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nlohmann
60 lines
3.0 KiB
Markdown
60 lines
3.0 KiB
Markdown
# Modules
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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>`.
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Please be aware that the module is experimental and a full test is outstanding, and the exported symbols are subject to change.
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## Requirements
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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.
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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.
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## Example
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When using modules rather than headers, the previous example for creating a `json` object through a JSON file, would instead be:
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```cpp
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import std;
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import nlohmann.json;
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using json = nlohmann::json;
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// ...
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std::ifstream f("example.json");
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json data = json::parse(f);
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```
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## Modules do not export macros
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It should be noted that as modules do not export macros, the `nlohmann.json` module will not export any macros.
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## Exported symbols
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Only the following symbols are exported from `nlohmann.json`:
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- `nlohmann::adl_serializer`
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- `nlohmann::basic_json`
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- `nlohmann::json`
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- `nlohmann::json_pointer`
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- `nlohmann::ordered_json`
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- `nlohmann::ordered_map`
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- `nlohmann::to_string`
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- `nlohmann::literals::json_literals::operator""_json`
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- `nlohmann::literals::json_literals::operator""_json_pointer`
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Additionally, the following `nlohmann::detail` symbols are exported, solely to work around an MSVC compilation issue
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([#3970](https://github.com/nlohmann/json/issues/3970)). They are implementation details, not part of the public API,
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and should not be used directly:
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- `nlohmann::detail::json_sax_dom_callback_parser`
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- `nlohmann::detail::unknown_size`
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## Known issues
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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`](../api/macros/json_has_ranges.md).
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!!! info "Known compiler issues"
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- **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))
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- **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))
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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.
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