[libc++][format] P3142R0: Printing Blank Lines with println (llvm#87277)

Implements https://wg21.link/P3142R0

Applied retroactively as DR, same as stdlibc++ and MS STL:

https://github.com/orgs/microsoft/projects/1143?pane=issue&itemId=57457187

Author: H-G-Hristov

libcxx/docs/ReleaseNotes/19.rst

@@ -44,6 +44,7 @@ Implemented Papers
 - P2495R3 - Interfacing ``stringstream``\s with ``string_view``
 - P2867R2 - Remove Deprecated ``strstream``\s From C++26
 - P2872R3 - Remove ``wstring_convert`` From C++26
+- P3142R0 - Printing Blank Lines with ``println`` (as DR against C++23)
 - P2302R4 - ``std::ranges::contains``
 - P1659R3 - ``std::ranges::starts_with`` and ``std::ranges::ends_with``
 

libcxx/docs/Status/Cxx2cPapers.csv

@@ -51,7 +51,7 @@
 "`P2869R4 <https://wg21.link/P2869R4>`__","LWG","Remove Deprecated ``shared_ptr`` Atomic Access APIs from C++26","Tokyo March 2024","","",""
 "`P2872R3 <https://wg21.link/P2872R3>`__","LWG","Remove ``wstring_convert`` From C++26","Tokyo March 2024","|Complete|","19.0",""
 "`P3107R5 <https://wg21.link/P3107R5>`__","LWG","Permit an efficient implementation of ``std::print``","Tokyo March 2024","","","|format| |DR|"
-"`P3142R0 <https://wg21.link/P3142R0>`__","LWG","Printing Blank Lines with ``println``","Tokyo March 2024","","","|format|"
+"`P3142R0 <https://wg21.link/P3142R0>`__","LWG","Printing Blank Lines with ``println``","Tokyo March 2024","|Complete|","19.0","|format|"
 "`P2845R8 <https://wg21.link/P2845R8>`__","LWG","Formatting of ``std::filesystem::path``","Tokyo March 2024","","","|format|"
 "`P0493R5 <https://wg21.link/P0493R5>`__","LWG","Atomic minimum/maximum","Tokyo March 2024","","",""
 "`P2542R8 <https://wg21.link/P2542R8>`__","LWG","``views::concat``","Tokyo March 2024","","","|ranges|"

libcxx/include/ostream

@@ -164,6 +164,7 @@ template<class... Args>
   void print(ostream& os, format_string<Args...> fmt, Args&&... args);
 template<class... Args>                                                                                // since C++23
   void println(ostream& os, format_string<Args...> fmt, Args&&... args);
+void println(ostream& os);                                                                             // since C++26
 
 void vprint_unicode(ostream& os, string_view fmt, format_args args);                                   // since C++23
 void vprint_nonunicode(ostream& os, string_view fmt, format_args args);                                // since C++23
@@ -1163,6 +1164,9 @@ _LIBCPP_HIDE_FROM_ABI void println(ostream& __os, format_string<_Args...> __fmt,
 #  endif // _LIBCPP_HAS_NO_UNICODE
 }
 
+template <class = void> // TODO PRINT template or availability markup fires too eagerly (http://llvm.org/PR61563).
+_LIBCPP_HIDE_FROM_ABI inline void println(ostream& __os) { std::print(__os, "\n"); }
+
 #endif // _LIBCPP_STD_VER >= 23
 
 _LIBCPP_END_NAMESPACE_STD

libcxx/include/print

@@ -15,8 +15,10 @@ namespace std {
   // [print.fun], print functions
   template<class... Args>
     void print(format_string<Args...> fmt, Args&&... args);
+  void println();                                                          // Since C++26
   template<class... Args>
     void print(FILE* stream, format_string<Args...> fmt, Args&&... args);
+  void println(FILE* stream);                                              // Since C++26
 
   template<class... Args>
     void println(format_string<Args...> fmt, Args&&... args);
@@ -356,6 +358,12 @@ _LIBCPP_HIDE_FROM_ABI void println(FILE* __stream, format_string<_Args...> __fmt
 #  endif // _LIBCPP_HAS_NO_UNICODE
 }
 
