format_as(basic_json)

template <typename BasicJsonType>
std::string format_as(const BasicJsonType& j);

This function implements the format_as customization point used by the {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

Return value

string containing the serialization of the JSON value (same as dump())

Exception safety

Strong guarantee: if an exception is thrown, there are no changes to any JSON value.

Exceptions

Throws type_error.316 if a string stored inside the JSON value is not UTF-8 encoded

Complexity

Linear.

Possible implementation

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"
```

Version history

Added in version 3.12.x.

3.0 KiB Raw Blame History