read json message with boost json - c++

Question

I'm trying to use boost json with property trees to decode a json message. I'm only interested about checking whether "mykey" is in the message and, if that is the case, get the corresponding values.
I'm a little lost in boost documentation, and I was trying to see what the actual code would be to parse a message such as the one below.
{
// some values
"mykey": [
{
"var1": "value1_str",
"var2" : "value2"
}
]
// some other values
}

Answer 1

I don't know about Boost ptree for JSON. I've tried it but it seemed... very clunky.
Here's a simple JSON parser based on the RFC, made in Spirit: https://github.com/sehe/spirit-v2-json/tree/q21356666
You could use it for your use case like test.cpp
#include <vector>
#include "json.hpp"
struct X {
static X from_json(JSON::Value const& v);
std::string var1;
double var2;
};
int main()
{
auto doc = as_object(JSON::parse(
"{\n"
" // some values\n"
" \"mykey\": [\n"
" {\n"
" \"var1\": \"value1_str\",\n"
" \"var2\" : 3.14\n"
" }\n"
" ]\n"
" // some other values\n"
"}\n"
));
if (doc.has_key("mykey"))
{
X data = X::from_json(doc["mykey"]);
std::cout << "X.var1: " << data.var1 << "\n";
std::cout << "X.var2: " << data.var2 << "\n";
}
std::cout << "doc: " << doc << "\n";
std::cout << "doc[\"mykey\"]: " << doc["mykey"] << "\n";
}
X X::from_json(JSON::Value const& v)
{
X result;
auto& o = as_object(as_array(v)[0]);
result.var1 = as_string(o["var1"]);
result.var2 = as_double(o["var2"]);
return result;
}
Output:
X.var1: value1_str
X.var2: 3.14
doc: {"mykey":[{"var1":"value1_str","var2":3.14}]}
doc["mykey"]: [{"var1":"value1_str","var2":3.14}]
There are other json libraries around. I suggest you select one to suit your needs.