[libcxx-commits] [libcxx] [libc++] Implement p0753r2 Manipulators for C++ Synchronized Buffered Ostream (PR #97955)

[Louis Dionne via libcxx-commits]

================
@@ -0,0 +1,92 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___OSTREAM_SYNCBUF_BASE_H
+#define _LIBCPP___OSTREAM_SYNCBUF_BASE_H
+
+#include <__config>
+#include <__ostream/basic_ostream.h>
+#include <streambuf>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 20 && !defined(_LIBCPP_HAS_NO_EXPERIMENTAL_SYNCSTREAM)
+
+template <class _CharT, class _Traits>
+class _LIBCPP_TEMPLATE_VIS __syncbuf_base : public basic_streambuf<_CharT, _Traits> {
+public:
+
+protected:
+  _LIBCPP_HIDE_FROM_ABI explicit __syncbuf_base(bool __b = false) : __emit_on_sync_(__b) {}
+
+private:
+  bool __emit_on_sync_{false};
+
+  virtual bool __emit() = 0;
+
+  template <class, class, class>
+  friend class basic_syncbuf;
+
+  friend struct __syncbuf_base_access;
+};
+
+struct __syncbuf_base_access {
+  template <class _CharT, class _Traits>
+  _LIBCPP_HIDE_FROM_ABI static void __set_emit_on_sync(__syncbuf_base<_CharT, _Traits>* __buf, bool __b) {
+    __buf->__emit_on_sync_ = __b;
+  }
+
+  template <class _CharT, class _Traits>
+  _LIBCPP_HIDE_FROM_ABI static bool __emit(__syncbuf_base<_CharT, _Traits>* __buf) {
+    return __buf->__emit();
+  }
+};
+
+template <class _CharT, class _Traits>
+_LIBCPP_HIDE_FROM_ABI basic_ostream<_CharT, _Traits>& emit_on_flush(basic_ostream<_CharT, _Traits>& __os) {
+  if (auto* __buf = dynamic_cast<__syncbuf_base<_CharT, _Traits>*>(__os.rdbuf())) {
+    __syncbuf_base_access::__set_emit_on_sync(__buf, true);
+  }
+  return __os;
+}
+
+template <class _CharT, class _Traits>
+_LIBCPP_HIDE_FROM_ABI basic_ostream<_CharT, _Traits>& noemit_on_flush(basic_ostream<_CharT, _Traits>& __os) {
+  if (auto* __buf = dynamic_cast<__syncbuf_base<_CharT, _Traits>*>(__os.rdbuf())) {
+    __syncbuf_base_access::__set_emit_on_sync(__buf, false);
+  }
+  return __os;
+}
+
+template <class _CharT, class _Traits>
+_LIBCPP_HIDE_FROM_ABI basic_ostream<_CharT, _Traits>& flush_emit(basic_ostream<_CharT, _Traits>& __os) {
+  __os.flush();
+  if (auto* __buf = dynamic_cast<__syncbuf_base<_CharT, _Traits>*>(__os.rdbuf())) {
+    // The standard specifies that:
+    // After constructing a sentry object, calls buf->emit().
+    // If that call returns false, calls os.setstate(ios_base::badbit).
+    //
+    // syncstream::emit already constructs a sentry
+    bool __emit_result = __syncbuf_base_access::__emit(__buf);
+    if (!__emit_result) {
+      __os.setstate(ios_base::badbit);
----------------
ldionne wrote:

How would we reach that state? We should have a test for that.

https://github.com/llvm/llvm-project/pull/97955