The json class provides an API for manipulating a JSON value. To create a json object by reading a JSON file:
#include <fstream>
#include <json.h>
using json = hades::json;
std::ifstream f("example.json");
json data(f);The json class provides an API for writing a JSON value. To write a json object to a JSON file:
#include <fstream>
#include <json.h>
using json = hades::json;
json j(std::ifstream("in.json"));
j.to_file("out.json");Assume you want to create hard-code this literal JSON value in a file, as a json object:
{
"pi": 3.141,
"happy": true
}There are various options:
// Using (raw) string literals and json::parse
json ex1 = json::parse(R"(
{
"pi": 3.141,
"happy": true
}
)");
// Using user-defined (raw) string literals
using namespace nlohmann::literals;
json ex2 = R"(
{
"pi": 3.141,
"happy": true
}
)"_json;
// Using initializer lists
json ex3 = {
{"happy", true},
{"pi", 3.141},
};Here are some examples to give you an idea how to use the class.
Assume you want to create the JSON object
{
"pi": 3.141,
"happy": true,
"name": "Niels",
"nothing": null,
"answer": {
"everything": 42
},
"list": [1, 0, 2],
"object": {
"currency": "USD",
"value": 42.99
}
}With this library, you could write:
// create an empty structure (null)
json j;
// add a number that is stored as double (note the implicit conversion of j to an object)
j["pi"] = 3.141;
// add a Boolean that is stored as bool
j["happy"] = true;
// add a string that is stored as std::string
j["name"] = "Niels";
// add another null object by passing nullptr
j["nothing"] = nullptr;
// add an object inside the object
j["answer"]["everything"] = 42;
// add an array that is stored as std::vector (using an initializer list)
j["list"] = { 1, 0, 2 };
// add another object (using an initializer list of pairs)
j["object"] = { {"currency", "USD"}, {"value", 42.99} };
// instead, you could also write (which looks very similar to the JSON above)
json j2 = {
{"pi", 3.141},
{"happy", true},
{"name", "Niels"},
{"nothing", nullptr},
{"answer", {
{"everything", 42}
}},
{"list", {1, 0, 2}},
{"object", {
{"currency", "USD"},
{"value", 42.99}
}}
};You can create a JSON value (deserialization) by appending _json to a string literal:
// create object from string literal
json j = "{ \"happy\": true, \"pi\": 3.141 }"_json;
// or even nicer with a raw string literal
auto j2 = R"(
{
"happy": true,
"pi": 3.141
}
)"_json;Note that without appending the _json suffix, the passed string literal is not parsed, but just used as JSON string
value. That is, json j = "{ \"happy\": true, \"pi\": 3.141 }" would just store the string
"{ "happy": true, "pi": 3.141 }" rather than parsing the actual object.
You can know more about the raw value using the helper functions provided by the json object
json j = {10.2, 23, 32};
j.size() // 3
j.push_back({2,3,4}) // [10.2, 23, 32, [2, 3, 4]]
j.describe() // Array[ Number ]
j.str() // [10.2, 23, 32, [2, 3, 4]] You can retrieve raw values by using get<>() value on a json object :
json j = {
{"number", 32.4},
{"arr", {
true, false, true
}},
{"str": "str"}
};
j["number"].get<double>() // 32.4
j["arr"][2].get<bool>() // true
j["str"].get<std::string>() // str
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