Add std::format and fmt support (#5224)

* ✨ add std::format and fmt support Signed-off-by: Niels Lohmann <mail@nlohmann.me> * ♻️ reorganize PR Signed-off-by: Niels Lohmann <mail@nlohmann.me> * 💚 fix build Signed-off-by: Niels Lohmann <mail@nlohmann.me> * 💚 fix build Signed-off-by: Niels Lohmann <mail@nlohmann.me> * 💚 fix build Signed-off-by: Niels Lohmann <mail@nlohmann.me> --------- Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2026-07-03 17:24:18 +00:00 · 2026-07-02 15:59:36 +02:00
parent ca49ab6123
commit 8d7e0046f4
21 changed files with 824 additions and 0 deletions
@@ -79,6 +79,10 @@
    #include <string_view>
 #endif

+#if JSON_HAS_STD_FORMAT
+    #include <format> // format_parse_context, format_context, formatter, format_error
+#endif
+
 /*!
@brief namespace for Niels Lohmann
@see https://github.com/nlohmann
@@ -5258,6 +5262,14 @@ std::string to_string(const NLOHMANN_BASIC_JSON_TPL& j)
    return j.dump();
 }

+/// @brief user-defined format_as function for JSON values (fmt <= 11.0.x support)
+/// @sa https://json.nlohmann.me/api/basic_json/format_as/
+NLOHMANN_BASIC_JSON_TPL_DECLARATION
+std::string format_as(const NLOHMANN_BASIC_JSON_TPL& j)
+{
+    return j.dump();
+}
+
 inline namespace literals
 {
 inline namespace json_literals
@@ -5361,6 +5373,84 @@ inline void swap(nlohmann::NLOHMANN_BASIC_JSON_TPL& j1, nlohmann::NLOHMANN_BASIC

 #endif

+#if JSON_HAS_STD_FORMAT
+
+/// @brief std::formatter specialization for JSON values
+/// @sa https://json.nlohmann.me/api/basic_json/std_formatter/
+NLOHMANN_BASIC_JSON_TPL_DECLARATION
+struct formatter<nlohmann::NLOHMANN_BASIC_JSON_TPL, char> // NOLINT(cert-dcl58-cpp)
+{
+    // -1 means compact output (dump()); any value >= 0 means pretty-printed
+    // output with that many spaces (or indent_char) per level (dump(indent, indent_char)).
+    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,
+        // e.g. "{:.>#4}" pretty-prints with '.' as the 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 for pretty-printing,
+        // e.g. "{:2}" or "{:#2}" pretty-print with an indent of 2; 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;
+            }
+        }
+
+        // sign, the '0' flag, precision, locale-specific formatting ('L'), dynamic
+        // width/precision ("{...}"), and type characters all have no meaning for
+        // JSON values; none of them are consumed above, so they all end up rejected
+        // by this single check along with any other unrecognized trailing spec.
+        if (it != end && *it != '}')
+        {
+            JSON_THROW(format_error("invalid format args for nlohmann::json"));
+        }
+
+        return it;
+    }
+
+    template<typename FormatContext>
+    auto format(const nlohmann::NLOHMANN_BASIC_JSON_TPL& j, FormatContext& ctx) const -> decltype(ctx.out())
+    {
+        // dump()'s own default (indent = -1) already means compact output, so this
+        // covers both the compact and pretty-printed cases without a branch.
+        const auto dumped = j.dump(indent, indent_char);
+        return std::copy(dumped.begin(), dumped.end(), ctx.out());
+    }
+};
+
+#endif
+
 }  // namespace std

 #if JSON_USE_GLOBAL_UDLS