c++ - std::unordered_set::load_factor，为什么是 float 而不是 double？

Question

据我所知，float 和 double 之间最快的类型取决于 native ALU 实现，它通常基于 double 。当你根据反向精度进行计算时，ALU必须一直做相应的精度转换。

那么，为什么标准选择 float 来表示 load_factor？我想这是为了节省对哈希表容器的内存思考，但我想知道是否有更充分的理由。

Answer 1

这发生在 revision 3 of the original proposal 中:

Changed load factor operations to use float instead of double

稍后给出基本原理(在“E. 哈希调整大小的控制”下):

Should the floating-point parameter be of type float, or of type double? It makes very little difference. On the one hand, double is typically the "natural" floating-point type that is used in the absence of a strong reason to the contrary. On the other hand, float may allow the hash table implementation to save some space, and may alert users to the fact that the value will not be used in any context that involves high precision. I have chosen float.

基本上就是你所说的。

至于性能，这里提到了这一点，并且它在宏伟的计划中并不重要(尽管在捍卫浮点与整数的使用的背景下):

The cost of a runtime floating-point parameter is one floating-point multiplication at every rehash (not every insertion). Even with incremental hashing, this is almost certain to be dwarfed by the cost of a rehash.