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std::unordered_map 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_map. If the unordered_map is empty, the returned iterator will be equal to begin().

uwaga

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

Example

Main.cpp
#include <cmath>
#include <iostream>
#include <unordered_map>

struct Node { double x, y; };

int main() {
Node nodes[3] = { {1, 0}, {2, 0}, {3, 0} };

//mag is a map mapping the address of a Node to its magnitude in the plane
std::unordered_map<Node *, double> mag = {
{ nodes, 1 },
{ nodes + 1, 2 },
{ nodes + 2, 3 }
};

//Change each y-coordinate from 0 to the magnitude
for(auto iter = mag.begin(); iter != mag.end(); ++iter){
auto cur = iter->first; // pointer to Node
cur->y = mag[cur]; // could also have used cur->y = iter->second;
}

//Update and print the magnitude of each node
for(auto iter = mag.begin(); iter != mag.end(); ++iter){
auto cur = iter->first;
mag[cur] = std::hypot(cur->x, cur->y);
std::cout << "The magnitude of (" << cur->x << ", " << cur->y << ") is ";
std::cout << iter->second << '\n';
}

//Repeat the above with the range-based for loop
for(auto i : mag) {
auto cur = i.first;
cur->y = i.second;
mag[cur] = std::hypot(cur->x, cur->y);
std::cout << "The magnitude of (" << cur->x << ", " << cur->y << ") is ";
std::cout << mag[cur] << '\n';
//Note that in contrast to std::cout << iter->second << '\n'; above,
// std::cout << i.second << '\n'; will NOT print the updated magnitude
}
}
Possible Output
The magnitude of (3, 3) is 4.24264
The magnitude of (1, 1) is 1.41421
The magnitude of (2, 2) is 2.82843
The magnitude of (3, 4.24264) is 5.19615
The magnitude of (1, 1.41421) is 1.73205
The magnitude of (2, 2.82843) is 3.4641
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_map 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_map. If the unordered_map is empty, the returned iterator will be equal to begin().

uwaga

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

Example

Main.cpp
#include <cmath>
#include <iostream>
#include <unordered_map>

struct Node { double x, y; };

int main() {
Node nodes[3] = { {1, 0}, {2, 0}, {3, 0} };

//mag is a map mapping the address of a Node to its magnitude in the plane
std::unordered_map<Node *, double> mag = {
{ nodes, 1 },
{ nodes + 1, 2 },
{ nodes + 2, 3 }
};

//Change each y-coordinate from 0 to the magnitude
for(auto iter = mag.begin(); iter != mag.end(); ++iter){
auto cur = iter->first; // pointer to Node
cur->y = mag[cur]; // could also have used cur->y = iter->second;
}

//Update and print the magnitude of each node
for(auto iter = mag.begin(); iter != mag.end(); ++iter){
auto cur = iter->first;
mag[cur] = std::hypot(cur->x, cur->y);
std::cout << "The magnitude of (" << cur->x << ", " << cur->y << ") is ";
std::cout << iter->second << '\n';
}

//Repeat the above with the range-based for loop
for(auto i : mag) {
auto cur = i.first;
cur->y = i.second;
mag[cur] = std::hypot(cur->x, cur->y);
std::cout << "The magnitude of (" << cur->x << ", " << cur->y << ") is ";
std::cout << mag[cur] << '\n';
//Note that in contrast to std::cout << iter->second << '\n'; above,
// std::cout << i.second << '\n'; will NOT print the updated magnitude
}
}
Possible Output
The magnitude of (3, 3) is 4.24264
The magnitude of (1, 1) is 1.41421
The magnitude of (2, 2) is 2.82843
The magnitude of (3, 4.24264) is 5.19615
The magnitude of (1, 1.41421) is 1.73205
The magnitude of (2, 2.82843) is 3.4641
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.