c++ - 为什么 unordered_map 由于 "reserve"有足够的桶时会增加大小？

Question

考虑这个代码。我为 unordered_map 保留 6 个点并插入 6 个元素。之后，有7个桶。为什么是这样？ max_load_factor 为 1，并且有足够的存储桶用于我插入的元素数量。

#include <iostream>
#include <unordered_map>
using namespace std;

int main () {
  unordered_map<std::string,std::string> mymap = { 
            {"house","maison"},
            {"apple","pomme"},
            {"tree","arbre"},
            {"book","livre"},
            {"door","porte"},
            {"grapefruit","pamplemousse"}
  };

    unordered_map<std::string,std::string> mymap2; // THIS ONE!!!
    mymap2.reserve(6);
    for (auto i:mymap) {
        mymap2[i.first] = i.second;
    }


    std::cout << "max_load factor " << mymap2.max_load_factor() << " mymap has " << mymap2.bucket_count() << " buckets.\n";

      for (unsigned i=0; i<mymap2.bucket_count(); ++i) {
        cout << "bucket #" << i << " contains: ";
        for (auto it = mymap2.begin(i); it!=mymap2.end(i); ++it)
            cout << "[" << it->first << ":" << it->second << "] ";
          cout << endl;
      }

  return 0;
}

输出:

max_load factor 1 mymap has 7 buckets.
bucket #0 contains: 
bucket #1 contains: [book:livre] 
bucket #2 contains: [tree:arbre] 
bucket #3 contains: [house:maison] [grapefruit:pamplemousse] 
bucket #4 contains: 
bucket #5 contains: [door:porte] 
bucket #6 contains: [apple:pomme] 

Answer 1

cplusplus.com website给出了这个解释:

void reserve (size_type n);

Request a capacity change

Sets the number of buckets in the container (bucket_count) to the most appropriate to contain at least n elements.

If n is greater than the current bucket_count multiplied by the max_load_factor, the container's bucket_count is increased and a rehash is forced.

If n is lower than that, the function may have no effect.

在您声明您的 unordered_map 时变量，它有一个 bucket_count的 1和一个 max_load_factor的 1 .
那你 reserve 6大于 max_load_factor 的桶乘以 bucket_count根据这个定义，在我看来，这种行为是正确的。
我在行 17 添加在您的代码中添加以下行以显示 bucket_count之前 reserve事实上，它是 1

 std::cout << "BEFORE RESERVE max_load factor " << mymap2.max_load_factor() << " mymap has " << mymap2.bucket_count() << " buckets.\n";

显示如下:

BEFORE RESERVE max_load factor 1 mymap has 1 buckets.

预留后:

AFTER RESERVE max_load factor 1 mymap has 7 buckets.

因此，在我看来，这种行为是正常的。