C++ unordered_multimap rehash()用法及代码示例

unordered_multimap::rehash(N)是C++ STL中的内置函数，该函数将容器中的存储桶数设置为N或更大。如果N大于容器中的当前存储桶数(bucket_count)，则会强制进行重新哈希处理。
新的存储区计数可以等于或大于N。如果n小于容器中当前存储区的数量(bucket_count)，该函数可能对存储区计数没有影响，也可能不会强制重新哈希。
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。每当容器重新进行修补时，容器都会自动执行负载系数在操作中将超越其max_load_factor。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引起的多次哈希。

用法:

unordered_multimap_name.rehash(N)

参数：该函数接受单个强制性参数N，该参数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。

以下示例程序旨在说明上述函数：

示例1:

// C++ program to illustrate the 
// unordered_multimap::rehash() 
#include <bits/stdc++.h> 
using namespace std; 
  
int main() 
{ 
  
    // declaration 
    unordered_multimap<int, int> 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}

示例2:

// C++ program to illustrate the 
// unordered_multimap::rehash() 
#include <bits/stdc++.h> 
using namespace std; 
  
int main() 
{ 
  
    // declaration 
    unordered_multimap<char, char> 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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注：本文由纯净天空筛选整理自gopaldave大神的英文原创作品 unordered_multimap rehash() function in C++ STL。非经特殊声明，原始代码版权归原作者所有，本译文未经允许或授权，请勿转载或复制。