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Merge pull request #363 from cppalliance/format
Support for <format>
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‎.github/workflows/ci.yml‎

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@@ -226,18 +226,26 @@ jobs:
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- "deb http://apt.llvm.org/jammy/ llvm-toolchain-jammy-17 main"
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source_keys:
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- "https://apt.llvm.org/llvm-snapshot.gpg.key"
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- name: UBSAN
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toolset: clang
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compiler: clang++-14
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- toolset: clang
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compiler: clang++-18
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cxxstd: "03,11,14,17,20,2b"
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cxxflags: -stdlib=libc++
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linkflags: -stdlib=libc++
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ubsan: 1
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os: ubuntu-22.04
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os: ubuntu-24.04
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install:
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- clang-14
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- libc++-14-dev
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- libc++abi-14-dev
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- clang-18
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sources:
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- "deb http://apt.llvm.org/noble/ llvm-toolchain-noble-18 main"
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source_keys:
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- "https://apt.llvm.org/llvm-snapshot.gpg.key"
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- toolset: clang
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compiler: clang++-19
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cxxstd: "03,11,14,17,20,2b"
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os: ubuntu-24.04
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install:
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- clang-19
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sources:
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- "deb http://apt.llvm.org/noble/ llvm-toolchain-noble-19 main"
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source_keys:
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- "https://apt.llvm.org/llvm-snapshot.gpg.key"
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- toolset: clang
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cxxstd: "03,11,14,17,20,2b"

‎doc/decimal.adoc‎

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@@ -33,6 +33,7 @@ include::decimal/numbers.adoc[]
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include::decimal/cmath.adoc[]
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include::decimal/cstdlib.adoc[]
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include::decimal/charconv.adoc[]
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include::decimal/format.adoc[]
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include::decimal/cfenv.adoc[]
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include::decimal/cfloat.adoc[]
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include::decimal/cstdio.adoc[]

‎doc/decimal/format.adoc‎

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////
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Copyright 2025 Matt Borland
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Distributed under the Boost Software License, Version 1.0.
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https://www.boost.org/LICENSE_1_0.txt
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////
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[#format]
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= format support
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:idprefix: format_
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== <format>
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Format is supported when using C++20 and a compiler with appropriate support: GCC >= 13, Clang >= 18, MSVC >= 19.40
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=== Type Modifiers
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The following type modifiers are the same as those used by built-in floating point values:
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- "g" or "G" for general format
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- "e" or "E" for scientific format
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- "f" for fixed format
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- "a" or "A" for hex format
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Example usage for scientific format would be: `{:e}`
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NOTE: The uppercase format will return with all applicable values in uppercase (e.g. 3.14E+02 vs 3.14e+02)
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=== Precision Modifiers
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Precision can be specified in the same way as built-in floating point values.
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For example a scientific format with 3 digits or precision would be: `{:.3e}`
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=== Padding Modifiers
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If you want all values to be printed with a fixed width padding is allowed before the precision modifier.
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For example with `{:10.3e}`:
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- 3.14 -> " 3.140e+00"
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- 3.141 -> " 3.141e+00"
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Note the space at the front of these string to keep with width at 10 characters
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=== Examples
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The example is padding modifiers can be done like so
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[source, c++]
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----
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#include <boost/decimal.hpp>
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#include <format>
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#include <iostream>
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int main()
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{
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constexpr boost::decimal::decimal64 val1 {314, -2};
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constexpr boost::decimal::decimal32 val2 {3141, -3};
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std::cout << std::format("{:10.3e}", val1) << '\n';
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std::cout << std::format("{:10.3e}", val2) << std::endl;
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return 0;
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}
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----

‎examples/format.cpp‎

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// Copyright 2025 Matt Borland
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// Distributed under the Boost Software License, Version 1.0.
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// https://www.boost.org/LICENSE_1_0.txt
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// MSVC 14.3 has a conversion error in <algorithm> so we need to try and supress that everywhere
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#ifdef _MSC_VER
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# pragma warning(push)
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# pragma warning(disable : 4244)
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#endif
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#include <boost/decimal.hpp>
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#include <iostream>
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#ifdef BOOST_CRYPT_HAS_FORMAT_SUPPORT
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#include <format>
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int main()
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{
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constexpr boost::decimal::decimal64 val1 {314, -2};
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constexpr boost::decimal::decimal32 val2 {3141, -3};
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std::cout << std::format("{:10.3e}", val1) << '\n';
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std::cout << std::format("{:10.3e}", val2) << std::endl;
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return 0;
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}
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#else
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int main()
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{
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std::cout << "<format> is unsupported" << std::endl;
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return 0;
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}
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#endif

‎examples/statistics.cpp‎

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#include <fstream>
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#include <sstream>
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// Warning suppression for boost.math
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#if defined(__clang__)
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# pragma clang diagnostic push
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# pragma clang diagnostic ignored "-Wfloat-equal"
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# pragma clang diagnostic ignored "-Wsign-conversion"
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# pragma clang diagnostic ignored "-Wundef"
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# pragma clang diagnostic ignored "-Wstring-conversion"
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#elif defined(__GNUC__)
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# pragma GCC diagnostic push
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# pragma GCC diagnostic ignored "-Wfloat-equal"

‎include/boost/decimal.hpp‎

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#include <boost/decimal/cfloat.hpp>
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#include <boost/decimal/charconv.hpp>
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#include <boost/decimal/detail/io.hpp>
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#include <boost/decimal/format.hpp>
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#include <boost/decimal/cstdio.hpp>
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#include <boost/decimal/bid_conversion.hpp>
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#include <boost/decimal/dpd_conversion.hpp>

