3.6 KiB
Object Order
The JSON standard defines objects as "an unordered collection of zero or more name/value pairs". As such, an implementation does not need to preserve any specific order of object keys.
Default behavior: sort keys
The default type nlohmann::json uses a std::map to store JSON objects, and thus stores object keys sorted alphabetically.
Example
#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
json j;
j["one"] = 1;
j["two"] = 2;
j["three"] = 3;
std::cout << j.dump(2) << '\n';
}
Output:
{
"one": 1,
"three": 3,
"two": 2
}
Alternative behavior: preserve insertion order
If you do want to preserve the insertion order, you can use the type nlohmann::ordered_json.
Example
#include <iostream>
#include <nlohmann/json.hpp>
using ordered_json = nlohmann::ordered_json;
int main()
{
ordered_json j;
j["one"] = 1;
j["two"] = 2;
j["three"] = 3;
std::cout << j.dump(2) << '\n';
}
Output:
{
"one": 1,
"two": 2,
"three": 3
}
Alternatively, nlohmann::fifo_map also preserves the insertion order and, unlike ordered_map, keeps a lookup index, so it does not have the quadratic cost described below. It is used through a small adapter (integration).
If the order does not matter and you only want faster lookup, boost::unordered_flat_map, absl::flat_hash_map, absl::node_hash_map, and several other hash maps work through an adapter that restores the template argument order basic_json expects; see Template Parameter Requirements. Note these are unordered, not insertion-ordered.
tsl::ordered_map cannot be used: its iterators expose the mapped value as const, while basic_json needs to modify it in place.
The ordered_map behind nlohmann::ordered_json is deliberately minimal and has no lookup index, so every key access is a linear scan and building an object of n keys costs O(n²). This is unnoticeable at typical object sizes but becomes significant for objects with many thousands of keys; see ordered_map complexity. The alternatives above keep a lookup index and do not have this cost.
Notes on parsing
Note that you also need to call the right parse function when reading from a file. Assume file input.json contains the JSON object above:
{
"one": 1,
"two": 2,
"three": 3
}
Right way
The following code correctly calls the parse function from nlohmann::ordered_json:
std::ifstream i("input.json");
auto j = nlohmann::ordered_json::parse(i);
std::cout << j.dump(2) << std::endl;
The output will be:
{
"one": 1,
"two": 2,
"three": 3
}
Wrong way
The following code incorrectly calls the parse function from nlohmann::json which does not preserve the insertion order, but sorts object keys. Assigning the result to nlohmann::ordered_json compiles, but does not restore the order from the input file.
std::ifstream i("input.json");
nlohmann::ordered_json j = nlohmann::json::parse(i);
std::cout << j.dump(2) << std::endl;
The output will be:
{
"one": 1,
"three": 3,
"two": 2
}