Add std::format and fmt support (#5224)

* ✨ add std::format and fmt support

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* ♻️ reorganize PR

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* 💚 fix build

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* 💚 fix build

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* 💚 fix build

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

Author: nlohmann

docs/mkdocs/docs/api/basic_json/format_as.md

@@ -0,0 +1,94 @@
+# 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
+
+## Version history
+
+- Added in version 3.12.x.

docs/mkdocs/docs/api/basic_json/index.md

@@ -301,13 +301,15 @@ Access to the JSON value
 - [**operator<<(std::ostream&)**](../operator_ltlt.md) - serialize to stream
 - [**operator>>(std::istream&)**](../operator_gtgt.md) - deserialize from stream
 - [**to_string**](to_string.md) - user-defined `to_string` function for JSON values
+- [**format_as**](format_as.md) - user-defined `format_as` function for JSON values (fmt support)
 
 ## Literals
 
 - [**operator""_json**](../operator_literal_json.md) - user-defined string literal for JSON values
 
 ## Helper classes
 
+- [**std::formatter&lt;basic_json&gt;**](std_formatter.md) - make JSON values formattable with `std::format`
 - [**std::hash&lt;basic_json&gt;**](std_hash.md) - return a hash value for a JSON object
 - [**std::swap&lt;basic_json&gt;**](std_swap.md) - exchanges the values of two JSON objects
 

docs/mkdocs/docs/api/basic_json/std_formatter.md

@@ -0,0 +1,56 @@
+# <small>std::</small>formatter<nlohmann::basic_json\>
+
+```cpp
+namespace std {
+    template <>
+    struct formatter<nlohmann::basic_json, char>;
+}
+```
+
+Specialization to make JSON values formattable with [`std::format`](https://en.cppreference.com/w/cpp/utility/format/format)
+(and the other members of C++20's `<format>` header, such as `std::format_to`).
+
+A subset of the [standard format spec grammar](https://en.cppreference.com/w/cpp/utility/format/spec) is
+supported, repurposed for JSON pretty-printing; any other spec component (sign, the `0` flag, precision,
+`L`, a dynamic width such as `#!cpp "{:{}}"`, or a trailing type character) throws
+[`std::format_error`](https://en.cppreference.com/w/cpp/utility/format/format_error):
+
+- `#!cpp "{}"` serializes the value the same way as [`dump()`](dump.md) (compact, no whitespace).
+- `#!cpp "{:#}"` ("alternate form") serializes the value the same way as `#!cpp dump(4)` (pretty-printed
+  with an indent of 4).
+- A width, with or without `#!cpp "#"` (e.g. `#!cpp "{:2}"` or `#!cpp "{:#2}"`), serializes the value the
+  same way as `#!cpp dump(width)` — a width on its own implies pretty-printing, since an indent size has
+  no meaning for compact output.
+- `fill-and-align` (e.g. `#!cpp "{:.>#}"` or `#!cpp "{:.>3}"`) picks a custom indent character, the same
+  way as `#!cpp dump(indent, indent_char)`. The alignment direction itself (`#!cpp '<'`, `#!cpp '>'`,
+  `#!cpp '^'`) has no separate meaning for JSON values — only the fill character before it is used, and
+  any of the three directions is accepted.
+
+This specialization is only available for `#!cpp char`-based JSON values and only if the standard library
+provides `<format>`, controlled by the [`JSON_HAS_STD_FORMAT`](../macros/json_has_std_format.md) macro.
+
+## Examples
+
+??? example
+
+    The example shows how to format JSON values with `std::format`.
+
+    ```cpp
+    --8<-- "examples/std_formatter.c++20.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/std_formatter.c++20.output"
+    ```
+
+## See also
+
+- [dump](dump.md) - serialization
+- [operator<<(std::ostream&)](../operator_ltlt.md) - serialize to stream
+- [format_as](format_as.md) - customization point used by `fmt::format` (fmtlib)
+
+## Version history
+
+- Added in version 3.12.x.

docs/mkdocs/docs/api/macros/index.md

@@ -19,6 +19,7 @@ header. See also the [macro overview page](../../features/macros.md).
 - [**JSON_HAS_CPP_11**<br>**JSON_HAS_CPP_14**<br>**JSON_HAS_CPP_17**<br>**JSON_HAS_CPP_20**](json_has_cpp_11.md) - set supported C++ standard
 - [**JSON_HAS_FILESYSTEM**<br>**JSON_HAS_EXPERIMENTAL_FILESYSTEM**](json_has_filesystem.md) - control `std::filesystem` support
 - [**JSON_HAS_RANGES**](json_has_ranges.md) - control `std::ranges` support
+- [**JSON_HAS_STD_FORMAT**](json_has_std_format.md) - control `std::format`/`std::formatter` support
 - [**JSON_HAS_THREE_WAY_COMPARISON**](json_has_three_way_comparison.md) - control 3-way comparison support
 - [**JSON_NO_IO**](json_no_io.md) - switch off functions relying on certain C++ I/O headers
 - [**JSON_SKIP_UNSUPPORTED_COMPILER_CHECK**](json_skip_unsupported_compiler_check.md) - do not warn about unsupported compilers

