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api/macros/json_no_automatic_udls.md

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+# JSON_NO_AUTOMATIC_UDLS
+
+```cpp
+#define JSON_NO_AUTOMATIC_UDLS
+```
+
+When defined, `<nlohmann/json.hpp>` does not include `<nlohmann/json_literals.hpp>`, so the user-defined string
+literals [`operator""_json`](../operator_literal_json.md) and
+[`operator""_json_pointer`](../operator_literal_json_pointer.md) are not declared. Include
+`<nlohmann/json_literals.hpp>` in the files that use them.
+
+The literals are ordinary inline functions whose bodies call the parser, so every translation unit that includes them
+instantiates the parser — even if it never parses anything itself. Defining `JSON_NO_AUTOMATIC_UDLS` for a whole project
+avoids this cost in translation units that do not parse (e.g., ones that only define types and conversions or pass
+`json` values around) and reduces their compile time.
+
+## Default definition
+
+By default, `#!cpp JSON_NO_AUTOMATIC_UDLS` is not defined, and `<nlohmann/json.hpp>` includes
+`<nlohmann/json_literals.hpp>`.
+
+```cpp
+#undef JSON_NO_AUTOMATIC_UDLS
+```
+
+## Notes
+
+!!! info "Header `<nlohmann/json_literals.hpp>`"
+
+    The header includes `<nlohmann/json.hpp>` itself and places the literals according to
+    [`JSON_USE_GLOBAL_UDLS`](json_use_global_udls.md). It is part of the multi-header sources (`include/nlohmann`)
+    and of the single-header sources (`single_include/nlohmann`), next to `json.hpp`.
+
+!!! info "C++ modules"
+
+    The `nlohmann.json` [module](../../features/modules.md) always exports the literals, regardless of this macro.
+
+## Examples
+
+??? example
+
+    The code below includes the library without the literals and adds them in a single translation unit.
+
+    ```cpp
+    // compiled with -DJSON_NO_AUTOMATIC_UDLS for the whole project
+    #include <nlohmann/json.hpp>
+
+    // this file uses the literals, so it includes them explicitly
+    #include <nlohmann/json_literals.hpp>
+
+    int main()
+    {
+        auto j = R"({"foo": 42})"_json;
+        return j.at("/foo"_json_pointer) == 42 ? 0 : 1;
+    }
+    ```
+
+    Without the include of `<nlohmann/json_literals.hpp>`, the code would fail to compile.
+
+## See also
+
+- [`operator""_json`](../operator_literal_json.md)
+- [`operator""_json_pointer`](../operator_literal_json_pointer.md)
+- [`JSON_USE_GLOBAL_UDLS`](json_use_global_udls.md) - place user-defined string literals (UDLs) into the global namespace
+
+## Version history
+
+- Added in version 3.13.0.