C++ unordered_map::swap() 函数

C++ 的std::unordered_map::swap()函数用于交换第一个unordered_map与另一个unordered_map的内容。当两个unordered_map的元素类型相同时，就会发生交换。

此函数不依赖于大小。如果两个unordered_map具有相同的类型，它将完美地交换unordered_map的元素，无论大小是否相同。

语法

以下是非成员std::unordered_map::swap()函数的语法。

void swap( unordered_map<Key,T,Hash,Pred,Alloc>& first,
           unordered_map<Key,T,Hash,Pred,Alloc>& second);

参数

• first - 第一个unordered_map对象。
• second - 第二个相同类型的unordered_map对象。

返回值

此函数不返回任何内容。

示例 1

在以下示例中，我们将对两个容器进行交换，一个是包含元素的，另一个是空的。

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_map<char, int> um1 = {
      {'a', 1},
      {'b', 2},
      {'c', 3},
      {'d', 4},
      {'e', 5}
   };
   unordered_map<char, int> um2;
   swap(um1, um2);
   cout << "Unordered map contains following elements" << endl;
   for (auto it = um2.begin(); it != um2.end(); ++it)
      cout << it->first << " = " << it->second << endl;
   return 0;
}

输出

让我们编译并运行上述程序，这将产生以下结果：

Unordered map contains following elements
e = 5
a = 1
b = 2
c = 3
d = 4

示例 2

考虑另一种情况，我们将使两个unordered_map相互交换。

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_map<char, int> um1 = {
      {'a', 1},
      {'b', 2},
      {'c', 3},
      {'d', 4},
      {'e', 5}
   };
   unordered_map<char, int> um2 = {
      {'A', 1},
      {'B', 2},
      {'C', 3},
      {'D', 4}
   };
   cout << "um1 contains following elements before swapping: " << endl;
   for (auto it = um1.begin(); it != um1.end(); ++it)
      cout << it->first << " = " << it->second << endl;
   
   cout << "um2 contains following elements before swapping: " << endl;
   for (auto it = um2.begin(); it != um2.end(); ++it)
      cout << it->first << " = " << it->second << endl;
      
   swap(um1, um2);
   
   cout << "um1 contains following elements after swapping: " << endl;
   for (auto it = um1.begin(); it != um1.end(); ++it)
      cout << it->first << " = " << it->second << endl;
   
   cout << "um2 contains following elements after swapping: " << endl;
   for (auto it = um2.begin(); it != um2.end(); ++it)
      cout << it->first << " = " << it->second << endl;
      
   return 0;
}

输出

以下是上述代码的输出：

um1 contains following elements before swapping: 
e = 5
d = 4
c = 3
b = 2
a = 1
um2 contains following elements before swapping: 
D = 4
C = 3
B = 2
A = 1
um1 contains following elements after swapping: 
D = 4
C = 3
B = 2
A = 1
um2 contains following elements after swapping: 
e = 5
d = 4
c = 3
b = 2
a = 1

示例 3

让我们看下面的例子，我们将创建两个unordered_map，John和Bob，它们存储科目的分数，现在我们将John和Bob的分数互相交换。

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_map<string, int> John = {
      {"CHE", 85},
      {"PHY", 88},
      {"MAT", 99},
      {"CMS", 86},
      {"ENG", 95}
   };
   unordered_map<string, int> Bob = {
      {"PHY", 90},
      {"CHE", 82},
      {"MAT", 98},
      {"CMS", 88}
   };
   cout<<"Swapping the marks of first unordered map with second unordered_map"<<endl;
   
   swap(John, Bob);
   
   cout <<"John contains following marks after swapping: " << endl;
   for (auto & it: John)
      cout << it.first << " = " << it.second << endl;
   
   cout <<"Bob contains following marks after swapping: " << endl;
   for (auto & it:  Bob)
      cout << it.first << " = " << it.second << endl;
   return 0;
}

输出

上述代码的输出如下：

Swapping the marks of first unordered map with second unordered_map
john contains following marks after swapping: 
CMS = 88
MAT = 98
CHE = 82
PHY = 90
Bob contains following marks after swapping: 
ENG = 95
CMS = 86
MAT = 99
PHY = 88
CHE = 85