JSON to C++ Struct Generator (nlohmann/json)

About the JSON to C++

Generate a C++17 header with structs and from_json/to_json functions for the popular nlohmann/json library. Nullable and optional fields use std::optional and arrays use std::vector.

Key features

• C++17 header with structs for each JSON object
• from_json / to_json functions for nlohmann/json
• std::optional for nullable and optional fields
• std::vector for arrays, int64_t and double for numbers
• Value-initialized members ({}), so no garbage values
• Optional namespace; C++ keywords avoided

C++ structs for nlohmann/json

C++ has no built-in reflection, so mapping JSON to structs means writing serialization code by hand. Paste a sample and get a complete header: structs plus from_json/to_json functions for nlohmann/json.

Example

[
  { "user_id": 7, "name": "Ada", "nickname": "ada", "score": 4.5 },
  { "user_id": 8, "name": "Alan", "nickname": null, "score": 3 }
]

struct User {
    int64_t user_id{};
    std::string name{};
    std::optional<std::string> nickname{};
    double score{};
};

inline void from_json(const nlohmann::json& j, User& x) {
    j.at("user_id").get_to(x.user_id);
    j.at("name").get_to(x.name);
    if (j.contains("nickname") && !j.at("nickname").is_null()) x.nickname = j.at("nickname").get<std::string>();
    j.at("score").get_to(x.score);
}

auto users = nlohmann::json::parse(text).get<std::vector<User>>();

Type mapping

Tips

• Errors: j.at() throws nlohmann::json::out_of_range when a required key is missing, so wrap parsing in try/catch.
• Performance: for very large documents, consider the SAX interface or simdjson for read-only access.
• Header-only: everything is inline, so the header can be included from multiple translation units.

Related tools

Prefer Rust? Use JSON to Rust. Need a schema for gRPC? Try JSON to Protobuf.

How to use the JSON to C++

1. Paste your data into the input box, upload a file, or click Sample to try it out.
2. Adjust the options above the editor if you need to (indentation, delimiter, and so on).
3. The C++ appears instantly in the output panel. Any errors are shown with their line and column.
4. Click Copy to copy the result, or Download to save it as a file.