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

od C++11

// Non const versioniterator begin() noexcept;// Const versionconst_iterator begin() const noexcept;// Const versionconst_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_map<std::string, int> map = {			{ "key1", 1 },			{ "key2", 2 },			{ "key3", 3 },		};    auto it = map.begin(); // Type: std::unordered_map<std::string, int>::iterator    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.cbegin(); // Type: std::unordered_map<std::string, int>::const_iterator        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.begin(); // Type: std::unordered_map<std::string, int>::const_iterator    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.cbegin(); // Type: std::unordered_map<std::string, int>::const_iterator    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 4.24264The magnitude of (1, 1) is 1.41421The magnitude of (2, 2) is 2.82843The magnitude of (3, 4.24264) is 5.19615The magnitude of (1, 1.41421) is 1.73205The magnitude of (2, 2.82843) is 3.4641