[clang] [llvm] [Support] Add VirtualOutputBackends to virtualize the output from tools (PR #68447)

[via cfe-commits]

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+//===- raw_ostream_proxy.h - Proxies for raw output streams -----*- 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 LLVM_SUPPORT_RAW_OSTREAM_PROXY_H
+#define LLVM_SUPPORT_RAW_OSTREAM_PROXY_H
+
+#include "llvm/Support/raw_ostream.h"
+
+namespace llvm {
+
+/// Common bits for \a raw_ostream_proxy_adaptor<>, split out to dedup in
+/// template instantions.
+class raw_ostream_proxy_adaptor_base {
+protected:
+  raw_ostream_proxy_adaptor_base() = delete;
+  raw_ostream_proxy_adaptor_base(const raw_ostream_proxy_adaptor_base &) =
+      delete;
+
+  explicit raw_ostream_proxy_adaptor_base(raw_ostream &OS)
+      : OS(&OS), PreferredBufferSize(OS.GetBufferSize()) {
+    // Drop OS's buffer to make this->flush() forward. This proxy will add a
+    // buffer in its place.
+    OS.SetUnbuffered();
+  }
+
+  ~raw_ostream_proxy_adaptor_base() {
+    assert(!OS && "Derived objects should call resetProxiedOS()");
+  }
+
+  /// Stop proxying the stream, taking the derived object by reference as \p
+  /// ThisProxyOS.  Updates \p ThisProxyOS to stop buffering before setting \a
+  /// OS to \c nullptr, ensuring that future writes crash immediately.
+  void resetProxiedOS(raw_ostream &ThisProxyOS) {
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zmodem wrote:

The parameter is confusing to me. Is ThisProxyOS always going to be == OS here? If not, how are they related? If yes, why take the parameter?

https://github.com/llvm/llvm-project/pull/68447