docs/mkdocs/docs/api/macros/json_has_std_format.md

@@ -0,0 +1,41 @@
+# JSON_HAS_STD_FORMAT
+
+```cpp
+#define JSON_HAS_STD_FORMAT /* value */
+```
+
+This macro indicates whether the standard library has support for `std::format`/`std::formatter` (that
+is, the `<format>` header). Possible values are `1` when supported or `0` when unsupported.
+
+## Default definition
+
+The default value is detected based on the preprocessor macros `#!cpp JSON_HAS_CPP_20` and
+`#!cpp __cpp_lib_format`.
+
+When the macro is not defined, the library will define it to its default value.
+
+## Notes
+
+!!! info "Enabled functionality"
+
+    When this macro evaluates to `1`, the library provides a
+    [`std::formatter<basic_json>`](../basic_json/std_formatter.md) specialization so JSON values can be
+    used directly with `std::format`.
+
+## Examples
+
+??? example
+
+    The code below forces the library to disable support for `std::format`, even if the standard library
+    would otherwise support it:
+
+    ```cpp
+    #define JSON_HAS_STD_FORMAT 0
+    #include <nlohmann/json.hpp>
+
+    ...
+    ```
+
+## Version history
+
+- Added in version 3.12.x.

docs/mkdocs/docs/examples/format_as.cpp

@@ -0,0 +1,16 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create a JSON value
+    json j = {{"one", 1}, {"two", 2}};
+
+    // format_as() is found via argument-dependent lookup, the same way
+    // fmt::format/fmt::print would find it
+    auto j_str = format_as(j);
+
+    std::cout << j_str << std::endl;
+}

docs/mkdocs/docs/examples/format_as.output

@@ -0,0 +1 @@
+{"one":1,"two":2}

docs/mkdocs/docs/examples/std_formatter.c++20.cpp

@@ -0,0 +1,22 @@
+#include <format>
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    json j = {{"one", 1}, {"two", 2}};
+
+    // compact formatting, like dump()
+    std::cout << std::format("{}", j) << "\n\n";
+
+    // pretty-printed formatting, like dump(4)
+    std::cout << std::format("{:#}", j) << "\n\n";
+
+    // a width sets the indent, like dump(2)
+    std::cout << std::format("{:2}", j) << "\n\n";
+
+    // fill-and-align sets the indent character, like dump(4, '.')
+    std::cout << std::format("{:.>#}", j) << std::endl;
+}

docs/mkdocs/docs/examples/std_formatter.c++20.output

@@ -0,0 +1,16 @@
+{"one":1,"two":2}
+
+{
+    "one": 1,
+    "two": 2
+}
+
+{
+  "one": 1,
+  "two": 2
+}
+
+{
+...."one": 1,
+...."two": 2
+}

docs/mkdocs/docs/home/faq.md

@@ -171,6 +171,31 @@ The library uses `std::numeric_limits<number_float_t>::digits10` (15 for IEEE `d
 
 See [this section](../features/types/number_handling.md#number-serialization) on the library's number handling for more information.
 
+### Using JSON values with `std::format` or `fmt`
+
+!!! question
+
+    - Can I use `std::format("{}", j)` on a JSON value?
+    - Can I use `fmt::format("{}", j)` or `fmt::print("{}", j)` (the [{fmt}](https://github.com/fmtlib/fmt) library) on a JSON value?
+
+`std::format` works out of the box since version 3.12.x, as long as the standard library provides
+`<format>` (see [`JSON_HAS_STD_FORMAT`](../api/macros/json_has_std_format.md)); see
+[`std::formatter<basic_json>`](../api/basic_json/std_formatter.md) for details, including the `#!cpp "{:#}"`
+pretty-print spec, indent widths (`#!cpp "{:2}"`), and custom indent characters (`#!cpp "{:.>#}"`).
+
+For `fmt`, the library ships [`format_as`](../api/basic_json/format_as.md), a small customization point
+`fmt` looks for via argument-dependent lookup. It only has an effect on fmt 10.0.0 through 11.0.2 — from
+fmt 11.1.0 onwards, `fmt` no longer picks up a `format_as` overload that returns a `std::string`. On such
+versions (or any version, if you also want the same `#!cpp "{:#}"`/width/fill-and-align spec support that
+`std::formatter<basic_json>` has), define your own `fmt::formatter` specialization; see
+[`format_as`](../api/basic_json/format_as.md) for a recipe that mirrors it.
+
+If you get ambiguous-overload errors when passing a JSON value to `fmt::format`/`fmt::print` without any
+`fmt::formatter<json>` specialization in scope, that's `fmt` picking up `basic_json`'s implicit
+`operator ValueType()` conversion operator (see [#964](https://github.com/nlohmann/json/issues/964) and
+[#958](https://github.com/nlohmann/json/issues/958)); disabling it via
+[`JSON_USE_IMPLICIT_CONVERSIONS 0`](../api/macros/json_use_implicit_conversions.md) avoids the ambiguity.
+
 ## Compilation issues
 
 ### Android SDK