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Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/../../cpp/utility/format/formatter.html below:

std::formatter - cppreference.com

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:

[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< class Traits, class Alloc > struct formatter<std::basic_string<CharT, Traits, Alloc>, CharT>; (7) (8)

Arithmetic formatters

template<> struct formatter<ArithmeticT, CharT>;

(9)

Pointer formatters

(10)

template<> struct formatter<void*, CharT>;

(11)

template<> struct formatter<const void*, CharT>;

(12)

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) (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 [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] class template that helps implementing std::formatter specializations for range types
(class template) [edit]

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