C++ STL中的unordered_map reserve()

众所周知，存储桶是容器内部哈希表中的一个插槽，所有元素均基于其键的哈希值分配给该插槽。存储桶的编号从0到bucket_count。现在，作为存储项的可变数量的存储桶。这个数字是基于术语Load Factor的，当Load Factor(load_factor)达到某个阈值时，容器会增加存储桶的数量并重新映射地图，但是当我们调用rehash(n)时，它将直接设置存储桶的数量到n并触发整个哈希表的重建。但是，当我们调用reserve(n)时，它将创建足够的存储桶以容纳至少n个项目。如果然后我们向地图中添加> n个项目，则可能会触发重新哈希操作，具体取决于负载系数。通过使用与我们预期的unordered_map容器大小相同的大小调用reserve，我们避免了容器大小增加可能产生的多次重排，并优化了哈希表的大小。 C++函数std :: unordered_map :: reserve()将容器中的存储桶数(bucket_count)设置为最适合包含至少n个元素的存储桶数。

句法：

unordered_map_name.reserve(N)

参数：该函数接受单个强制性参数N ，该参数N将所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。

下面的程序说明了上述函数：

程序1：

// C++ program to illustrate the
// unordered_map::reserve()
#include 
using namespace std;
  
int main()
{
  
    // declaration
    unordered_map 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: 2
Key and Elements of Sample2 are:{30, 300} {20, 200}

程式2：

// C++ program to illustrate the
// unordered_map::reserve()
#include 
using namespace std;
  
int main()
{
  
    // declaration
    unordered_map 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}

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