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>
2026-07-08 03:25:10 +00:00 · 2026-07-02 15:59:36 +02:00
parent ca49ab6123
commit 8d7e0046f4
21 changed files with 824 additions and 0 deletions
@@ -0,0 +1,96 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// cmake/test.cmake selects the C++ standard versions with which to build a
+// unit test based on the presence of JSON_HAS_CPP_<VERSION> macros.
+// When using macros that are only defined for particular versions of the standard
+// (e.g., JSON_HAS_FILESYSTEM for C++17 and up), please mention the corresponding
+// version macro in a comment close by, like this:
+// JSON_HAS_CPP_<VERSION> (do not remove; see note at top of file)
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+using json = nlohmann::json;
+
+// JSON_HAS_CPP_20 (do not remove; see note at top of file)
+#if JSON_HAS_STD_FORMAT
+
+#include <iterator>
+#include <string>
+
+TEST_CASE("std::formatter<nlohmann::json>")
+{
+    SECTION("compact formatting matches dump()")
+    {
+        CHECK(std::format("{}", json(nullptr)) == json(nullptr).dump());
+        CHECK(std::format("{}", json(true)) == json(true).dump());
+        CHECK(std::format("{}", json(42)) == json(42).dump());
+        CHECK(std::format("{}", json(42.23)) == json(42.23).dump());
+        CHECK(std::format("{}", json("foo")) == json("foo").dump());
+        CHECK(std::format("{}", json::array({1, 2, 3})) == json::array({1, 2, 3}).dump());
+
+        const json j = {{"foo", 1}, {"bar", {1, 2, 3}}};
+        CHECK(std::format("{}", j) == j.dump());
+    }
+
+    SECTION("'#' triggers pretty-printing with an indent of 4, like dump(4)")
+    {
+        const json j = {{"foo", 1}, {"bar", {1, 2, 3}}};
+        CHECK(std::format("{:#}", j) == j.dump(4));
+        CHECK(std::format("{:#}", json::array()) == json::array().dump(4));
+    }
+
+    SECTION("a width sets the indent, like dump(width), with or without '#'")
+    {
+        const json j = {{"foo", 1}, {"bar", {1, 2, 3}}};
+        CHECK(std::format("{:2}", j) == j.dump(2));
+        CHECK(std::format("{:#2}", j) == j.dump(2));
+        CHECK(std::format("{:8}", j) == j.dump(8));
+    }
+
+    SECTION("fill-and-align sets the indent character, like dump(indent, indent_char)")
+    {
+        const json j = {{"foo", 1}, {"bar", {1, 2, 3}}};
+        CHECK(std::format("{:.>#}", j) == j.dump(4, '.'));
+        CHECK(std::format("{:.>#3}", j) == j.dump(3, '.'));
+        CHECK(std::format("{:.>3}", j) == j.dump(3, '.'));
+        // the alignment direction itself ('<', '>', '^') has no separate meaning for
+        // JSON values -- only the fill character before it is used as the indent character
+        CHECK(std::format("{:.<3}", j) == j.dump(3, '.'));
+        CHECK(std::format("{:.^3}", j) == j.dump(3, '.'));
+    }
+
+    SECTION("format args with no meaning for JSON values are rejected")
+    {
+        // std::vformat parses the format string at runtime (unlike std::format, whose
+        // format_string type is checked at compile time), so it lets us verify that an
+        // invalid spec throws std::format_error without needing a compile-time-illegal
+        // format string.
+        const json j = 42;
+        CHECK_THROWS_AS(std::vformat("{:x}", std::make_format_args(j)), std::format_error);
+        CHECK_THROWS_AS(std::vformat("{:+}", std::make_format_args(j)), std::format_error);   // sign
+        CHECK_THROWS_AS(std::vformat("{:-}", std::make_format_args(j)), std::format_error);   // sign
+        CHECK_THROWS_AS(std::vformat("{: }", std::make_format_args(j)), std::format_error);   // sign
+        CHECK_THROWS_AS(std::vformat("{:04}", std::make_format_args(j)), std::format_error);  // '0' flag
+        CHECK_THROWS_AS(std::vformat("{:.2}", std::make_format_args(j)), std::format_error);  // precision
+        CHECK_THROWS_AS(std::vformat("{:L}", std::make_format_args(j)), std::format_error);   // locale
+        const int dynamic_width = 4;
+        CHECK_THROWS_AS(std::vformat("{:{}}", std::make_format_args(j, dynamic_width)), std::format_error); // dynamic width
+    }
+
+    SECTION("std::format_to writes through an arbitrary output iterator")
+    {
+        const json j = {{"foo", 1}, {"bar", {1, 2, 3}}};
+        std::string out;
+        std::format_to(std::back_inserter(out), "{}", j);
+        CHECK(out == j.dump());
+    }
+}
+
+#endif