C++ STL中的unordered_map交换

std :: unordered_map :: swap()是C++ STL中的内置函数，可将容器的元素交换到另一个容器。调用此函数后，调用者unordered_map的元素将成为被称为unordered_map的元素，而被调用unordered_map的元素将成为调用者unordered_map的元素。
内部元素交换未完成，仅更改了两个unordered_map的引用类型。
句法

unordered_map.swap ( unordered_map& ump )

返回类型：此函数的返回类型无效。
参数：另一个具有相同元素类型的unordered_map。
复杂度：其复杂度是恒定的。

例子1

// C++ code to illustrate the method
// unordered_map swap
#include 
  
using namespace std;
  
int main()
{
    unordered_map sample1, sample2;
  
    // Map initialization
    sample1 = { { 2, 2 }, { 3, 4 }, { 4, 6 }, { 5, 8 } };
    sample2 = { { 10, 11 }, { 12, 13 }, { 14, 15 }, { 26, 17 } };
  
    // printing details before calling swap
  
    cout << " Elements of maps before swap \n";
    cout << " Elements of first map are : \n";
    for (auto& x : sample1)
        cout << x.first << " : " << x.second << endl;
  
    cout << " Elements of second map are : \n";
    for (auto& x : sample2)
        cout << x.first << " : " << x.second << endl;
  
    // swapping
    sample1.swap(sample2);
  
    cout << " Elements of maps after swap \n";
    cout << " Elements of first map are : \n";
    for (auto& x : sample1)
        cout << x.first << " : " << x.second << endl;
  
    cout << " Elements of second map are : \n";
    for (auto& x : sample2)
        cout << x.first << " : " << x.second << endl;
    return 0;
}

输出：

Elements of maps before swap 
 Elements of first map are : 
5 : 8
4 : 6
3 : 4
2 : 2
 Elements of second map are : 
14 : 15
26 : 17
12 : 13
10 : 11
 Elements of maps after swap 
 Elements of first map are : 
14 : 15
26 : 17
12 : 13
10 : 11
 Elements of second map are : 
5 : 8
4 : 6
3 : 4
2 : 2

例子2

// C++ code to illustrate the method
// unordered_map swap
#include 
  
using namespace std;
  
int main()
{
    unordered_map sample1, sample2;
  
    // Map initialization
    sample1 = { { 'a', 2 }, { 'b', 4 }, { 'c', 6 }, { 'd', 8 } };
    sample2 = { { 'e', 11 }, { 'f', 13 }, { 'h', 15 } };
  
    // printing details before calling swap
  
    cout << " Elements of maps before swap \n";
    cout << " Elements of first map are : \n";
    for (auto& x : sample1)
        cout << x.first << " : " << x.second << endl;
  
    cout << " Elements of second map are : \n";
    for (auto& x : sample2)
        cout << x.first << " : " << x.second << endl;
  
    // swapping
    sample1.swap(sample2);
  
    cout << " Elements of maps after swap \n";
    cout << " Elements of first map are : \n";
    for (auto& x : sample1)
        cout << x.first << " : " << x.second << endl;
  
    cout << " Elements of second map are : \n";
    for (auto& x : sample2)
        cout << x.first << " : " << x.second << endl;
    return 0;
}

输出：

Elements of maps before swap 
 Elements of first map are : 
d : 8
c : 6
b : 4
a : 2
 Elements of second map are : 
h : 15
f : 13
e : 11
 Elements of maps after swap 
 Elements of first map are : 
h : 15
f : 13
e : 11
 Elements of second map are : 
d : 8
c : 6
b : 4
a : 2

注意：调用方和被调用的unordered_map都应包含相同类型的元素，否则我们将得到编译时错误。

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