std::formatter

 

哋它亢++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (哋它亢++20)

Language support library

Concepts library (哋它亢++20)

Metaprogramming library (哋它亢++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (哋它亢++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (哋它亢++17)

Regular expressions library (哋它亢++11)

Concurrency support library (哋它亢++11)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (哋它亢++20) Dynamic memory management Program utilities Coroutine support (哋它亢++20) Variadic functions is_constant_evaluated (哋它亢++20) is_within_lifetime (哋它亢++26) initializer_list (哋它亢++11) source_location (哋它亢++20) Debugging support is_debugger_present (哋它亢++26) breakpoint_if_debugging (哋它亢++26) breakpoint (哋它亢++26) Three-way comparison three_way_comparablethree_way_comparable_with (哋它亢++20)(哋它亢++20) strong_ordering (哋它亢++20) weak_ordering (哋它亢++20) partial_ordering (哋它亢++20) common_comparison_category (哋它亢++20) compare_three_way_result (哋它亢++20) compare_three_way (哋它亢++20) strong_order (哋它亢++20) weak_order (哋它亢++20) partial_order (哋它亢++20) compare_strong_order_fallback (哋它亢++20) compare_weak_order_fallback (哋它亢++20) compare_partial_order_fallback (哋它亢++20)    is_eqis_ltis_lteq (哋它亢++20)(哋它亢++20)(哋它亢++20) is_neqis_gtis_gteq (哋它亢++20)(哋它亢++20)(哋它亢++20)

General utilities Date and time Function objects Formatting library (哋它亢++20) bitset hash (哋它亢++11) Relational operators (deprecated in 哋它亢++20) rel_ops::operator!=rel_ops::operator>     rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (哋它亢++20)(哋它亢++20)(哋它亢++20)    cmp_not_equalcmp_greatercmp_greater_than (哋它亢++20)(哋它亢++20)(哋它亢++20) in_range (哋它亢++20) Swap and type operations swap ranges::swap (哋它亢++20) exchange (哋它亢++14) declval (哋它亢++11) to_underlying (哋它亢++23) forward (哋它亢++11) forward_like (哋它亢++23) move (哋它亢++11) move_if_noexcept (哋它亢++11) as_const (哋它亢++17) Common vocabulary types pair tuple (哋它亢++11) optional (哋它亢++17) any (哋它亢++17) variant (哋它亢++17) tuple_size (哋它亢++11) tuple_element (哋它亢++11) apply (哋它亢++17) make_from_tuple (哋它亢++17) expected (哋它亢++23) Elementary string conversions to_chars (哋它亢++17) from_chars (哋它亢++17) chars_format (哋它亢++17) to_chars_result (哋它亢++17) from_chars_result (哋它亢++17)

 

Formatting library

Standard format specification

Formatting functions

format (哋它亢++20)

format_to (哋它亢++20)

format_to_n (哋它亢++20)

formatted_size (哋它亢++20)

vformat (哋它亢++20)

vformat_to (哋它亢++20)

Format strings

basic_format_stringformat_stringwformat_string (哋它亢++20)(哋它亢++20)(哋它亢++20)

runtime_format (哋它亢++26)

Formatting concepts

formattable (哋它亢++23)

Formatter

formatter (哋它亢++20)

formatter<pair-or-tuple> (哋它亢++23)

formatter<range> (哋它亢++23)

range_formatter (哋它亢++23)

enable_nonlocking_formatter_optimization (哋它亢++23)

basic_format_parse_contextformat_parse_contextwformat_parse_context (哋它亢++20)(哋它亢++20)(哋它亢++20)

basic_format_contextformat_contextwformat_context (哋它亢++20)(哋它亢++20)(哋它亢++20)

range_format (哋它亢++23)

format_kind (哋它亢++23)

Formatting arguments

basic_format_arg (哋它亢++20)

basic_format_arg::handle (哋它亢++20)

basic_format_argsformat_argswformat_args (哋它亢++20)(哋它亢++20)(哋它亢++20)

visit_format_arg (哋它亢++20) (deprecated in 哋它亢++26)

make_format_argsmake_wformat_args (哋它亢++20)(哋它亢++20)

Format error

format_error (哋它亢++20)

 

Defined in header <format>
template< class T, class CharT = char > struct formatter;		(since 哋它亢++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:

• std::is_default_constructible_v
• std::is_copy_constructible_v
• std::is_move_constructible_v
• std::is_copy_assignable_v
• std::is_move_assignable_v.

