std::unordered_multiset begin()/cbegin() method

od C++11

// Non const version
iterator end() noexcept;

// Const version
const_iterator end() const noexcept;

// Const version
const_iterator cend() const noexcept;

Returns an iterator

to the element past-the-end of the unordered_multiset. If the unordered_multiset is empty, the returned iterator will be equal to begin().

uwaga

Attempting to dereference a past-the-end iterator is undefined behaviour

.

Parameters

(none)

Return value

Iterator to the first element.

Complexity

Constant - O(1).

Difference between begin and cbegin

For a const container c, begin and cbegin are the same - c.end() == c.cend()

For non-const container of type c they return different iterators:

Non const container

begin

#include <unordered_map>
#include <string>

int main()
{
	std::unordered_multiset<std::string> multiset = { "abc", "Hello World!", "abc", "asd" };
    auto it = multiset.end(); // Type: std::unordered_multiset<std::string>::iterator
    std::prev(it)->second = 5; // ✔ Ok
}

cbegin

#include <unordered_map>
#include <string>

int main()
{
	std::unordered_multiset<std::string> multiset = { "abc", "Hello World!", "abc", "asd" };
    auto it = multiset.cend(); // Type: std::unordered_multiset<std::string>::iterator
    std::prev(it)->second = 5; // ❌ Error! 
}

Const container

begin

#include <unordered_map>
#include <string>

int main()
{
	std::unordered_multiset<std::string> multiset = { "abc", "Hello World!", "abc", "asd" };
    auto it = multiset.end(); // Type: std::unordered_multiset<std::string>::iterator
    std::prev(it)->second = 5; // ❌ Error! 
}

cbegin

#include <unordered_map>
#include <string>

int main()
{
	std::unordered_multiset<std::string> multiset = { "abc", "Hello World!", "abc", "asd" };
    auto it = multiset.cend(); // Type: std::unordered_multiset<std::string>::iterator
    std::prev(it)->second = 5; // ❌ Error! 
}

Example

Main.cpp

#include <iostream>
#include <iterator>
#include <string>
#include <unordered_set>
 
int main() {
    const std::unordered_multiset<std::string> words = {
        "some", "words", "to", "count",
        "count", "these", "words"
    };
 
    for (auto it = words.begin(); it != words.end(); ) {
        auto cnt = words.count(*it);
        std::cout << *it << ":\t" << cnt << '\n';
        std::advance(it, cnt); // all cnt elements have equivalent keys
    }
}

Possible Output

some:   1
words:  2
to:     1
count:  2
these:  1