std::unordered_map 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_map. If the unordered_map 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_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
}

cbegin

#include <unordered_map>
#include <string>

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

Const container

begin

#include <unordered_map>
#include <string>

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

cbegin

#include <unordered_map>
#include <string>

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

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