C++ unordered_multiset::swap() Function

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Description

The unordered_multiset::swap() function in C++ STL is used to exchange the content of one unordered_multiset with the content of another unordered_multiset of same type. It swaps all the elements, hash functions, key equality predicates, and allocators of one unordered_multiset with the other unordered_multiset.

Syntax

Following is the syntax of unordered_multiset::swap() function −

unordered_multiset1.swap(unordered_multiset2);

Parameters

The swap() function accepts one parameter that represents the unordered_multiset with which we want to swap the unordered_multiset.

Return value

The swap() function does not return any value. It has a return type of void.

Exceptions

If an exception is thrown, the container remains unchanged.

Time complexity

The time complexity of unordered_multiset::swap() function is O(1).

Examples of unordered_multiset::swap() Function

The following examples demonstrate the usage of unordered_multiset::swap() function in unordered_multiset −

Swapping Two Unordered_multisets

Below is an example to swap the contents of two unordered_multisets set1 and set2 using the unordered_multiset::swap() function −

#include <iostream>
#include <unordered_set>
using namespace std;
void printEle(const string& name, const unordered_multiset<int> &ums){
    cout << name << ": ";
    for (auto it = ums.begin(); it != ums.end(); ++it)
        cout << *it << " ";
    cout << endl;
}
int main() {
    unordered_multiset<int> set1 = {10, 20, 30, 40, 50};
    unordered_multiset<int> set2 = {100, 200, 300};
    cout << "Before swap:" << endl;
    printEle("set1", set1);
    printEle("set2", set2);
    set1.swap(set2);
    cout << "\nAfter swap:" << endl;
    printEle("set1", set1);
    printEle("set2", set2);
    return 0;
}

The output of the above code is given below −

Before swap:
set1: 50 40 30 20 10 
set2: 300 200 100 

After swap:
set1: 300 200 100 
set2: 50 40 30 20 10

Swapping Empty and Non-Empty Unordered_multisets

You can use swap() function to swap an empty unordered_multiset with a non empty unordered_multiset. The empty unordered_multiset becomes a non empty unordered_multiset and non empty unordered_multiset becomes an empty unordered_multiset after swapping.

#include <iostream>
#include <unordered_set>
using namespace std;
int main() {
    unordered_multiset<int> numbers = {1, 2, 3, 4, 5};
    unordered_multiset<int> empty_set;
    cout << "Before swap:" << endl;
    cout << "numbers size: " << numbers.size() << endl;
    cout << "empty_set size: " << empty_set.size() << endl;
    numbers.swap(empty_set);
    cout << "\nAfter swap:" << endl;
    cout << "numbers size: " << numbers.size() << endl;
    cout << "empty_set size: " << empty_set.size() << endl;
    cout << "empty_set contents: ";
    for (int n : empty_set)
        cout << n << " ";
    cout << endl;
    return 0;
}

The output of the code is given below −

Before swap:
numbers size: 5
empty_set size: 0

After swap:
numbers size: 0
empty_set size: 5
empty_set contents: 5 4 3 2 1

Effect of unordered_multiset::swap() on Iterators

The following example demonstrates that after swapping the iterators remain valid but they point to swapped elements now. For example: if itr1 was pointing to an element of unordered_multiset1 then after swapping, itr1 will be pointing to that same element which is now in unordered_multiset2.

#include <iostream>
#include <unordered_set>
using namespace std;
int main() {
    unordered_multiset<int> set1 = {10, 20, 30};
    unordered_multiset<int> set2 = {40, 50, 60};
    auto it1 = set1.begin();
    auto it2 = set2.begin();
    cout << "Before swap:" << endl;
    cout << "it1 points to: " << *it1 << " (in set1)" << endl;
    cout << "it2 points to: " << *it2 << " (in set2)" << endl;
    set1.swap(set2);
    cout << "\nAfter swap:" << endl;
    cout << "it1 points to: " << *it1 << " (now in set2)" << endl;
    cout << "it2 points to: " << *it2 << " (now in set1)" << endl;
    cout << "\nCurrent contents:" << endl;
    cout << "set1: ";
    for (int n : set1)
        cout << n << " ";
    cout << endl;
    cout << "set2: ";
    for (int n : set2)
        cout << n << " ";
    cout << endl;
    return 0;
}

The output of the code is given below −

Before swap:
it1 points to: 30 (in set1)
it2 points to: 60 (in set2)

After swap:
it1 points to: 30 (now in set2)
it2 points to: 60 (now in set1)

Current contents:
set1: 60 50 40 
set2: 30 20 10

Swapping Unordered_multisets of Custom Structures

In this example, we have swapped value of two unordered_multisets that store user defined structures −

#include <iostream>
#include <unordered_set>
using namespace std;
struct Product {
    string name;
    int price;
    bool operator==(const Product &p) const {
        return name == p.name && price == p.price;
    }
};
struct ProductHash {
    size_t operator()(const Product &p) const {
        return hash<string>()(p.name) ^ hash<int>()(p.price);
    }
};
void printUnorderedMultiset(const unordered_multiset<Product, ProductHash> &products) {
    int count = 0, n = products.size();
    for (const auto &p : products) {
        cout << p.name << " " << p.price;
        if (++count < n) 
            cout << ", ";
    }
    cout << endl;
}
int main() {
    unordered_multiset<Product, ProductHash> shopA = {{"Pen", 10}, {"Book", 30}};
    unordered_multiset<Product, ProductHash> shopB = {{"Pencil", 5}, {"Notebook", 25}};
    cout << "Shop A before swap:\n";
    printUnorderedMultiset(shopA);
    cout << "Shop B before swap:\n";
    printUnorderedMultiset(shopB);
    shopA.swap(shopB);
    cout << "\nShop A after swap:\n";
    printUnorderedMultiset(shopA);
    cout << "Shop B after swap:\n";
    printUnorderedMultiset(shopB);
}

The output of the above code is given below −

Shop A before swap:
Book 30, Pen 10
Shop B before swap:
Notebook 25, Pencil 5

Shop A after swap:
Notebook 25, Pencil 5
Shop B after swap:
Book 30, Pen 10

Difference Between multiset::swap and unordered_multiset::swap

The differences between multiset::swap and unordered_multiset::swap functions are given below −