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112 righe
3.9 KiB
112 righe
3.9 KiB
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file. See the AUTHORS file for names of contributors.
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//
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// An iterator yields a sequence of key/value pairs from a source.
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// The following class defines the interface. Multiple implementations
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// are provided by this library. In particular, iterators are provided
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// to access the contents of a Table or a DB.
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//
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// Multiple threads can invoke const methods on an Iterator without
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// external synchronization, but if any of the threads may call a
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// non-const method, all threads accessing the same Iterator must use
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// external synchronization.
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#ifndef STORAGE_LEVELDB_INCLUDE_ITERATOR_H_
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#define STORAGE_LEVELDB_INCLUDE_ITERATOR_H_
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#include "leveldb/export.h"
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#include "leveldb/slice.h"
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#include "leveldb/status.h"
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namespace leveldb {
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class LEVELDB_EXPORT Iterator {
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public:
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Iterator();
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Iterator(const Iterator&) = delete;
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Iterator& operator=(const Iterator&) = delete;
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virtual ~Iterator();
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// An iterator is either positioned at a key/value pair, or
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// not valid. This method returns true iff the iterator is valid.
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virtual bool Valid() const = 0;
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// Position at the first key in the source. The iterator is Valid()
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// after this call iff the source is not empty.
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virtual void SeekToFirst() = 0;
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// Position at the last key in the source. The iterator is
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// Valid() after this call iff the source is not empty.
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virtual void SeekToLast() = 0;
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// Position at the first key in the source that is at or past target.
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// The iterator is Valid() after this call iff the source contains
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// an entry that comes at or past target.
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virtual void Seek(const Slice& target) = 0;
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// Moves to the next entry in the source. After this call, Valid() is
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// true iff the iterator was not positioned at the last entry in the source.
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// REQUIRES: Valid()
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virtual void Next() = 0;
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// Moves to the previous entry in the source. After this call, Valid() is
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// true iff the iterator was not positioned at the first entry in source.
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// REQUIRES: Valid()
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virtual void Prev() = 0;
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// Return the key for the current entry. The underlying storage for
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// the returned slice is valid only until the next modification of
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// the iterator.
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// REQUIRES: Valid()
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virtual Slice key() const = 0;
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// Return the value for the current entry. The underlying storage for
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// the returned slice is valid only until the next modification of
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// the iterator.
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// REQUIRES: Valid()
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virtual Slice value() const = 0;
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// If an error has occurred, return it. Else return an ok status.
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virtual Status status() const = 0;
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// Clients are allowed to register function/arg1/arg2 triples that
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// will be invoked when this iterator is destroyed.
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//
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// Note that unlike all of the preceding methods, this method is
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// not abstract and therefore clients should not override it.
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using CleanupFunction = void (*)(void* arg1, void* arg2);
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void RegisterCleanup(CleanupFunction function, void* arg1, void* arg2);
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private:
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// Cleanup functions are stored in a single-linked list.
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// The list's head node is inlined in the iterator.
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struct CleanupNode {
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// True if the node is not used. Only head nodes might be unused.
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bool IsEmpty() const { return function == nullptr; }
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// Invokes the cleanup function.
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void Run() {
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assert(function != nullptr);
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(*function)(arg1, arg2);
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}
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// The head node is used if the function pointer is not null.
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CleanupFunction function;
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void* arg1;
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void* arg2;
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CleanupNode* next;
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};
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CleanupNode cleanup_head_;
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};
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// Return an empty iterator (yields nothing).
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LEVELDB_EXPORT Iterator* NewEmptyIterator();
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// Return an empty iterator with the specified status.
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LEVELDB_EXPORT Iterator* NewErrorIterator(const Status& status);
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} // namespace leveldb
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#endif // STORAGE_LEVELDB_INCLUDE_ITERATOR_H_
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