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
template <>
struct fmt::formatter<nlohmann::json>
{
    // -1 means compact output (dump()); any value >= 0 means pretty-printed
    // output with that many spaces (or indent_char) per level.
    int indent = -1;
    char indent_char = ' ';

    constexpr auto parse(format_parse_context& ctx) -> format_parse_context::iterator
    {
        auto it = ctx.begin();
        const auto end = ctx.end();
        constexpr auto is_align = [](char c)
        {
            return c == '<' || c == '>' || c == '^';
        };

        // [[fill] align] - repurposed here to pick a custom indent character
        if (it != end && it + 1 != end && is_align(it[1]))
        {
            indent_char = *it;
            it += 2;
        }
        else if (it != end && is_align(*it))
        {
            ++it;
        }

        // ['#'] - "alternate form", used here to request pretty-printing with a
        // default indent of 4 (overridden by an explicit width below, if given)
        if (it != end && *it == '#')
        {
            indent = 4;
            ++it;
        }

        // [width] - repurposed here to pick the indent size; a width without '#'
        // implies pretty-printing since an indent otherwise has no meaning
        if (it != end && *it >= '1' && *it <= '9')
        {
            indent = 0;
            while (it != end && *it >= '0' && *it <= '9')
            {
                indent = (indent * 10) + (*it - '0');
                ++it;
            }
        }

        if (it != end && *it != '}')
        {
            throw fmt::format_error("invalid format args for nlohmann::json");
        }

        return it;
    }

    auto format(const nlohmann::json& j, format_context& ctx) const
    {
        const auto dumped = j.dump(indent, indent_char);
        return fmt::format_to(ctx.out(), "{}", dumped);
    }
};
```

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
#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;
}

```

Output:

```json
{"one":1,"two":2}

```

Version history

Added in version 3.13.0.

5.3 KiB Raw Blame History