Add format specialization for all types

Author: mborland

include/boost/decimal/format.hpp

@@ -0,0 +1,133 @@
+// Copyright 2023 Matt Borland
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#ifndef BOOST_DECIMAL_FORMAT_HPP
+#define BOOST_DECIMAL_FORMAT_HPP
+
+#if __cplusplus >= 202002L || (defined(_MSVC_LANG) && _MSVC_LANG >= 202002L) && __has_include(<format>) && !defined(BOOST_DECIMAL_DISABLE_CLIB)
+
+#include <boost/decimal/decimal32.hpp>
+#include <boost/decimal/decimal64.hpp>
+#include <boost/decimal/decimal128.hpp>
+#include <format>
+#include <iostream>
+#include <iomanip>
+#include <string>
+#include <tuple>
+
+// See for general impl
+// https://en.cppreference.com/w/cpp/utility/format/formatter
+
+namespace boost::decimal::detail {
+
+template <typename ParseContext>
+constexpr auto parse_impl(ParseContext& ctx)
+{
+    auto it {ctx.begin()};
+    int precision = 5;
+
+    if (it == ctx.end())
+    {
+        return std::make_tuple(precision, it);
+    }
+
+    if (*it == '.')
+    {
+        ++it;
+        precision = 0;
+        while (*it >= '0' && *it <= '9')
+        {
+            precision = precision * 10 + *it;
+            ++it;
+        }
+
+        if (*it != 'f')
+        {
+            throw std::format_error("Invalid format");
+        }
+        ++it;
+    }
+
+    if (*it != '}')
+    {
+        throw std::format_error("Invalid format");
+    }
+
+    return std::make_tuple(precision, it);
+};
+};
+
+template <>
+struct std::formatter<boost::decimal::decimal32>
+{
+    int precision {};
+
+    template <typename ParseContext>
+    constexpr auto parse(ParseContext& ctx)
+    {
+        auto res {boost::decimal::detail::parse_impl(ctx)};
+        precision = std::get<0>(res);
+        return std::get<1>(res);
+    }
+
+    template <typename FormatContext>
+    auto format(const boost::decimal::decimal32& v, FormatContext& ctx)
+    {
+        std::ostringstream out;
+        out << std::setprecision(precision) << v;
+
+        return std::ranges::copy(std::move(out).str(), ctx.out()).out;
+    }
+};
+
+template <>
+struct std::formatter<boost::decimal::decimal64>
+{
+    int precision;
+
+    template <typename ParseContext>
+    constexpr auto parse(ParseContext& ctx)
+    {
+        auto res {boost::decimal::detail::parse_impl(ctx)};
+        precision = std::get<0>(res);
+        return std::get<1>(res);
+    }
+
+    template <typename FormatContext>
+    auto format(const boost::decimal::decimal32& v, FormatContext& ctx)
+    {
+        std::ostringstream out;
+        out << std::setprecision(precision) << v;
+
+        return std::ranges::copy(std::move(out).str(), ctx.out()).out;
+    }
+};
+
+template <>
+struct std::formatter<boost::decimal::decimal128>
+{
+    int precision;
+
+    template <typename ParseContext>
+    constexpr auto parse(ParseContext& ctx)
+    {
+        auto res {boost::decimal::detail::parse_impl(ctx)};
+        precision = std::get<0>(res);
+        return std::get<1>(res);
+    }
+
+    template <typename FormatContext>
+    auto format(const boost::decimal::decimal32& v, FormatContext& ctx)
+    {
+        std::ostringstream out;
+        out << std::setprecision(precision) << v;
+
+        return std::ranges::copy(std::move(out).str(), ctx.out()).out;
+    }
+};
+
+
+#endif
+
+#endif //BOOST_DECIMAL_FORMAT_HPP