7248
...
}
""
{
plate T701486C
}
}
you should read all objects of list by pt.get_child("results") and then for each subtree you can call v.second.get<string>("XXX") to read member of object.
Test code:
BOOST_FOREACH(boost::property_tree::ptree::value_type &v, pt.get_child("results")) {
cout << v.second.get<string>("plate") << endl;
cout << v.second.get<string>("confidence") << endl;
}