unordered_map swap in C++ STL (original) (raw)

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.