+template <class = void> // TODO PRINT template or availability markup fires too eagerly (http://llvm.org/PR61563).
+_LIBCPP_HIDE_FROM_ABI inline void println(FILE* __stream) { std::print(__stream, "\n"); }
+
+template <class = void> // TODO PRINT template or availability markup fires too eagerly (http://llvm.org/PR61563).
+_LIBCPP_HIDE_FROM_ABI inline void println() { println(stdout); }
+
 template <class... _Args>
 _LIBCPP_HIDE_FROM_ABI void println(format_string<_Args...> __fmt, _Args&&... __args) {
   std::println(stdout, __fmt, std::forward<_Args>(__args)...);

libcxx/test/std/input.output/iostream.format/output.streams/ostream.formatted/ostream.formatted.print/locale-specific_form.pass.cpp

@@ -26,6 +26,7 @@
 //   void print(ostream& os, format_string<Args...> fmt, Args&&... args);
 // template<class... Args>
 //   void println(ostream& os, format_string<Args...> fmt, Args&&... args);
+// void println(ostream& os);                                                // since C++26
 //
 // void vprint_unicode(ostream& os, string_view fmt, format_args args);
 // void vprint_nonunicode(ostream& os, string_view fmt, format_args args);
@@ -67,7 +68,7 @@ test(std::stringstream& stream, std::string expected, test_format_string<char, A
   // *** vprint_unicode ***
   {
     stream.str("");
-    ;
+
     std::vprint_unicode(stream, fmt.get(), std::make_format_args(args...));
     std::string out = stream.str();
     TEST_REQUIRE(out == expected,
@@ -77,7 +78,7 @@ test(std::stringstream& stream, std::string expected, test_format_string<char, A
   // *** vprint_nonunicode ***
   {
     stream.str("");
-    ;
+
     std::vprint_nonunicode(stream, fmt.get(), std::make_format_args(args...));
     std::string out = stream.str();
     TEST_REQUIRE(out == expected,
@@ -88,7 +89,7 @@ test(std::stringstream& stream, std::string expected, test_format_string<char, A
   {
     expected += '\n'; // Tested last since it changes the expected value.
     stream.str("");
-    ;
+
     std::println(stream, fmt, std::forward<Args>(args)...);
     std::string out = stream.str();
     TEST_REQUIRE(out == expected,
@@ -111,6 +112,7 @@ static void test(std::string expected, std::locale loc, test_format_string<char,
 }
 
 #ifndef TEST_HAS_NO_UNICODE
+
 struct numpunct_unicode : std::numpunct<char> {
   string_type do_truename() const override { return "gültig"; }
   string_type do_falsename() const override { return "ungültig"; }
@@ -2188,12 +2190,47 @@ static void test_floating_point() {
   test_floating_point_default_precision<F>();
 }
 
+static void test_println_blank_line(std::stringstream& stream) {
+  std::string expected{'\n'};
+  stream.str("");
+
+  std::println(stream);
+  std::string out = stream.str();
+  TEST_REQUIRE(out == expected,
+               TEST_WRITE_CONCATENATED("\nExpected output (blank line) ", expected, "\nActual output   ", out, '\n'));
+}
+
+static void test_println_blank_line(std::locale loc) {
+  std::stringstream stream;
+  stream.imbue(loc);
+  test_println_blank_line(stream);
+}
+
+static void test_println_blank_line() {
+  std::locale::global(std::locale(LOCALE_en_US_UTF_8));
+  assert(std::locale().name() == LOCALE_en_US_UTF_8);
+  std::stringstream stream;
+  test_println_blank_line(stream);
+
+  std::locale loc = std::locale(std::locale(), new numpunct<char>());
+  std::locale::global(loc);
+  test_println_blank_line(std::locale(LOCALE_en_US_UTF_8));
+
+#ifndef TEST_HAS_NO_UNICODE
+
+  std::locale loc_unicode = std::locale(std::locale(), new numpunct_unicode());
+  test_println_blank_line(loc_unicode);
+
+#endif // TEST_HAS_NO_UNICODE
+}
+
 int main(int, char**) {
   test_bool();
   test_integer();
   test_floating_point<float>();
   test_floating_point<double>();
   test_floating_point<long double>();
+  test_println_blank_line();
 
   return 0;
 }

libcxx/test/std/input.output/iostream.format/output.streams/ostream.formatted/ostream.formatted.print/println.pass.cpp

@@ -17,6 +17,7 @@
 
 // template<class... Args>
 //   void println(ostream& os, format_string<Args...> fmt, Args&&... args);
+// void println(ostream& os);                                                // since C++26
 
 // [ostream.formatted.print]/3
 //   If the function is vprint_unicode and os is a stream that refers to
@@ -55,8 +56,20 @@ auto test_exception = []<class... Args>(std::string_view, std::string_view, Args
   // The exceptions are tested by other functions that don't use the basic-format-string as fmt argument.
 };
 
