c++, JSON array parsing using nlohmann::json

Question

In a json file imported into my c++ program there is such structure as:

{
 "a":"1",
 "ec":[
       {
        "a":"0x00",
        "s":[
             {"a":"0xA0"},
             {"a":"0xA1"},
            ],
        "b":"v1"
       },
       {
        "a":"0x01",
        "s":[
             {"a":"0xB0"},
             {"a":"0xB1"},
            ],
        "b":"v2"
       }
      ]
}

I want to iterate over the "ec" array and get the values of all "a" and for each "a" the same for its s array

vector<string> ec_a; // should contain {"0x00","0x01"}
vector<string> a1_s; // should contain {"0xA0", "0xA1"}
vector<string> a2_s; // should contain {"0xB0","0xB1"}

first I get the size of ec but from the docs I understood should use iterator for the rest

int n=j["ec"].size() // n = 2
for(auto it=j["ec"].begin();it!=j["ec"].end();++it){
  if(it.key() == "a") ec_a.push_back(it.value());
}

but get this exception nlohmann::detail::invalid_iterator at memory location I assume that j["ec"].begin() is incorrect.

how should I do that, Thanks.

score 7 · Answer 1 · answered Apr 22 '19 at 11:20

it is an iterator to a compound type. A compound type doesn't, by itself, have a "key."

What you are trying to achieve is much easier than you may have thought. You can try this:

std::vector<std::string> ec_a;
for (auto& elem : j["ec"])
    ec_a.push_back(elem["a"]);

(live demo)

c++, JSON array parsing using nlohmann::json

1 Answers1