JSON parsing using nlohmann json - c++

Question

I am trying to parse the JSON structure using nlohmann's json.hpp
. But I am not to create the JSON structure from the string. I have tried all the way, but still it is failing.
My requirement is to:
1) Create the JSON structure from the string.
2) Find the value of "statusCode" from it.
After trying for so long time, I am really getting doubt, does nlohmann's json parser support nested JSON or not.
#include "json.hpp"
using namespace std;
int main(){
// giving error 1
nlohmann::json strjson = nlohmann::json::parse({"statusResp":{"statusCode":"S001","message":"Registration Success","snStatus":"Active","warrantyStart":"00000000","warrantyEnd":"00000000","companyBPID":"0002210887","siteBPID":"0002210888","contractStart":"00000000","contractEnd":"00000000"}});
// Giving error 2:
auto j= "{
"statusResp": {
"statusCode": "S001",
"message": "Registration Success",
"snStatus": "Active",
"warrantyStart": "20170601",
"warrantyEnd": "20270601",
"companyBPID": "0002210887",
"siteBPID": "0002210888",
"contractStart": "00000000",
"contractEnd": "00000000"
}
}"_json;
// I actually want to get the value of "statusCode" code from the JSOn structure. But no idea how to parse the nested value.
return 1;
}
Below are the error for both the initialisations:
//ERROR 1:
test.cpp: In function 'int main()':
test.cpp:17:65: error: expected '}' before ':' token
nlohmann::json strjson = nlohmann::json::parse({"statusResp":{"statusCode":"S001","message":"Registration Success","snStatus":"Active","warrantyStart":"00000000","warrantyEnd":"00000000","companyBPID":"0002210887","siteBPID":"0002210888","contractStart":"00000000","contractEnd":"00000000"}});
// ERROR 2:
hemanty#sLinux:/u/hemanty/workspaces/avac/cb-product/mgmt/framework/src/lib/libcurl_cpp$g++ test.cpp -std=gnu++11
test.cpp: In function 'int main()':
test.cpp:27:17: error: expected '}' before ':' token
"statusResp": {

Answer 1

Since " is the character to begin and end a string literal you can not have a " character inside a string without putting a \ before it.
std::string str = " "statusCode":"5001" "; //This does not work
std::string str = " \"statusCode\":\"5001\" "; //This will work
An easier alternative when you want to make strings with a lot of " in them is to use the R"" string literal. Then you can write it like so.
std::string str = R"("statusCode":"5001")";
If we now transfear this to your json example, the correct way to parse the strings would be one of the following.
auto j3 = json::parse("{ \"happy\": true, \"pi\": 3.141 }");
// and below the equivalent with raw string literal
auto j3 = json::parse(R"({"happy": true, "pi": 3.141 })");
//Here we use the `_json` suffix
auto j2 = "
{
\"happy\": true,
\"pi\": 3.141
}"_json;
// Here we combine the R"" with _json suffix to do the same thing.
auto j2 = R"(
{
"happy": true,
"pi": 3.141
}
)"_json;
Examples taken from the readme

Answer 2

If this is what you need:
std::string ss= R"(
{
"test-data":
[
{
"name": "tom",
"age": 11
},
{
"name": "jane",
"age": 12
}
]
}
)";
json myjson = json::parse(ss);
auto &students = myjson["test-data"];
for(auto &student : students) {
cout << "name=" << student["name"].get<std::string>() << endl;
}
Or:
json myjson = { {"name", "tom"}, {"age", 11} };
cout << "name=" << myjson["name"].get<std::string>() << endl;