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api/basic_json/format_as.md

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+# format_as(basic_json)
+
+```cpp
+template <typename BasicJsonType>
+std::string format_as(const BasicJsonType& j);
+```
+
+This function implements the [`format_as`](https://fmt.dev/latest/api/#formatting-user-defined-types)
+customization point used by the [{fmt}](https://github.com/fmtlib/fmt) library (fmtlib). It has no
+dependency on any `fmt` header and no effect at all unless a caller's translation unit also includes
+`fmt` and calls `fmt::format`/`fmt::print` on a JSON value.
+
+## Template parameters
+
+`BasicJsonType`
+:   a specialization of [`basic_json`](index.md)
+
+## Return value
+
+string containing the serialization of the JSON value (same as [`dump()`](dump.md))
+
+## Exception safety
+
+Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
+
+## Exceptions
+
+Throws [`type_error.316`](../../home/exceptions.md#jsonexceptiontype_error316) if a string stored inside the JSON value
+is not UTF-8 encoded
+
+## Complexity
+
+Linear.
+
+## Possible implementation
+
+```cpp
+template <typename BasicJsonType>
+std::string format_as(const BasicJsonType& j)
+{
+    return j.dump();
+}
+```
+
+## Notes
+
+!!! warning "Version-dependent effect on fmt"
+
+    `fmt` only picks up a `format_as` overload that returns a `std::string` in fmt **10.0.0 through
+    11.0.2**. Starting with fmt **11.1.0**, `fmt` restricts automatic `format_as` pickup to overloads that
+    return an arithmetic type, so this function has no effect there (it is simply unused, not a compile
+    error).
+
+    If you use fmt \>= 11.1.0, or want the same pretty-print spec support that
+    [`std::formatter<basic_json>`](std_formatter.md) has (`#!cpp "{:#}"`, a width to set the indent such
+    as `#!cpp "{:2}"`/`#!cpp "{:#2}"`, and fill-and-align to pick the indent character such as
+    `#!cpp "{:.>#}"`), define your own `fmt::formatter` specialization mirroring the same logic:
+
+    ```cpp
+    --8<-- "../../../tests/fmt_formatter/project/main.cpp:formatter_recipe"
+    ```
+
+    This recipe isn't shipped by the library itself, since doing so would make `fmt` a build dependency
+    (see the FAQ entry on
+    [using JSON values with `std::format` or `fmt`](../../home/faq.md#using-json-values-with-stdformat-or-fmt)
+    for more background) — but it *is* compiled and exercised against a real, current `fmt` release as
+    part of the library's own test suite (`tests/fmt_formatter`, via CMake `FetchContent`), so it's kept in
+    sync with `std::formatter<basic_json>` and verified to actually work, not just illustrative.
+
+## Examples
+
+??? example
+
+    The following code shows how the library's `format_as()` function integrates with `fmt::format`,
+    allowing argument-dependent lookup.
+
+    ```cpp
+    --8<-- "examples/format_as.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/format_as.output"
+    ```
+
+## See also
+
+- [dump](dump.md)
+- [std::formatter<basic_json>](std_formatter.md) - the `std::format` (C++20) equivalent
+- [Serialization](../../features/serialization.md) - the serialization article
+
+## Version history
+
+- Added in version 3.12.x.