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C++ unordered_map::reserve() 函数
C++ 的 std::unordered_map::reserve() 函数用于将容器中的桶数量设置为最适合容纳至少 n 个元素且不超过最大负载因子的数量。
如果 n 大于当前的 bucket_count(),则容器的桶数量会以相同的方式增加;如果 n 小于当前的 bucket_count(),则不会产生任何影响。
语法
以下是 std::unordered_map::reserve() 函数的语法。
void reserve(size_type n);
参数
n - 表示容器的新容量。
返回值
此函数为 void 类型,因此不返回任何值。
示例 1
在以下示例中,让我们看看 reserve() 函数的用法。
#include <iostream> #include <unordered_map> using namespace std; int main(void) { unordered_map<char, int> um; cout << "Initial bucket count = " << um.bucket_count() << endl; um.reserve(5); cout << "Bucket count after reserve = "<< um.bucket_count() << endl; return 0; }
输出
以上代码的输出如下:
Initial bucket count = 1 Bucket count after reserve = 5
示例 2
考虑以下示例,我们将使桶数量至少可以存储 5 个元素。
#include <iostream> #include <unordered_map> using namespace std; int main () { unordered_map<string, string> uMap; cout << "Bucket Count: " << uMap.bucket_count() << endl; uMap.reserve(5); cout << "Bucket Count after reserve(): " << uMap.bucket_count() << endl; uMap["Fname"] = "tutorials"; uMap["Lname"] = "Point"; uMap["Country"] = "India"; uMap["Locaton"] = "Hyderabad"; for (auto& it: uMap) cout << it.first << "->" << it.second << endl; return 0; }
输出
如果我们运行以上代码,它将生成以下输出:
Bucket Count: 1 Bucket Count after reserve(): 5 Country->India Lname->Point Locaton->Hyderabad Fname->tutorials
示例 3
让我们看一下以下示例,其中我们在使用 reserve() 函数之前和之后显示桶及其元素。
#include <iostream> #include <unordered_map> using namespace std; int main () { unordered_map<string, string> uMap={{"Hyderabad", "India"}, {"Delhi", "India"}, {"Bangalore", "India"}}; cout<<"Unordered_map contains "<<uMap.bucket_count()<<" buckets:"; for(unsigned int i = 0; i < uMap.bucket_count(); i++) { cout<<"\nThe bucket "<<i<<" contains: "; for(auto it = uMap.begin(i); it != uMap.end(i); ++it) { cout<<it->first<<":"<<it->second<<" "; } } cout<<"\nCapacity is changed using reserve function.\n"; uMap.reserve(5); cout<<"Unordered_map contains "<<uMap.bucket_count()<<" buckets:"; for(unsigned int i = 0; i < uMap.bucket_count(); i++) { cout<<"\nThe bucket "<<i<<" contains: "; for(auto it = uMap.begin(i); it != uMap.end(i); ++it) { cout<<it->first<<":"<<it->second<<" "; } } return 0; }
输出
让我们编译并运行以上程序,这将产生以下结果:
Unordered_map contains 13 buckets: The bucket 0 contains: The bucket 1 contains: Bangalore:India The bucket 2 contains: Hyderabad:India The bucket 3 contains: Delhi:India The bucket 4 contains: The bucket 5 contains: The bucket 6 contains: The bucket 7 contains: The bucket 8 contains: The bucket 9 contains: The bucket 10 contains: The bucket 11 contains: The bucket 12 contains: Capacity is changed using reserve function. Unordered_map contains 5 buckets: The bucket 0 contains: Banglore:India The bucket 1 contains: The bucket 2 contains: The bucket 3 contains: Delhi:India The bucket 4 contains: Hyderabad:India
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