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>
2026-08-19 07:33:27 +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 +1,22 @@
+# fmt's CMakeLists.txt unconditionally sets the VERSION/SOVERSION/DEBUG_POSTFIX
+# properties on its (always-configured) INTERFACE_LIBRARY "fmt-header-only" target;
+# CMake rejected those properties on INTERFACE_LIBRARY targets before CMake 3.19
+if (${CMAKE_VERSION} VERSION_GREATER_EQUAL "3.19.0")
+  add_test(NAME fmt_formatter_configure
+    COMMAND ${CMAKE_COMMAND}
+    -G "${CMAKE_GENERATOR}"
+    -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
+    ${CMAKE_CURRENT_SOURCE_DIR}/project
+  )
+  add_test(NAME fmt_formatter_build
+    COMMAND ${CMAKE_COMMAND} --build .
+  )
+  set_tests_properties(fmt_formatter_configure PROPERTIES
+    FIXTURES_SETUP fmt_formatter
+    LABELS "git_required;not_reproducible"
+  )
+  set_tests_properties(fmt_formatter_build PROPERTIES
+    FIXTURES_REQUIRED fmt_formatter
+    LABELS "git_required;not_reproducible"
+  )
+endif()
@@ -0,0 +1,36 @@
+cmake_minimum_required(VERSION 3.14)
+
+project(FmtFormatterTest CXX)
+
+include(FetchContent)
+
+get_filename_component(GIT_REPOSITORY_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/../../.. ABSOLUTE)
+FetchContent_Declare(json GIT_REPOSITORY ${GIT_REPOSITORY_DIRECTORY} GIT_TAG HEAD)
+
+set(FMT_TEST OFF CACHE BOOL "" FORCE)
+set(FMT_DOC OFF CACHE BOOL "" FORCE)
+set(FMT_INSTALL OFF CACHE BOOL "" FORCE)
+FetchContent_Declare(fmt
+  GIT_REPOSITORY https://github.com/fmtlib/fmt.git
+  GIT_TAG 12.2.0
+  GIT_SHALLOW TRUE
+)
+
+FetchContent_MakeAvailable(json fmt)
+
+if(MSVC)
+  add_compile_options(/EHsc)
+endif()
+
+add_executable(fmt_formatter_test main.cpp)
+target_link_libraries(fmt_formatter_test PRIVATE nlohmann_json::nlohmann_json fmt::fmt)
+target_compile_features(fmt_formatter_test PRIVATE cxx_std_17)
+
+# Fail the build itself (and therefore the ctest "build" step that drives it) if the
+# recipe's runtime assertions don't hold -- there is no separate "run" step, since the
+# executable's location varies by platform/generator, but CMake resolves a target name
+# passed to a custom command's COMMAND for us.
+add_custom_command(TARGET fmt_formatter_test POST_BUILD
+  COMMAND fmt_formatter_test
+  COMMENT "Running fmt::formatter<nlohmann::json> recipe test"
+)
@@ -0,0 +1,94 @@
+#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 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);
+    }
+};
+// --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);
+}