c++ - boost::unordered_multimap 是否调整大小

Question

在初始化 boost::unordered_multimap 时，我们定义 HashMap 的大小。

explicit unordered_multimap(size_type n = implementation-defined, 
                            hasher const& hf = hasher(), 
                            key_equal const& eq = key_equal(), 
                            allocator_type const& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eq as the key equality predicate, a as the allocator and a maximum load factor of 1.0.

当 hash map 快满时，它会调整大小吗？它似乎没有这样的API。它会自动执行吗？定义 unordered_multimap 大小的最佳做法是什么？

Answer 1

它会自动完成，as documented :

The number of buckets can be automatically increased by a call to insert, or as the result of calling rehash.

但请记住，收缩时它不会重新散列:Why does C++11/Boost `unordered_map` not rehash when erasing?

您可以通过手动强制降低负载系数并将最大负载系数设置得足够高(http://www.boost.org/doc/libs/1_64_0/doc/html/boost/unordered_map.html#idp776321712-bb、http://www.boost.org/doc/libs/1_64_0/doc/html/boost/unordered_map.html#idp776318000-bb)来防止不必要的重新散列