ClickHouse/dbms/include/DB/IO/IReadableWriteBuffer.h

#pragma once
#include <memory>
#include <DB/IO/ReadBuffer.h>

namespace DB
{

struct IReadableWriteBuffer
{
	/// At the first time returns getReadBufferImpl(). Next calls return nullptr.
	inline std::shared_ptr<ReadBuffer> tryGetReadBuffer()
	{
		if (!can_reread)
			return nullptr;

		can_reread = false;
		return getReadBufferImpl();
	}

	virtual ~IReadableWriteBuffer() {}

protected:

	/// Creates read buffer from current write buffer.
	/// Returned buffer points to the first byte of original buffer.
	/// Original stream becomes invalid.
	virtual std::shared_ptr<ReadBuffer> getReadBufferImpl() = 0;

	bool can_reread = true;
};

}