+void test_println_blank_line() {
+  std::string expected{'\n'};
+
+  std::stringstream sstr;
+  std::println(sstr);
+
+  std::string out = sstr.str();
+  TEST_REQUIRE(out == expected,
+               TEST_WRITE_CONCATENATED("\nExpected output (blank line) ", expected, "\nActual output   ", out, '\n'));
+};
+
 int main(int, char**) {
   print_tests(test_file, test_exception);
+  test_println_blank_line();
 
   return 0;
 }

libcxx/test/std/input.output/iostream.format/print.fun/no_file_description.pass.cpp

@@ -25,8 +25,10 @@
 
 // template<class... Args>
 //   void print(FILE* stream, format_string<Args...> fmt, Args&&... args);
+// void println();                                                          // Since C++26
 // template<class... Args>
 //   void println(FILE* stream, format_string<Args...> fmt, Args&&... args);
+// void println(FILE* stream);                                              // Since C++26
 // void vprint_unicode(FILE* stream, string_view fmt, format_args args);
 // void vprint_nonunicode(FILE* stream, string_view fmt, format_args args);
 
@@ -63,6 +65,20 @@ static void test_println() {
   assert(std::string_view(buffer.data(), pos) == "hello world!\n");
 }
 
+static void test_println_blank_line() {
+  std::array<char, 100> buffer{0};
+
+  FILE* file = fmemopen(buffer.data(), buffer.size(), "wb");
+  assert(file);
+
+  std::println(file);
+  long pos = std::ftell(file);
+  std::fclose(file);
+
+  assert(pos > 0);
+  assert(std::string_view(buffer.data(), pos) == "\n");
+}
+
 static void test_vprint_unicode() {
   std::array<char, 100> buffer{0};
 
@@ -96,6 +112,7 @@ static void test_vprint_nonunicode() {
 int main(int, char**) {
   test_print();
   test_println();
+  test_println_blank_line();
   test_vprint_unicode();
   test_vprint_nonunicode();
 

libcxx/test/std/input.output/iostream.format/print.fun/println.blank_line.sh.cpp

@@ -0,0 +1,48 @@
+//===----------------------------------------------------------------------===//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+// UNSUPPORTED: no-filesystem
+// UNSUPPORTED: executor-has-no-bash
+// UNSUPPORTED: GCC-ALWAYS_INLINE-FIXME
+
+// FIXME PRINT How to test println on Windows?
+// XFAIL: msvc, target={{.+}}-windows-gnu
+
+// XFAIL: availability-fp_to_chars-missing
+
+// <print>
+
+//   void println();
+
+// Testing this properly is quite hard; the function unconditionally
+// writes to stdout. When stdout is redirected to a file it is no longer
+// considered a terminal. The function is a small wrapper around
+//
+// template<class... Args>
+//   void println(FILE* stream, format_string<Args...> fmt, Args&&... args);
+//
+// So do minimal tests for this function and rely on the FILE* overload
+// to do more testing.
+//
+// The testing is based on the testing for std::cout.
+
+// TODO PRINT Use lit builtin echo
+
+// FILE_DEPENDENCIES: echo.sh
+// RUN: %{build}
+// RUN: %{exec} bash echo.sh -ne "\n" > %t.expected
+// RUN: %{exec} "%t.exe" > %t.actual
+// RUN: diff -u %t.actual %t.expected
+
+#include <print>
+
+int main(int, char**) {
+  std::println();
+
+  return 0;
+}

libcxx/test/std/input.output/iostream.format/print.fun/println.file.pass.cpp

@@ -129,6 +129,29 @@ static void test_new_line() {
   }
 }
 
+static void test_println_blank_line() {
+  // Text does newline translation.
+  {
+    FILE* file = fopen(filename.c_str(), "w");
+    assert(file);
+
+    std::println(file);
+#ifndef _WIN32
+    assert(std::ftell(file) == 1);
+#else
+    assert(std::ftell(file) == 2);
+#endif
+  }
+  // Binary no newline translation.
+  {
+    FILE* file = fopen(filename.c_str(), "wb");
+    assert(file);
+
+    std::println(file);
+    assert(std::ftell(file) == 1);
+  }
+}
+
 int main(int, char**) {
   print_tests(test_file, test_exception);
 
@@ -137,6 +160,7 @@ int main(int, char**) {
 #endif
   test_read_only();
   test_new_line();
+  test_println_blank_line();
 
   return 0;
 }