使用std::format格式化chrono时间并对齐时解析失败的修复方法

我想用std::format打印%H:%M:%S格式的时间值，并且把每个“单元格”对齐到width的特定倍数。当前代码如下：

tps_headings += std::format("{1:%H:%M:%S:<{0}} ", width, tp_seconds);

其中tp_seconds是std::chrono的秒时长，width是size_t类型。运行时std::format解析字符串失败，报错信息如下：

/home/myuser/.espressif/tools/xtensa-esp-elf/esp-13.2.0_20230928/xtensa-esp-elf/xtensa-esp-elf/include/c++/13.2.0/bits/chrono_io.h:303:35: error: call to non-'constexpr' function 'void std::__throw_format_error(const char*)'
  303 |               __throw_format_error("chrono format error: '{' in chrono-specs");
      |               ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /home/myuser/.espressif/tools/xtensa-esp-elf/esp-13.2.0_20230928/xtensa-esp-elf/xtensa-esp-elf/include/c++/13.2.0/bits/chrono_io.h:39:
/home/myuser/.espressif/tools/xtensa-esp-elf/esp-13.2.0_20230928/xtensa-esp-elf/xtensa-esp-elf/include/c++/13.2.0/format:156:3: note: 'void std::__throw_format_error(const char*)' declared here
  156 |   __throw_format_error(const char* __what)
      |   ^~~~~~~~~~~~~~~~~~~~
/home/myuser/dev2/aurora-light-controller/components/a_light_emitter/tables.ipp:243:43:   in 'constexpr' expansion of 'std::basic_format_string<char, const unsigned int&, const double&>("{1:.2f:<{0}} ")'
/home/myuser/.espressif/tools/xtensa-esp-elf/esp-13.2.0_20230928/xtensa-esp-elf/xtensa-esp-elf/include/c++/13.2.0/format:3672:19:   in 'constexpr' expansion of '__scanner.std::__format::_Checking_scanner<char, unsigned int, double>::<anonymous>.std::__format::_Scanner<char>::_M_scan()'
/home/myuser/.espressif/tools/xtensa-esp-elf/esp-13.2.0_20230928/xtensa-esp-elf/xtensa-esp-elf/include/c++/13.2.0/format:3482:30:   in 'constexpr' expansion of '((std::__format::_Scanner<char>*)this)->std::__format::_Scanner<char>::_M_on_replacement_field()'
/home/myuser/.espressif/tools/xtensa-esp-elf/esp-13.2.0_20230928/xtensa-esp-elf/xtensa-esp-elf/include/c++/13.2.0/format:3534:15:   in 'constexpr' expansion of '((std::__format::_Scanner<char>*)this)->std::__format::_Scanner<char>::_M_format_arg(__id)'
/home/myuser/.espressif/tools/xtensa-esp-elf/esp-13.2.0_20230928/xtensa-esp-elf/xtensa-esp-elf/include/c++/13.2.0/format:3610:33:   in 'constexpr' expansion of '((std::__format::_Checking_scanner<char, unsigned int, double>*)this)->std::__format::_Checking_scanner<char, unsigned int, double>::_M_parse_format_spec<unsigned int, double>(__id)'
/home/myuser/.espressif/tools/xtensa-esp-elf/esp-13.2.0_20230928/xtensa-esp-elf/xtensa-esp-elf/include/c++/13.2.0/format:3627:41:   in 'constexpr' expansion of '((std::__format::_Checking_scanner<char, unsigned int, double>*)this)->std::__format::_Checking_scanner<char, unsigned int, double>::_M_parse_format_spec<double>((__id - 1))'
/home/myuser/.espressif/tools/xtensa-esp-elf/esp-13.2.0_20230928/xtensa-esp-elf/xtensa-esp-elf/include/c++/13.2.0/format:3624:40:   in 'constexpr' expansion of '__f.std::formatter<double, char>::parse(((std::__format::_Checking_scanner<char, unsigned int, double>*)this)->std::__format::_Checking_scanner<char, unsigned int, double>::<anonymous>.std::__format::_Scanner<char>::_M_pc)'
/home/myuser/.espressif/tools/xtensa-esp-elf/esp-13.2.0_20230928/xtensa-esp-elf/xtensa-esp-elf/include/c++/13.2.0/format:2025:26:   in 'constexpr' expansion of '((std::formatter<double, char>*)this)->std::formatter<double, char>::_M_f.std::__format::__formatter_fp<char>::parse((* & __pc))'
/home/myuser/.espressif/tools/xtensa-esp-elf/esp-13.2.0_20230928/xtensa-esp-elf/xtensa-esp-elf/include/c++/13.2.0/format:1410:48: error: call to non-'constexpr' function 'void std::__format::__failed_to_parse_format_spec()'
 1410 |         __format::__failed_to_parse_format_spec();
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
/home/myuser/.espressif/tools/xtensa-esp-elf/esp-13.2.0_20230928/xtensa-esp-elf/xtensa-esp-elf/include/c++/13.2.0/format:185:3: note: 'void std::__format::__failed_to_parse_format_spec()' declared here
 185 |   __failed_to_parse_format_spec()
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

问题根源是格式字符串的写法错误：std::chrono的格式说明（%H:%M:%S）和对齐/宽度参数的嵌套写法冲突，编译器会把<{0}当成chrono格式的一部分，导致解析失败。

以下两种方法可以解决问题：

方法一：拆分格式步骤 #

先将时间格式化为字符串，再对字符串做对齐处理：

auto time_str = std::format("{:%H:%M:%S}", tp_seconds);
tps_headings += std::format("{:<{}} ", time_str, width);

方法二：调整格式字符串结构 #

将对齐和宽度参数放在chrono格式说明的前面，作为整个替换字段的格式修饰，而非chrono格式的一部分：

tps_headings += std::format("{1:<{0}:%H:%M:%S} ", width, tp_seconds);

内容的提问来源于stack exchange，提问作者glades