std::unordered_set begin() method

since C++11

// Non const version
iterator begin() noexcept;

// Const version
const_iterator begin() const noexcept;

// Const version
const_iterator cbegin() const noexcept;

Returns an iterator

to the first element of the unordered_set. If the array is empty, the returned iterator will be equal to end().

Parameters

(none)

Return value

Iterator to the first element.

Exceptions

(none)

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_set>

int main()
{
    std::unordered_set<int> unordered_set = {1, 2, 3, 4, 5};
    auto it = unordered_set.begin(); // Type: std::unordered_set<int>::iterator
    *it = 5; // ✔ Ok
}

cbegin

#include <unordered_set>

int main()
{
    std::unordered_set<int> unordered_set = {1, 2, 3, 4, 5};
    auto it = unordered_set.cbegin(); // Type: std::unordered_set<int>::const_iterator
    *it = 5; // ❌ Error! 
}

Const container

begin

#include <unordered_set>

int main()
{
    const std::unordered_set<int> unordered_set = {1, 2, 3, 4, 5};
    auto it = unordered_set.begin(); // Type: std::unordered_set<int>::const_iterator
    *it = 5; // ❌ Error! 
}

cbegin

#include <unordered_set>

int main()
{
    const std::unordered_set<int> unordered_set = {1, 2, 3, 4, 5};
    auto it = unordered_set.cbegin(); // Type: std::unordered_set<int>::const_iterator
    *it = 5; // ❌ Error! 
}

Example

Main.cpp

#include <iostream>
#include <unordered_set>
 
struct Point { double x, y; };
 
int main() {
    Point pts[3] = { {1, 0}, {2, 0}, {3, 0} };
 
    //points is a set containing the addresses of points
    std::unordered_set<Point *> points = { pts, pts + 1, pts + 2 };
 
    //Change each y-coordinate of (i, 0) from 0 into i^2 and print the point
    for(auto iter = points.begin(); iter != points.end(); ++iter){
        (*iter)->y = ((*iter)->x) * ((*iter)->x); //iter is a pointer-to-Point*
        std::cout << "(" << (*iter)->x << ", " << (*iter)->y << ") ";
    }
    std::cout << '\n';
 
    //Now using the range-based for loop, we increase each y-coordinate by 10
    for(Point * i : points) {
        i->y += 10;
        std::cout << "(" << i->x << ", " << i->y << ") ";
    }
}

Possible output

(3, 9) (1, 1) (2, 4) 
(3, 19) (1, 11) (2, 14)