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cppdraft/format/formatter/locking.md

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+[format.formatter.locking]
+
+# 28 Text processing library [[text]](./#text)
+
+## 28.5 Formatting [[format]](format#formatter.locking)
+
+### 28.5.6 Formatter [[format.formatter]](format.formatter#locking)
+
+#### 28.5.6.2 Formatter locking [format.formatter.locking]
+
+[🔗](#lib:enable_nonlocking_formatter_optimization)
+
+`template<class T>
+  constexpr bool enable_nonlocking_formatter_optimization = false;
+`
+
+[1](#1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/text.tex#L7086)
+
+*Remarks*: Pursuant to [[namespace.std]](namespace.std "16.4.5.2.1 Namespace std"),
+users may specialize enable_nonlocking_formatter_optimization for
+cv-unqualified program-defined types[.](#1.sentence-1)
+
+Such specializations shall be usable in constant expressions ([[expr.const]](expr.const "7.7 Constant expressions"))
+and have type const bool[.](#1.sentence-2)

cppdraft/format/formatter/spec.md

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+[format.formatter.spec]
+
+# 28 Text processing library [[text]](./#text)
+
+## 28.5 Formatting [[format]](format#formatter.spec)
+
+### 28.5.6 Formatter [[format.formatter]](format.formatter#spec)
+
+#### 28.5.6.4 Formatter specializations [format.formatter.spec]
+
+[1](#1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/text.tex#L7130)
+
+The functions defined in [[format.functions]](format.functions "28.5.5 Formatting functions") use
+specializations of the class template formatter to format
+individual arguments[.](#1.sentence-1)
+
+[2](#2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/text.tex#L7136)
+
+Let charT be either char or wchar_t[.](#2.sentence-1)
+
+Each specialization of formatter is either enabled or disabled,
+as described below[.](#2.sentence-2)
+
+A [*debug-enabled*](#def:debug-enabled "28.5.6.4 Formatter specializations [format.formatter.spec]") specialization of formatter additionally provides
+a public, constexpr, non-static member function set_debug_format() which modifies the state of the formatter to be as if
+the type of the *std-format-spec* parsed by the last call to parse were ?[.](#2.sentence-3)
+
+Each header that declares the template formatter provides the following enabled specializations:
+
+- [(2.1)](#2.1)
+
+The debug-enabled specializationstemplate<> struct formatter<char, char>;template<> struct formatter<char, wchar_t>;template<> struct formatter<wchar_t, wchar_t>;
+
+- [(2.2)](#2.2)
+
+For each charT,
+the debug-enabled string type specializationstemplate<> struct formatter<charT*, charT>;template<> struct formatter<const charT*, charT>;template<size_t N> struct formatter<charT[N], charT>;template<class traits, class Allocator>struct formatter<basic_string<charT, traits, Allocator>, charT>;template<class traits>struct formatter<basic_string_view<charT, traits>, charT>;
+
+- [(2.3)](#2.3)
+
+For each charT,
+for each cv-unqualified arithmetic type ArithmeticT other thanchar,wchar_t,char8_t,char16_t, orchar32_t,
+a specializationtemplate<> struct formatter<ArithmeticT, charT>;
+
+- [(2.4)](#2.4)
+
+For each charT,
+the pointer type specializationstemplate<> struct formatter<nullptr_t, charT>;template<> struct formatter<void*, charT>;template<> struct formatter<const void*, charT>;
+
+The parse member functions of these formatters
+interpret the format specification
+as a *std-format-spec* as described in [[format.string.std]](format.string.std "28.5.2.2 Standard format specifiers")[.](#2.sentence-4)
+
+[3](#3)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/text.tex#L7204)
+
+Unless specified otherwise, for each type T for which
+a formatter specialization is provided by the library,
+each of the headers provides the following specialization:template<> inline constexpr bool enable_nonlocking_formatter_optimization<T> = true;
+
+[*Note [1](#note-1)*:
+
+Specializations such as formatter<wchar_t, char> that would require implicit
+multibyte / wide string or character conversion are disabled[.](#3.sentence-1)
+
+— *end note*]
+
+[4](#4)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/text.tex#L7217)
+
+The header <format> provides
+the following disabled specializations:
+
+- [(4.1)](#4.1)
+
+The string type specializationstemplate<> struct formatter<char*, wchar_t>;template<> struct formatter<const char*, wchar_t>;template<size_t N> struct formatter<char[N], wchar_t>;template<class traits, class Allocator>struct formatter<basic_string<char, traits, Allocator>, wchar_t>;template<class traits>struct formatter<basic_string_view<char, traits>, wchar_t>;
+
+[5](#5)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/text.tex#L7234)
+
+For any types T and charT for which
+neither the library nor the user provides
+an explicit or partial specialization of
+the class template formatter,formatter<T, charT> is disabled[.](#5.sentence-1)
+
+[6](#6)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/text.tex#L7241)
+
+If the library provides an explicit or partial specialization offormatter<T, charT>, that specialization is enabled
+and meets the *Formatter* requirements
+except as noted otherwise[.](#6.sentence-1)
+
+[7](#7)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/text.tex#L7247)
+
+If F is a disabled specialization of formatter, these
+values are false:
+
+- [(7.1)](#7.1)
+
+is_default_constructible_v<F>,
+
+- [(7.2)](#7.2)
+
+is_copy_constructible_v<F>,
+
+- [(7.3)](#7.3)
+
+is_move_constructible_v<F>,
+
+- [(7.4)](#7.4)
+
+is_copy_assignable_v<F>, and
+
+- [(7.5)](#7.5)
+
+is_move_assignable_v<F>[.](#7.sentence-1)
+
+[8](#8)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/text.tex#L7258)
+
+An enabled specialization formatter<T, charT> meets the*BasicFormatter* requirements ([[formatter.requirements]](formatter.requirements "28.5.6.1 Formatter requirements"))[.](#8.sentence-1)
+
+[*Example [1](#example-1)*: #include <format>#include <string>enum color { red, green, blue };const char* color_names[] = { "red", "green", "blue" };
+
+template<> struct std::formatter<color> : std::formatter<const char*> {auto format(color c, format_context& ctx) const {return formatter<const char*>::format(color_names[c], ctx); }};
+
+struct err {};
+
+std::string s0 = std::format("{}", 42); // OK, library-provided formatter std::string s1 = std::format("{}", L"foo"); // error: disabled formatter std::string s2 = std::format("{}", red); // OK, user-provided formatter std::string s3 = std::format("{}", err{}); // error: disabled formatter — *end example*]