‎include/boost/decimal/format.hpp‎

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// Copyright 2023 - 2024 Matt Borland
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// Distributed under the Boost Software License, Version 1.0.
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// https://www.boost.org/LICENSE_1_0.txt
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#ifndef BOOST_DECIMAL_FORMAT_HPP
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#define BOOST_DECIMAL_FORMAT_HPP
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// Many compilers seem to have <format> with completly broken support so narrow down our support range
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#if (__cplusplus >= 202002L || (defined(_MSVC_LANG) && _MSVC_LANG >= 202002L)) && !defined(BOOST_DECIMAL_DISABLE_CLIB) && \
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((defined(__GNUC__) && __GNUC__ >= 13) || (defined(__clang__) && __clang_major__ >= 18) || (defined(_MSC_VER) && _MSC_VER >= 1940))
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#define BOOST_CRYPT_HAS_FORMAT_SUPPORT
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#include <boost/decimal/charconv.hpp>
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#include <algorithm>
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#include <format>
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#include <iostream>
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#include <iomanip>
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#include <string>
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#include <tuple>
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#include <cctype>
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// Default :g
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// Fixed :f
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// Scientific :e
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// Hex :a
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//
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// Capital letter for any of the above leads to all characters being uppercase
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namespace boost::decimal::detail {
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template <typename ParseContext>
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constexpr auto parse_impl(ParseContext &ctx)
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{
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auto it {ctx.begin()};
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int ctx_precision = 6;
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boost::decimal::chars_format fmt = boost::decimal::chars_format::general;
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bool is_upper = false;
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int padding_digits = 0;
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// Check for a padding character
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while (it != ctx.end() && *it >= '0' && *it <= '9')
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{
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padding_digits = padding_digits * 10 + (*it - '0');
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++it;
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}
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// If there is a . then we need to capture the precision argument
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if (*it == '.')
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{
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++it;
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ctx_precision = 0;
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while (it != ctx.end() && *it >= '0' && *it <= '9')
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{
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ctx_precision = ctx_precision * 10 + (*it - '0');
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++it;
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}
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}
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// Lastly we capture the format to include if it's upper case
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if (it != ctx.end() && *it != '}')
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{
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switch (*it)
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{
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case 'G':
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is_upper = true;
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[[fallthrough]];
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case 'g':
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fmt = chars_format::general;
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break;
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case 'F':
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[[fallthrough]];
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case 'f':
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fmt = chars_format::fixed;
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break;
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case 'E':
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is_upper = true;
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[[fallthrough]];
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case 'e':
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fmt = chars_format::scientific;
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break;
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case 'A':
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is_upper = true;
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[[fallthrough]];
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case 'a':
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fmt = chars_format::hex;
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break;
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// LCOV_EXCL_START
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default:
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throw std::format_error("Invalid format specifier");
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// LCOV_EXCL_STOP
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}
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}
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++it;
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return std::make_tuple(ctx_precision, fmt, is_upper, padding_digits, it);
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}
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} // Namespace boost::decimal::detail
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namespace std {
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template <boost::decimal::concepts::decimal_floating_point_type T>
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struct formatter<T>
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{
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constexpr formatter() : ctx_precision(6),
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fmt(boost::decimal::chars_format::general),
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is_upper(false),
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padding_digits(0)
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{}
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int ctx_precision;
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boost::decimal::chars_format fmt;
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bool is_upper;
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int padding_digits;
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constexpr auto parse(format_parse_context &ctx)
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{
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const auto res {boost::decimal::detail::parse_impl(ctx)};
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ctx_precision = std::get<0>(res);
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fmt = std::get<1>(res);
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is_upper = std::get<2>(res);
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padding_digits = std::get<3>(res);
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return std::get<4>(res);
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}
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template <typename FormatContext>
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auto format(const T &v, FormatContext &ctx) const
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{
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auto out = ctx.out();
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std::array<char, 128> buffer {};
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const auto r = boost::decimal::to_chars(buffer.data(), buffer.data() + buffer.size(), v, fmt, ctx_precision);
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std::string_view sv(buffer.data(), static_cast<std::size_t>(r.ptr - buffer.data()));
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std::string s(sv);
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if (is_upper)
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{
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std::transform(s.begin(), s.end(), s.begin(),
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[](unsigned char c)
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{ return std::toupper(c); });
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}
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if (s.size() < static_cast<std::size_t>(padding_digits))
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{
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s.insert(s.begin(), static_cast<std::size_t>(padding_digits) - s.size(), ' ');
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}
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return std::copy(s.begin(), s.end(), out);
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}
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};
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} // Namespace std
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#endif
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#endif //BOOST_DECIMAL_FORMAT_HPP

‎test/Jamfile‎

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@@ -111,6 +111,7 @@ run test_fast_math.cpp ;
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run test_fenv.cpp ;
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run test_fixed_width_trunc.cpp ;
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run test_float_conversion.cpp ;
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run test_format.cpp ;
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run-fail test_fprintf.cpp ;
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run test_frexp_ldexp.cpp ;
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run test_from_chars.cpp /boost/charconv//boost_charconv ;
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run ../examples/moving_average.cpp ;
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run ../examples/currency_conversion.cpp ;
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run ../examples/statistics.cpp ;
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run ../examples/format.cpp ;

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