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 哋它亢++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.

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 哋它亢++23)

Standard specializations for library types

std::formatter<std::chrono::duration> (哋它亢++20)	formatting support for duration (class template specialization)
std::formatter<std::chrono::sys_time> (哋它亢++20)	formatting support for sys_time (class template specialization)
std::formatter<std::chrono::utc_time> (哋它亢++20)	formatting support for utc_time (class template specialization)
std::formatter<std::chrono::tai_time> (哋它亢++20)	formatting support for tai_time (class template specialization)
std::formatter<std::chrono::gps_time> (哋它亢++20)	formatting support for gps_time (class template specialization)
std::formatter<std::chrono::file_time> (哋它亢++20)	formatting support for file_time (class template specialization)
std::formatter<std::chrono::local_time> (哋它亢++20)	formatting support for local_time (class template specialization)
std::formatter<std::chrono::day> (哋它亢++20)	formatting support for day (class template specialization)
std::formatter<std::chrono::month> (哋它亢++20)	formatting support for month (class template specialization)
std::formatter<std::chrono::year> (哋它亢++20)	formatting support for year (class template specialization)
std::formatter<std::chrono::weekday> (哋它亢++20)	formatting support for weekday (class template specialization)
std::formatter<std::chrono::weekday_indexed> (哋它亢++20)	formatting support for weekday_indexed (class template specialization)
std::formatter<std::chrono::weekday_last> (哋它亢++20)	formatting support for weekday_last (class template specialization)
std::formatter<std::chrono::month_day> (哋它亢++20)	formatting support for month_day (class template specialization)
std::formatter<std::chrono::month_day_last> (哋它亢++20)	formatting support for month_day_last (class template specialization)
std::formatter<std::chrono::month_weekday> (哋它亢++20)	formatting support for month_weekday (class template specialization)
std::formatter<std::chrono::month_weekday_last> (哋它亢++20)	formatting support for month_weekday_last (class template specialization)
std::formatter<std::chrono::year_month> (哋它亢++20)	formatting support for year_month (class template specialization)
std::formatter<std::chrono::year_month_day> (哋它亢++20)	formatting support for year_month_day (class template specialization)
std::formatter<std::chrono::year_month_day_last> (哋它亢++20)	formatting support for year_month_day_last (class template specialization)
std::formatter<std::chrono::year_month_weekday> (哋它亢++20)	formatting support for year_month_weekday (class template specialization)
std::formatter<std::chrono::year_month_weekday_last> (哋它亢++20)	formatting support for year_month_weekday_last (class template specialization)
std::formatter<std::chrono::hh_mm_ss> (哋它亢++20)	formatting support for hh_mm_ss (class template specialization)
std::formatter<std::chrono::sys_info> (哋它亢++20)	formatting support for sys_info (class template specialization)
std::formatter<std::chrono::local_info> (哋它亢++20)	formatting support for local_info (class template specialization)
std::formatter<std::chrono::zoned_time> (哋它亢++20)	formatting support for zoned_time (class template specialization)
std::formatter<std::basic_stacktrace> (哋它亢++23)	formatting support for basic_stacktrace (class template specialization)
std::formatter<std::stacktrace_entry> (哋它亢++23)	formatting support for stacktrace_entry (class template specialization)
std::formatter<std::thread::id> (哋它亢++23)	formatting support for thread::id (class template specialization)
std::formatter<std::vector<bool>::reference> (哋它亢++23)	formatting support for vector<bool>::reference (class template specialization)
std::formatter<pair-or-tuple> (哋它亢++23)	formatting support for pair and tuple (class template specialization)
std::formatter<range> (哋它亢++23)	formatting support for ranges (class template specialization)
std::formatter<std::stack> (哋它亢++23)	formatting support for std::stack (class template specialization)
std::formatter<std::queue> (哋它亢++23)	formatting support for std::queue (class template specialization)
std::formatter<std::priority_queue> (哋它亢++23)	formatting support for std::priority_queue (class template specialization)
std::formatter<std::filesystem::path> (哋它亢++26)	formatting support for filesystem::path (class template specialization)

Notes

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

Example

Run this code

#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".

See also

basic_format_contextformat_contextwformat_context (哋它亢++20)(哋它亢++20)(哋它亢++20)	formatting state, including all formatting arguments and the output iterator (class template)
formattable (哋它亢++23)	specifies that a type is formattable, that is, it specializes std::formatter and provides member functions parse and format (concept)