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 tobegin()
.
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
- Const container
- begin
- cbegin
#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
}
#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!
}
- begin
- 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.end(); // Type: std::unordered_multimap<std::string, int>::const_iterator
std::prev(it)->second = 5; // ❌ Error!
}
#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
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