Merge 2024-03 LWG Motion 6

P3107R5 Permit an efficient implementation of std::print

Author: tkoeppe

source/iostreams.tex

@@ -7706,13 +7706,19 @@
 \begin{itemdescr}
 \pnum
 \effects
+Let \tcode{locksafe} be
+\tcode{(enable_nonlocking_formatter_optimization<remove_cvref_t<Args>> \&\& ...)}.
 If the ordinary literal encoding\iref{lex.charset} is UTF-8, equivalent to:
 \begin{codeblock}
-vprint_unicode(stream, fmt.@\exposid{str}@, make_format_args(args...));
+locksafe
+  ? vprint_unicode_locking(stream, fmt.str, make_format_args(args...))
+  : vprint_unicode(stream, fmt.str, make_format_args(args...));
 \end{codeblock}
 Otherwise, equivalent to:
 \begin{codeblock}
-vprint_nonunicode(stream, fmt.@\exposid{str}@, make_format_args(args...));
+locksafe
+  ? vprint_nonunicode_locking(stream, fmt.str, make_format_args(args...))
+  : vprint_nonunicode(stream, fmt.str, make_format_args(args...));
 \end{codeblock}
 \end{itemdescr}
 
@@ -7742,7 +7748,7 @@
 \effects
 Equivalent to:
 \begin{codeblock}
-print(stream, "{}\n", format(fmt, std::forward<Args>(args)...));
+print(stream, runtime_format(string(fmt.get()) + '\n'), std::forward<Args>(args)...);
 \end{codeblock}
 \end{itemdescr}
 
@@ -7765,17 +7771,32 @@
 void vprint_unicode(FILE* stream, string_view fmt, format_args args);
 \end{itemdecl}
 
+\begin{itemdescr}
+\pnum
+\effects
+Equivalent to:
+\begin{codeblock}
+string out = vformat(fmt, args);
+vprint_unicode_locking(stream, "{}", make_format_args(out));
+\end{codeblock}
+\end{itemdescr}
+
+\indexlibraryglobal{vprint_unicode_locking}%
+\begin{itemdecl}
+void vprint_unicode_locking(FILE* stream, string_view fmt, format_args args);
+\end{itemdecl}
+
 \begin{itemdescr}
 \pnum
 \expects
 \tcode{stream} is a valid pointer to an output C stream.
 
 \pnum
 \effects
-The function initializes an automatic variable via
-\begin{codeblock}
-string out = vformat(fmt, args);
-\end{codeblock}
+Locks \tcode{stream}.
+Let \tcode{out} denote the character representation of
+formatting arguments provided by \tcode{args}
+formatted according to specifications given in \tcode{fmt}.
 If \tcode{stream} refers to a terminal capable of displaying Unicode,
 writes \tcode{out} to the terminal using the native Unicode API;
 if \tcode{out} contains invalid code units,
@@ -7785,6 +7806,10 @@
 Otherwise writes \tcode{out} to \tcode{stream} unchanged.
 If the native Unicode API is used,
 the function flushes \tcode{stream} before writing \tcode{out}.
+Unconditionally unlocks \tcode{stream} on function exit.
+
+\xrefc{7.21.2}.
+
 \begin{note}
 On POSIX and Windows, \tcode{stream} referring to a terminal means that,
 respectively,
@@ -7829,14 +7854,32 @@
 void vprint_nonunicode(FILE* stream, string_view fmt, format_args args);
 \end{itemdecl}
 
+\begin{itemdescr}
+\pnum
+\effects
+Equivalent to:
+\begin{codeblock}
+string out = vformat(fmt, args);
+vprint_nonunicode_locking("{}", make_format_args(out));
+\end{codeblock}
+\end{itemdescr}
+
+\indexlibraryglobal{vprint_nonunicode_locking}%
+\begin{itemdecl}
+void vprint_nonunicode_locking(FILE* stream, string_view fmt, format_args args);
+\end{itemdecl}
+
 \begin{itemdescr}
 \pnum
 \expects
 \tcode{stream} is a valid pointer to an output C stream.
 
 \pnum
 \effects
-Writes the result of \tcode{vformat(fmt, args)} to \tcode{stream}.
+While holding the lock on \tcode{stream},
+writes the character representation of
+formatting arguments provided by \tcode{args}
+formatted according to specifications given in \tcode{fmt} to \tcode{stream}.
 
 \pnum
 \throws

source/support.tex

@@ -722,7 +722,7 @@
 #define @\defnlibxname{cpp_lib_out_ptr}@                           202311L // freestanding, also in \libheader{memory}
 #define @\defnlibxname{cpp_lib_parallel_algorithm}@                201603L // also in \libheader{algorithm}, \libheader{numeric}
 #define @\defnlibxname{cpp_lib_polymorphic_allocator}@             201902L // also in \libheader{memory_resource}
-#define @\defnlibxname{cpp_lib_print}@                             202207L // also in \libheader{print}, \libheader{ostream}
+#define @\defnlibxname{cpp_lib_print}@                             202403L // also in \libheader{print}, \libheader{ostream}
 #define @\defnlibxname{cpp_lib_quoted_string_io}@                  201304L // also in \libheader{iomanip}
 #define @\defnlibxname{cpp_lib_ranges}@                            202302L
   // also in \libheader{algorithm}, \libheader{functional}, \libheader{iterator}, \libheader{memory}, \libheader{ranges}

source/utilities.tex

@@ -15670,6 +15670,10 @@
   // \ref{format.formatter}, formatter
   template<class T, class charT = char> struct formatter;
 
+  // \ref{format.formatter.locking}, formatter locking
+  template<class T>
+    constexpr bool enable_nonlocking_formatter_optimization = false;
+
   // \ref{format.formattable}, concept \libconcept{formattable}
   template<class T, class charT>
     concept formattable = @\seebelow@;
@@ -16913,6 +16917,24 @@
 \\
 \end{concepttable}
 
+\rSec3[format.formatter.locking]{Formatter locking}
+
+\indexlibraryglobal{enable_nonlocking_formatter_optimization}%
+\begin{itemdecl}
+template<class T>
+  constexpr bool enable_nonlocking_formatter_optimization = false;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\remarks
+Pursuant to \ref{namespace.std},
+users may specialize \tcode{enable_nonlocking_formatter_optimization} for
+cv-unqualified program-defined types.
+Such specializations shall be usable in constant expressions\iref{expr.const}
+and have type \tcode{const bool}.
+\end{itemdescr}
+
 \rSec3[format.formattable]{Concept \cname{formattable}}
 
 \pnum
@@ -17021,6 +17043,13 @@
 interpret the format specification
 as a \fmtgrammarterm{std-format-spec}
 as described in \ref{format.string.std}.
+In addition,
+for each type \tcode{T} for which
+a \tcode{formatter} specialization is provided above,
+each of the headers provides the following specialization:
+\begin{codeblock}
+template<> inline constexpr bool enable_nonlocking_formatter_optimization<T> = true;
+\end{codeblock}
 \begin{note}
 Specializations such as \tcode{formatter<wchar_t, char>}
 and \tcode{formatter<const char*, wchar_t>}