[libcxx-commits] [libcxx] WIP - [libc++][spanstream] P0448R4: A `strstream` replacement using `span<charT>` as buffer (PR #83541)

[Hristo Hristov via libcxx-commits]

================
@@ -0,0 +1,456 @@
+// -*- 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_SPANSTREAM
+#define _LIBCPP_SPANSTREAM
+
+// clang-format off
+
+/*
+  Span-based streams [span.streams]
+
+  template<class charT, class traits = char_traits<charT>>
+    class basic_spanbuf;
+
+  template<class charT, class traits>
+    void swap(basic_spanbuf<charT, traits>& x, basic_spanbuf<charT, traits>& y);
+
+  using spanbuf = basic_spanbuf<char>;
+  using wspanbuf = basic_spanbuf<wchar_t>;
+
+  template<class charT, class traits = char_traits<charT>>
+    class basic_ispanstream;
+
+  template<class charT, class traits>
+    void swap(basic_ispanstream<charT, traits>& x, basic_ispanstream<charT, traits>& y);
+
+  using ispanstream = basic_ispanstream<char>;
+  using wispanstream = basic_ispanstream<wchar_t>;
+
+  template<class charT, class traits = char_traits<charT>>
+    class basic_ospanstream;
+
+  template<class charT, class traits>
+    void swap(basic_ospanstream<charT, traits>& x, basic_ospanstream<charT, traits>& y);
+
+  using ospanstream = basic_ospanstream<char>;
+  using wospanstream = basic_ospanstream<wchar_t>;
+
+  template<class charT, class traits = char_traits<charT>>
+    class basic_spanstream;
+
+  template<class charT, class traits>
+    void swap(basic_spanstream<charT, traits>& x, basic_spanstream<charT, traits>& y);
+
+  using spanstream = basic_spanstream<char>;
+  using wspanstream = basic_spanstream<wchar_t>;
+*/
+
+// clang-format on
+
+#include <__assert> // all public C++ headers provide the assertion handler
+#include <__availability>
+#include <__concepts/convertible_to.h>
+#include <__config>
+#include <__fwd/spanstream.h>
+#include <__memory/addressof.h>
+#include <__ranges/concepts.h>
+#include <__utility/cmp.h>
+#include <__utility/forward.h>
+#include <__utility/move.h>
+#include <__utility/swap.h>
+#include <iostream>
+#include <span>
+#include <streambuf>
+#include <version>
+
+// #include <print>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 23
+
+// Class template basic_spanbuf [spanbuf]
+
+template <class _CharT, class _Traits>
+class _LIBCPP_TEMPLATE_VIS basic_spanbuf : public basic_streambuf<_CharT, _Traits> {
+public:
+  using char_type   = _CharT;
+  using int_type    = typename _Traits::int_type;
+  using pos_type    = typename _Traits::pos_type;
+  using off_type    = typename _Traits::off_type;
+  using traits_type = _Traits;
+
+  // [spanbuf.cons], constructors
+
+  _LIBCPP_HIDE_FROM_ABI basic_spanbuf() : basic_spanbuf(ios_base::in | ios_base::out) {}
+
+  _LIBCPP_HIDE_FROM_ABI explicit basic_spanbuf(ios_base::openmode __which)
+      : basic_spanbuf(std::span<_CharT>(), __which) {}
+
+  _LIBCPP_HIDE_FROM_ABI explicit basic_spanbuf(std::span<_CharT> __s,
+                                               ios_base::openmode __which = ios_base::in | ios_base::out)
+      : basic_streambuf<_CharT, _Traits>{}, __mode_(__which) {
+    span(__s);
+  }
+
+  basic_spanbuf(const basic_spanbuf&) = delete;
+
+  _LIBCPP_HIDE_FROM_ABI basic_spanbuf(basic_spanbuf&& __rhs)
+      : basic_streambuf<_CharT, _Traits>{std::move(__rhs)},
+        __mode_{std::move(__rhs.__mode_)},
+        __buf_{std::move(__rhs.__buf_)} {}
+  // __buf_{std::exchange(__rhs.__buf_, {})} {}
+
----------------
H-G-Hristov wrote:

Alternative implementation with `std::exchange`, which sets an empty `span` (`data() == nullptr`):

```suggestion
  _LIBCPP_HIDE_FROM_ABI basic_spanbuf(basic_spanbuf&& __rhs)
      : basic_streambuf<_CharT, _Traits>{std::move(__rhs)},
        __mode_{std::move(__rhs.__mode_)},
        __buf_{std::exchange(__rhs.__buf_, {})} {}

```

https://github.com/llvm/llvm-project/pull/83541