Add fmt library integration example

example/fmtlib_formatter.cpp

@@ -0,0 +1,70 @@
+// Copyright (c) 2023 Denis Mikhailov
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/pfr.hpp>
+#include <fmt/format.h>
+#include <type_traits>
+
+namespace boost::pfr {
+    struct fmtlib_tag;
+}
+
+template<class T> struct fmt::formatter<T, char, std::enable_if_t<
+    boost::pfr::is_reflectable<T, boost::pfr::fmtlib_tag>::value>>
+{
+    constexpr auto parse(format_parse_context& ctx) -> format_parse_context::iterator {
+        auto it = ctx.begin(), end = ctx.end();
+
+        if (it != end && *it != '}') throw_format_error("invalid format");
+
+        return it;
+    }
+
+    auto format(const T& t, format_context& ctx) const -> format_context::iterator {
+        using namespace boost::pfr;
+
+        auto out = ctx.out();
+
+        *out++ = '{';
+
+        const char* div = "";
+        constexpr auto names = names_as_array<T>();
+
+        for_each_field(t, [&](const auto& field, auto I) {
+            out = fmt::format_to(out, "{}", std::exchange(div, ","));
+            out = fmt::format_to(out, " .{}={}", names[I], field);
+            });
+
+        out = fmt::format_to(out, "{}", std::strlen(div) == 0 ? "}" : " }");
+
+        return out;
+    }
+};
+
+struct point
+{
+    int x, y;
+};
+
+struct color
+{
+    unsigned char r, g, b;
+};
+
+struct line
+{
+    color clr;
+    point first, last;
+};
+
+template<> struct boost::pfr::is_reflectable<point, boost::pfr::fmtlib_tag> : std::integral_constant<bool, true> {};
+template<> struct boost::pfr::is_reflectable<color, boost::pfr::fmtlib_tag> : std::integral_constant<bool, true> {};
+template<> struct boost::pfr::is_reflectable<line, boost::pfr::fmtlib_tag> : std::integral_constant<bool, true> {};
+
+int main()
+{
+    fmt::print("{}\n", line{ { 255, 192, 16 }, { 1, 2 }, { 3, 4 } });
+}
+