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std::formatter

From cppreference.com
< cpp‎ | utility‎ | format
 
 
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Defined in header <format>
template< class T, class CharT = char >
struct formatter;
(since C++20)

The enabled specializations of std::formatter define formatting rules for a given type. Enabled specializations meet the BasicFormatter requirements, and, unless otherwise specified, also meet the Formatter requirements.

For all types T and CharT for which no specialization std::formatter<T, CharT> is enabled, that specialization is a complete type and is disabled.

Disabled specializations do not meet the Formatter requirements, and the following are all false:

Contents

[edit] Basic standard specializations

In the following list, CharT is either char or wchar_t, ArithmeticT is any cv-unqualified arithmetic type other than char, wchar_t, char8_t, char16_t, or char32_t:

Character formatters
template<>
struct formatter<char, char>;
(1)
template<>
struct formatter<char, wchar_t>;
(2)
template<>
struct formatter<wchar_t, wchar_t>;
(3)
String formatters
template<>
struct formatter<CharT*, CharT>;
(4)
template<>
struct formatter<const CharT*, CharT>;
(5)
template< std::size_t N >
struct formatter<CharT[N], CharT>;
(6)
template< std::size_t N >
struct formatter<const CharT[N], CharT>;
(7) (until C++23)
template< class Traits, class Alloc >
struct formatter<std::basic_string<CharT, Traits, Alloc>, CharT>;
(8)
template< class Traits >
struct formatter<std::basic_string_view<CharT, Traits>, CharT>;
(9)
Arithmetic formatters
template<>
struct formatter<ArithmeticT, CharT>;
(10)
Pointer formatters
template<>
struct formatter<std::nullptr_t, CharT>;
(11)
template<>
struct formatter<void*, CharT>;
(12)
template<>
struct formatter<const void*, CharT>;
(13)

Formatters for other pointers and pointers to members are disabled.

Specializations such as std::formatter<wchar_t, char> and std::formatter<const char*, wchar_t> that would require encoding conversions are disabled.

The following specialization are still disabled in C++23 to avoid formatting some char sequences as ranges of wchar_t:

Disabled formatters for wchar_t
template<>
struct formatter<char*, wchar_t>;
(1)
template<>
struct formatter<const char*, wchar_t>;
(2)
template< std::size_t N >
struct formatter<char[N], wchar_t>;
(3)
template< class Traits, class Allocator >
struct formatter<std::basic_string<char, Traits, Allocator>, wchar_t>;
(4)
template< class Traits >
struct formatter<std::basic_string_view<char, Traits>, wchar_t>;
(5)

A debug-enabled formatter specialization additionally provides a public non-static member function constexpr void set_debug_format(); which modifies the state of the formatter object so that it will format the values as escaped and quoted, as if the type of the format specifier parsed by the last call to parse were ?.

Each formatter specialization for string or character type is debug-enabled.

(since C++23)

[edit] Standard format specification

[edit] Standard specializations for library types

formatting support for duration
(class template specialization) [edit]
formatting support for sys_time
(class template specialization) [edit]
formatting support for utc_time
(class template specialization) [edit]
formatting support for tai_time
(class template specialization) [edit]
formatting support for gps_time
(class template specialization) [edit]
formatting support for file_time
(class template specialization) [edit]
formatting support for local_time
(class template specialization) [edit]
formatting support for day
(class template specialization) [edit]
formatting support for month
(class template specialization) [edit]
formatting support for year
(class template specialization) [edit]
formatting support for weekday
(class template specialization) [edit]
formatting support for weekday_indexed
(class template specialization) [edit]
formatting support for weekday_last
(class template specialization) [edit]
formatting support for month_day
(class template specialization) [edit]
formatting support for month_day_last
(class template specialization) [edit]
formatting support for month_weekday
(class template specialization) [edit]
formatting support for month_weekday_last
(class template specialization) [edit]
formatting support for year_month
(class template specialization) [edit]
formatting support for year_month_day
(class template specialization) [edit]
formatting support for year_month_day_last
(class template specialization) [edit]
formatting support for year_month_weekday
(class template specialization) [edit]
formatting support for year_month_weekday_last
(class template specialization) [edit]
formatting support for hh_mm_ss
(class template specialization) [edit]
formatting support for sys_info
(class template specialization) [edit]
formatting support for local_info
(class template specialization) [edit]
formatting support for zoned_time
(class template specialization) [edit]
formatting support for basic_stacktrace
(class template specialization) [edit]
formatting support for stacktrace_entry
(class template specialization) [edit]
formatting support for thread::id
(class template specialization) [edit]
formatting support for vector<bool>::reference
(class template specialization) [edit]
formatting support for pair and tuple
(class template specialization) [edit]
formatting support for ranges
(class template specialization) [edit]
formatting support for std::stack
(class template specialization) [edit]
formatting support for std::queue
(class template specialization) [edit]
formatting support for std::priority_queue
(class template specialization) [edit]
formatting support for filesystem::path
(class template specialization) [edit]

[edit] Notes

Feature-test macro Value Std Feature
__cpp_lib_format_uchar 202311L (C++20)
(DR)
Formatting of code units as unsigned integers

[edit] Example

#include <algorithm>
#include <format>
#include <iomanip>
#include <iostream>
#include <sstream>
#include <string_view>
 
struct QuotableString : std::string_view
{};
 
template<>
struct std::formatter<QuotableString, char>
{
    bool quoted = false;
 
    template<class ParseContext>
    constexpr ParseContext::iterator parse(ParseContext& ctx)
    {
        auto it = ctx.begin();
        if (it == ctx.end())
            return it;
 
        if (*it == '#')
        {
            quoted = true;
            ++it;
        }
        if (it != ctx.end() && *it != '}')
            throw std::format_error("Invalid format args for QuotableString.");
 
        return it;
    }
 
    template<class FmtContext>
    FmtContext::iterator format(QuotableString s, FmtContext& ctx) const
    {
        std::ostringstream out;
        if (quoted)
            out << std::quoted(s);
        else
            out << s;
 
        return std::ranges::copy(std::move(out).str(), ctx.out()).out;
    }
};
 
int main()
{
    QuotableString a("be"), a2(R"( " be " )");
    QuotableString b("a question");
    std::cout << std::format("To {0} or not to {0}, that is {1}.\n", a, b);
    std::cout << std::format("To {0:} or not to {0:}, that is {1:}.\n", a, b);
    std::cout << std::format("To {0:#} or not to {0:#}, that is {1:#}.\n", a2, b);
}

Output:

To be or not to be, that is a question.
To be or not to be, that is a question.
To " \" be \" " or not to " \" be \" ", that is "a question".

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3944 C++23 some char sequences were formattable as ranges of wchar_t disable specializations added

[edit] See also

formatting state, including all formatting arguments and the output iterator
(class template) [edit]
specifies that a type is formattable, that is, it specializes std::formatter and provides member functions parse and format
(concept) [edit]