cpp-btree – C++ B-tree – Google Project Hosting

cpp-btree - C++ B-tree - Google Project Hosting

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C++ B-tree is a template library that implements ordered in-memory containers based on a B-tree data structure. Similar to the STL map, set, multimap, and multiset templates, this library provides btree_map, btree_set, btree_multimap, and btree_multiset.

C++ B-tree containers have a few advantages compared with the standard containers, which are typically implemented using Red-Black trees. Nodes in a Red-Black tree require three pointers per entry (plus 1 bit), whereas B-trees on average make use of fewer than one pointer per entry, leading to significant memory savings. For example, a set<int32_t> has an overhead of 16 bytes for every 4 byte set element (on a 32-bit operating system); the corresponding btree_set<int32_t> has an overhead of around 1 byte per set element.

B-trees are widely known as data structures for secondary storage, because they keep disk seeks to a minimum. For an in-memory data structure, the same property yields a performance boost by keeping cache-line misses to a minimum. C++ B-tree containers make better use of the cache by performing multiple key-comparisons per node when searching the tree. Although B-tree algorithms are more complex, compared with the Red-Black tree algorithms, the improvement in cache behavior may account for a significant speedup in accessing large containers.

The C++ B-tree containers are not without drawbacks, however. Unlike the standard STL containers, modifying a C++ B-tree container invalidates all outstanding iterators on that container. For this reason, the library also contains "safe" variations on the four containers: iterators on safe B-tree containers keep a copy of the current key and automatically reposition the iterator whenever it is used.