json/docs/mkdocs/docs/features/modules.md

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.

Importing the module instead of including the header reduces the compile time of every translation unit that uses the library, at the price of building the module once. The Compile times page has measurements and the one-off cost.

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 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::basic_json_document
• nlohmann::basic_json_view
• nlohmann::json
• nlohmann::json_document
• nlohmann::json_pointer
• nlohmann::json_view
• nlohmann::ordered_json
• nlohmann::ordered_json_document
• nlohmann::ordered_json_view
• nlohmann::ordered_map
• nlohmann::to_string
• nlohmann::literals::json_literals::operator""_json
• nlohmann::literals::json_literals::operator""_json_pointer

The module always exports the two user-defined string literals, even if JSON_NO_AUTOMATIC_UDLS is defined when building it.

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.

!!! info "Known compiler issues"

- **GCC** may emit "redefinition" errors when `#include <nlohmann/json.hpp>` appears in a module preamble together with other imports. This is an upstream GCC bug, not yet resolved as of GCC 16. Workarounds: include `nlohmann/json.hpp` before other `#include`s, use `import nlohmann.json;` instead, or upgrade GCC. ([issue #5103](https://github.com/nlohmann/json/issues/5103))
- **MSVC** could fail with `C2039: 'json_sax_dom_callback_parser' is not a member of ... detail`; fixed by exporting the required internal symbols from `json.cppm` (see [Exported symbols](#exported-symbols) above). ([issue #3970](https://github.com/nlohmann/json/issues/3970))

If you hit a different module-related build failure, search [existing issues](https://github.com/nlohmann/json/issues?q=is%3Aissue+modules) before filing a new one.

See also

• Supported Macros - macros are not exported by modules
• Macros - reference of all macros
• nlohmann Namespace - the library namespace
• CMake - enable the module build in CMake
• Compile times - reduce compile times
• basic_json - the main class and all its member functions