Modules¶

This library has experimental support for C++ modules, introduced in C++20. The library can be imported by writing import nlohmann.json; instead of #include <nlohmann/json.hpp>.

Please be aware that the module is experimental and a full test is outstanding, and the exported symbols are subject to change.

Requirements¶

The nlohmann.json module requires that the build system is configured to build and resolve modules when imported. Obviously, as modules were introduced in C++20, this feature can only be used in C++20 and subsequent versions.

To enable building the nlohmann.json module (which is not done by default), the macro NLOHMANN_JSON_BUILD_MODULES must be passed to the build system.

Example¶

When using modules rather than headers, the previous example for creating a json object through a JSON file, would instead be:

import std;
import nlohmann.json;

using json = nlohmann::json;

// ...

std::ifstream f("example.json");
json data = json::parse(f);

Modules do not export macros¶

It should be noted that as modules do not export macros, the nlohmann.json module will not export any macros.

Exported symbols¶

Only the following symbols are exported from nlohmann.json:

• nlohmann::adl_serializer
• nlohmann::basic_json
• nlohmann::json
• nlohmann::json_pointer
• nlohmann::ordered_json
• nlohmann::ordered_map
• nlohmann::to_string
• nlohmann::literals::json_literals::operator""_json
• nlohmann::literals::json_literals::operator""_json_pointer

Additionally, the following nlohmann::detail symbols are exported, solely to work around an MSVC compilation issue (#3970). They are implementation details, not part of the public API, and should not be used directly:

• nlohmann::detail::json_sax_dom_callback_parser
• nlohmann::detail::unknown_size

Known issues¶

C++20 modules support is exercised in CI against current GCC and Clang on Ubuntu, and the default MSVC toolset on Windows Server 2022 — there is no documented minimum compiler version, unlike feature-test-macro-gated features such as JSON_HAS_RANGES.