Skip to main content

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

// 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:

#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
}

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
This article originates from this CppReference page. It was likely altered for improvements or editors' preference. Click "Edit this page" to see all changes made to this document.
Hover to see the original license.

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

// 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:

#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
}

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
This article originates from this CppReference page. It was likely altered for improvements or editors' preference. Click "Edit this page" to see all changes made to this document.
Hover to see the original license.