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

unordered_multimap::reserve()是C++ STL中的内置函数，该函数将容器中的存储桶数(bucket_count)设置为最合适的数字，以使其至少包含n个元素。如果n大于当前的bucket_count乘以max_load_factor，则该容器的bucket_count会增加，并强制进行重新哈希处理。通过使用unordered_multimap容器预期的大小调用reserve，可以避免多次重复。多次重复是由于容器大小增加而产生的，从而优化了哈希表的大小。如果n小于该值，则该函数可能无效。

用法:

unordered_multimap_name.reserve(N)

参数：该函数接受单个强制性参数N，该参数将请求的元素数指定为最小容量。

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::reserve() 
#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.reserve(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.reserve(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::reserve() 
#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.reserve(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.reserve(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}

相关用法

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 reserve() 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。