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

since 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_multimap. If the unordered_multimap is empty, the returned iterator will be equal to begin().

danger

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_multimap<std::string, int> map = {
			{ "key1", 1 },
			{ "key2", 2 },
			{ "key3", 3 },
		};
    auto it = map.end(); // Type: std::unordered_multimap<std::string, int>::iterator
    std::prev(it)->second = 5; // ✔ Ok
}

cbegin

#include <unordered_map>
#include <string>

int main()
{
    std::unordered_multimap<std::string, int> map = {
			{ "key1", 1 },
			{ "key2", 2 },
			{ "key3", 3 },
		};
    auto it = map.cend(); // Type: std::unordered_multimap<std::string, int>::const_iterator    
    std::prev(it)->second = 5; // ❌ Error! 
}

Const container

begin

#include <unordered_map>
#include <string>

int main()
{
    const std::unordered_multimap<std::string, int> map = {
			{ "key1", 1 },
			{ "key2", 2 },
			{ "key3", 3 },
		};
    auto it = map.end(); // Type: std::unordered_multimap<std::string, int>::const_iterator
    std::prev(it)->second = 5; // ❌ Error! 
}

cbegin

#include <unordered_map>
#include <string>

int main()
{
    const std::unordered_multimap<std::string, int> map = {
			{ "key1", 1 },
			{ "key2", 2 },
			{ "key3", 3 },
		};
    auto it = map.cend(); // Type: std::unordered_multimap<std::string, int>::const_iterator
    std::prev(it)->second = 5; // ❌ Error! 
}

Example

Main.cpp

#include <unordered_map>
#include <algorithm>
#include <cassert>
#include <iostream>
#include <string>
#include <utility>
 
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());   // OK
    assert(lemmas.cbegin() == lemmas.cend()); // OK
 
    lemmas.insert({ "1. ∀x ∈ N ∃y ∈ N", "x ≤ y" });
    show_node(*lemmas.cbegin());
    assert(lemmas.begin() != lemmas.end());   // OK
    assert(lemmas.cbegin() != lemmas.cend()); // OK
    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