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Showing content from https://www.geeksforgeeks.org/cpp/unordered_map-swap-in-c-stl/ below:

unordered_map swap in C++ STL

Last Updated : 14 Dec, 2018

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 CPP


 // 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;
}

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 CPP


 // 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;
}

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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