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96 lines
3.0 KiB
Markdown
96 lines
3.0 KiB
Markdown
# <small>std::</small>formatter<nlohmann::basic_json\>
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```cpp
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namespace std {
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template <>
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struct formatter<nlohmann::basic_json, char>;
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}
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```
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Specialization to make JSON values formattable with [`std::format`](https://en.cppreference.com/w/cpp/utility/format/format)
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(and the other members of C++20's `<format>` header, such as `std::format_to`).
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A subset of the [standard format spec grammar](https://en.cppreference.com/w/cpp/utility/format/spec) is
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supported, repurposed for JSON pretty-printing; any other spec component (sign, the `0` flag, precision,
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`L`, a dynamic width such as `#!cpp "{:{}}"`, or a trailing type character) throws
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[`std::format_error`](https://en.cppreference.com/w/cpp/utility/format/format_error):
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- `#!cpp "{}"` serializes the value the same way as [`dump()`](dump.md) (compact, no whitespace).
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- `#!cpp "{:#}"` ("alternate form") serializes the value the same way as `#!cpp dump(4)` (pretty-printed
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with an indent of 4).
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- A width, with or without `#!cpp "#"` (e.g. `#!cpp "{:2}"` or `#!cpp "{:#2}"`), serializes the value the
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same way as `#!cpp dump(width)` — a width on its own implies pretty-printing, since an indent size has
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no meaning for compact output.
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- `fill-and-align` (e.g. `#!cpp "{:.>#}"` or `#!cpp "{:.>3}"`) picks a custom indent character, the same
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way as `#!cpp dump(indent, indent_char)`. The alignment direction itself (`#!cpp '<'`, `#!cpp '>'`,
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`#!cpp '^'`) has no separate meaning for JSON values — only the fill character before it is used, and
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any of the three directions is accepted.
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This specialization is only available for `#!cpp char`-based JSON values and only if the standard library
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provides `<format>`, controlled by the [`JSON_HAS_STD_FORMAT`](../macros/json_has_std_format.md) macro.
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## Examples
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??? example
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The example shows how to format JSON values with `std::format`.
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```cpp
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#include <format>
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#include <iostream>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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json j = {{"one", 1}, {"two", 2}};
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// compact formatting, like dump()
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std::cout << std::format("{}", j) << "\n\n";
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// pretty-printed formatting, like dump(4)
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std::cout << std::format("{:#}", j) << "\n\n";
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// a width sets the indent, like dump(2)
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std::cout << std::format("{:2}", j) << "\n\n";
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// fill-and-align sets the indent character, like dump(4, '.')
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std::cout << std::format("{:.>#}", j) << std::endl;
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}
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```
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Output:
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```json
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{"one":1,"two":2}
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{
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"one": 1,
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"two": 2
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}
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{
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"one": 1,
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"two": 2
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}
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{
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...."one": 1,
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...."two": 2
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}
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```
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## See also
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- [dump](dump.md) - serialization
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- [operator<<(std::ostream&)](../operator_ltlt.md) - serialize to stream
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- [format_as](format_as.md) - customization point used by `fmt::format` (fmtlib)
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- [Serialization](../../features/serialization.md) - the serialization article
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## Version history
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- Added in version 3.13.0.
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