# std::unordered_multimap<Key,T,Hash,KeyEqual,Allocator>::begin, std::unordered_multimap<Key,T,Hash,KeyEqual,Allocator>::cbegin

From cppreference.com
< cpp‎ | container‎ | unordered multimap

iterator begin() noexcept; | (1) | (since C++11)
const_iterator begin() const noexcept; | (2) | (since C++11)
const_iterator cbegin() const noexcept; | (3) | (since C++11)

Returns an iterator to the first element of the `unordered_multimap`.

If the `unordered_multimap` is empty, the returned iterator will be equal to end().

### Parameters

(none)

### Return value

Iterator to the first element.

### Complexity

Constant.

### Example

Run this code

    #include <algorithm>
    #include <cassert>
    #include <iostream>
    #include <string>
    #include <utility>
    #include <unordered_map>

    int main()
    {
        auto show_node = [](const std::pair<std::string, std::string>& node)
        {
            std::cout << node.first << " : " << node.second << '\n';
        };

        std::unordered_multimap<std::string, std::string> lemmas;
        assert(lemmas.begin() == lemmas.end());
        assert(lemmas.cbegin() == lemmas.cend());

        lemmas.insert({ "1. ∀x ∈ N ∃y ∈ N", "x ≤ y" });
        show_node(*lemmas.cbegin());
        assert(lemmas.begin() != lemmas.end());
        assert(lemmas.cbegin() != lemmas.cend());

        lemmas.begin()->second = "x < y";
        show_node(*lemmas.cbegin());

        lemmas.insert({ "2. ∀x, y ∈ N    ", "x = y V x ≠ y" });
        show_node(*lemmas.cbegin());

        lemmas.insert({ "3. ∀x ∈ N ∃y ∈ N", "y = x + 1" });
        show_node(*lemmas.cbegin());

        std::cout << "Lemmas: \n";
        std::for_each(lemmas.cbegin(), lemmas.cend(), [&](const auto& n)
        {
            show_node(n);
        });
        std::cout << '\n';
    }

Possible output:

    1. ∀x ∈ N ∃y ∈ N : x ≤ y
    1. ∀x ∈ N ∃y ∈ N : x < y
    2. ∀x, y ∈ N     : x = y V x ≠ y
    3. ∀x ∈ N ∃y ∈ N : y = x + 1
    Lemmas:
    3. ∀x ∈ N ∃y ∈ N : y = x + 1
    1. ∀x ∈ N ∃y ∈ N : x < y
    2. ∀x, y ∈ N     : x = y V x ≠ y

### See also

end cend (C++11) | returns an iterator to the end (public member function)
begin cbegin (C++11)(C++14) | returns an iterator to the beginning of a container or array (function template)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/container/unordered_multimap/begin&oldid=136059"