unordered_map swap in C++ STL

The std::unordered_map::swap() is a built in function in C++ STL which swaps the elements of a container to an other container. After the call of this function elements of the caller unordered_map will be elements of called unordered_map while elements of called unordered_map will be elements of caller unordered_map.
Internally swapping of elements is not done only reference type of both unordered_map is changed.
Syntax

unordered_map.swap ( unordered_map& ump )

Return type : Return type of this function is void.
Parameters : An other unordered_map with same type of elements.
Complexity : Its complexity is constant.

Example 1



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// C++ code to illustrate the method
// unordered_map swap
#include <bits/stdc++.h>
  
using namespace std;
  
int main()
{
    unordered_map<int, int> 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;
}

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Output:

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

Example 2

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// C++ code to illustrate the method
// unordered_map swap
#include <bits/stdc++.h>
  
using namespace std;
  
int main()
{
    unordered_map<char, int> 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;
}

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Output:

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

Note : Caller and called unordered_map both should contain same type of elements otherwise we will get compile time error.




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