♻️ reorganize PR

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

tests/fmt_formatter/project/main.cpp

@@ -0,0 +1,98 @@
+#include <cassert>
+#include <fmt/format.h>
+#include <nlohmann/json.hpp>
+
+// A fmt::formatter<nlohmann::json> specialization mirroring std::formatter<basic_json>
+// (see docs/mkdocs/docs/api/basic_json/std_formatter.md), for use with fmt versions that
+// no longer pick up format_as() (fmt >= 11.1.0), or to get the same "{:#}"/width/
+// fill-and-align spec support with any fmt version.
+// --8<-- [start:formatter_recipe]
+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
+        bool pretty = false;
+        if (it != end && *it == '#')
+        {
+            pretty = true;
+            ++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;
+            }
+        }
+        else if (pretty)
+        {
+            indent = 4;
+        }
+
+        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);
+    }
+};
+// --8<-- [end:formatter_recipe]
+
+int main()
+{
+    const nlohmann::json j = {{"foo", 1}, {"bar", {1, 2, 3}}};
+
+    assert(fmt::format("{}", j) == j.dump());
+    assert(fmt::format("{:#}", j) == j.dump(4));
+    assert(fmt::format("{:2}", j) == j.dump(2));
+    assert(fmt::format("{:#2}", j) == j.dump(2));
+    assert(fmt::format("{:.>#}", j) == j.dump(4, '.'));
+
+    bool threw = false;
+    try
+    {
+        (void)fmt::vformat("{:x}", fmt::make_format_args(j));
+    }
+    catch (const fmt::format_error&)
+    {
+        threw = true;
+    }
+    assert(threw);
+}