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

since C++11

// Non const version
iterator begin() noexcept;

// Const version
const_iterator begin() const noexcept;

// Const version
const_iterator cbegin() const noexcept;

Returns an iterator

to the first element of the vector. If the array is empty, the returned iterator will be equal to end().

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.begin() == c.cbegin()

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.begin(); // Type: std::unordered_multimap<std::string, int>::iterator
    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.cbegin(); // Type: std::unordered_multimap<std::string, int>::const_iterator    
    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.begin(); // Type: std::unordered_multimap<std::string, int>::const_iterator
    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.cbegin(); // Type: std::unordered_multimap<std::string, int>::const_iterator
    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