json/tests/src/unit-std-format.cpp

//     __ _____ _____ _____
//  __|  |   __|     |   | |  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 builds a unit test with C++ standards beyond C++11 only if the
// source file mentions the corresponding version macro. This whole file tests
// std::format (C++20), so it cannot be split: it is built for C++20 as well:
// JSON_HAS_CPP_20 (do not remove)

#include "doctest_compatibility.h"

#include <nlohmann/json.hpp>
using json = nlohmann::json;
using ordered_json = nlohmann::ordered_json;

#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));
        // multi-digit widths must accumulate every digit, not just the first
        CHECK(std::format("{:12}", j) == j.dump(12));
        CHECK(std::format("{:#12}", j) == j.dump(12));
        CHECK(std::format("{:10}", j) == j.dump(10));
    }

    SECTION("bare alignment with no fill character defaults to a space indent character")
    {
        const json j = {{"foo", 1}, {"bar", {1, 2, 3}}};
        // without a preceding fill character, the alignment character itself must not
        // be mistaken for the indent character -- the default space is kept
        CHECK(std::format("{:<}", j) == j.dump());
        CHECK(std::format("{:>}", j) == j.dump());
        CHECK(std::format("{:^}", j) == j.dump());
        CHECK(std::format("{:<3}", j) == j.dump(3, ' '));
        CHECK(std::format("{:>3}", j) == j.dump(3, ' '));
        CHECK(std::format("{:^3}", j) == j.dump(3, ' '));
    }

    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("a format spec may run to the end of the parse context")
    {
        // std::format always hands parse() a range that still holds the closing
        // '}', but a parse context may also end right after the spec
        const auto parse = [](const char* spec)
        {
            std::format_parse_context ctx(spec);
            std::formatter<json> f;
            CHECK(f.parse(ctx) == ctx.end());
            return f;
        };

        CHECK(parse("").indent == -1);
        CHECK(parse(">").indent == -1);
        CHECK(parse("#").indent == 4);
        CHECK(parse("3").indent == 3);
        CHECK(parse("#12").indent == 12);

        const auto f = parse(".>");
        CHECK(f.indent == -1);
        CHECK(f.indent_char == '.');
    }

    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());
    }
}

TEST_CASE("std::formatter<nlohmann::ordered_json>")
{
    // spot-check a non-default basic_json instantiation, since the formatter
    // is written against the generic NLOHMANN_BASIC_JSON_TPL_DECLARATION
    // template and must actually instantiate (and behave correctly) for
    // template arguments other than nlohmann::json
    const ordered_json j = {{"foo", 1}, {"bar", {1, 2, 3}}};
    CHECK(std::format("{}", j) == j.dump());
    CHECK(std::format("{:#}", j) == j.dump(4));
    CHECK(std::format("{:2}", j) == j.dump(2));
}

#endif