unordered_map swap in C++ STL Last Updated : 14 Dec, 2018 Comments Improve Suggest changes Like Article Like Report 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. Comment More infoAdvertise with us Next Article unordered_multiset swap() in C++ STL A ankit15697 Follow Improve Article Tags : Technical Scripter C++ cpp-unordered_map cpp-unordered_map-functions Practice Tags : CPP Similar Reads Unordered Map in C++ STL In C++, unordered_map is an unordered associative container that stores data in the form of unique key-value pairs. But unlike map, unordered map stores its elements using hashing. This provides average constant-time complexity O(1) for search, insert, and delete operations but the elements are not 7 min read unordered_set swap() in C++ STL The swap() method of “unordered_set†swaps the contents of two containers. It is public member function. This function: Exchanges the content of the container by the content of variable, which is another unordered_set object containing elements of the same type but the sizes may differ. 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