C++ STL中的unordered_multimap rehash()函数

unordered_multimap :: rehash(N)是C++ STL中的内置函数，可将容器中的存储桶数设置为N或更大。如果N大于容器中当前的存储桶数(bucket_count)，则将强制进行重新哈希处理。
新的存储桶计数可以等于或大于N。如果n小于容器中当前存储桶的数量(bucket_count)，则该函数可能对存储桶计数没有影响，并且可能不会强制进行重新哈希。
重新哈希是哈希表的重建：容器中的所有元素根据其哈希值重新排列到新的存储桶集中。尽管可以保留具有等效键的元素的相对顺序，但是这可能会更改容器内元素的迭代顺序。只要容器的负载因子在操作中超过其max_load_factor，容器就会自动执行重整。通过调用rehash在哈希表中保留一定数量的最小存储桶，我们避免了容器扩展可能导致的多次rehash。

句法：

unordered_multimap_name.rehash(N)

参数：该函数接受单个强制性参数N ，该参数N指定容器哈希表的最小存储桶数。

返回值：该函数不返回任何内容。

下面的程序说明了上述函数：

程序1：

// C++ program to illustrate the
// unordered_multimap::rehash()
#include 
using namespace std;
  
int main()
{
  
    // declaration
    unordered_multimap sample1, sample2;
  
    // the sample1 size is reserved for
    // the bucket to contain a minimum of
    // one elements
    sample1.rehash(1);
  
    // inserts key and element
    // in sample1
    sample1.insert({ 10, 100 });
    sample1.insert({ 50, 500 });
  
    // inserts key and element
    // in sample1
  
    // the sample1 size is reserved for
    // the bucket to contain a minimum of
    // three elements
    sample2.rehash(3);
  
    sample2.insert({ 20, 200 });
    sample2.insert({ 30, 300 });
    sample2.insert({ 30, 150 });
  
    cout << "The size of Sample1 is: " << sample1.size();
  
    cout << "\nKey and Elements of Sample1 are:";
    for (auto it = sample1.begin(); it != sample1.end(); it++) {
        cout << "{" << it->first << ", " << it->second << "} ";
    }
  
    cout << "\n\nThe size of Sample2 is: " << sample2.size();
  
    cout << "\nKey and Elements of Sample2 are:";
    for (auto it = sample2.begin(); it != sample2.end(); it++) {
        cout << "{" << it->first << ", " << it->second << "} ";
    }
  
    return 0;
}

输出：

The size of Sample1 is: 2
Key and Elements of Sample1 are:{50, 500} {10, 100} 

The size of Sample2 is: 3
Key and Elements of Sample2 are:{30, 150} {30, 300} {20, 200}

程式2：

// C++ program to illustrate the
// unordered_multimap::rehash()
#include 
using namespace std;
  
int main()
{
  
    // declaration
    unordered_multimap sample1, sample2;
  
    // the sample1 size is reserved for
    // the bucket to contain a minimum of
    // one elements
    sample1.rehash(1);
  
    // inserts key and element
    // in sample1
    sample1.insert({ 'a', 'A' });
    sample1.insert({ 'g', 'G' });
  
    // inserts key and element
    // in sample1
  
    // the sample1 size is reserved for
    // the bucket to contain a minimum of
    // three elements
    sample2.rehash(3);
  
    sample2.insert({ 'b', 'B' });
    sample2.insert({ 'c', 'C' });
    sample2.insert({ 'd', 'D' });
  
    cout << "The size of Sample1 is: " << sample1.size();
  
    cout << "\nKey and Elements of Sample1 are:";
    for (auto it = sample1.begin(); it != sample1.end(); it++) {
        cout << "{" << it->first << ", " << it->second << "} ";
    }
  
    cout << "\n\nThe size of Sample2 is: " << sample2.size();
  
    cout << "\nKey and Elements of Sample2 are:";
    for (auto it = sample2.begin(); it != sample2.end(); it++) {
        cout << "{" << it->first << ", " << it->second << "} ";
    }
  
    return 0;
}

输出：

The size of Sample1 is: 2
Key and Elements of Sample1 are:{g, G} {a, A} 

The size of Sample2 is: 3
Key and Elements of Sample2 are:{d, D} {c, C} {b, B}

参考： http : //www.cplusplus.com/reference/unordered_map/unordered_multimap/rehash/

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