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Niels Lohmann
521a084827
* 📝 Fix documentation gaps for 3.13.0 release (todos 138-142) - Todo 138: Add "Known issues" section to modules.md with compiler-specific troubleshooting (GCC redefinition, MSVC symbol export). Add pointer note to quality_assurance.md. - Todo 139: Document CBOR/MessagePack half-precision float encoding for NaN/Infinity (0xF9/0xCA with exact byte sequences). Explain pre-3.13.0 double-precision bug mechanism without issue citations. - Todo 140: Document CBOR negative-integer-overflow rejection (parse_error.112) for magnitudes exceeding int64_t range (already implemented in rev 1). - Todo 141: Update version history in value.md and operator[].md with behavior-change details, removing issue citations per citation policy (prose is self-contained). - Todo 142: Global sed replace of 3.12.x → 3.13.0 placeholder across all 20 documentation files. Revision 2 incorporates feedback to reduce changelog-like issue citations. Only citations that add unique troubleshooting value are retained (#5103 for GCC workaround, #3970 for MSVC symbol export). "Known issues" section follows PR #5252's visual pattern (info admonition with bold-bullet format). Signed-off-by: Niels Lohmann <mail@nlohmann.me> * 📝 Document integer type selection, type_name() invalid value, and std::optional get() fix - number_handling.md: clarify that positive/negative integers select unsigned/signed storage based on the leading minus sign (todo 143). - type_name.md: document the new "invalid" return value for corrupted JSON values (todo 145). - get.md: note that get<std::optional<T>>() was unreachable in every configuration prior to 3.13.0 due to an internal macro-guard bug, unrelated to JSON_USE_IMPLICIT_CONVERSIONS's actual effect (todo 144). Signed-off-by: Niels Lohmann <mail@nlohmann.me> --------- Signed-off-by: Niels Lohmann <mail@nlohmann.me>
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2.3 KiB
std::formatternlohmann::basic_json\
namespace std {
template <>
struct formatter<nlohmann::basic_json, char>;
}
Specialization to make JSON values formattable with std::format
(and the other members of C++20's <format> header, such as std::format_to).
A subset of the standard format spec grammar 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:
#!cpp "{}"serializes the value the same way asdump()(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 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 - serialization
- operator<<(std::ostream&) - serialize to stream
- format_as - customization point used by
fmt::format(fmtlib) - Serialization - the serialization article
Version history
- Added in version 3.13.0.