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

since C++11

// Non-const version
iterator begin() noexcept;

// Const version
const_iterator begin() const noexcept;
const_iterator cbegin() const noexcept;

Returns an iterator to the first element of the forward list.

note

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).

Notes

For a container c, the expression *c.begin() is equivalent to c.front().

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 <forward_list>

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

cbegin

#include <forward_list>

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

Const container

begin

#include <forward_list>

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

cbegin

#include <forward_list>

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

Example

Main.cpp

#include <algorithm>
#include <iostream>
#include <numeric>
#include <string>
#include <forward_list>

int main()
{
    std::forward_list<int> nums {1, 2, 4, 8, 16};
    std::forward_list<std::string> fruits {"orange", "apple", "raspberry"};
    std::forward_list<char> empty;

    // Print forward_list.
    std::for_each(nums.begin(), nums.end(), [](const int n) { std::cout << n << ' '; });
    std::cout << '\n';

    // Sums all integers in the forward_list nums (if any), printing only the result.
    std::cout << "Sum of nums: "
              << std::accumulate(nums.begin(), nums.end(), 0) << '\n';

    // Prints the first fruit in the forward_list fruits, checking if there is any.
    if (!fruits.empty())
        std::cout << "First fruit: " << *fruits.begin() << '\n';

    if (empty.begin() == empty.end())
        std::cout << "forward_list 'empty' is indeed empty.\n";
}

Output

1 2 4 8 16
Sum of nums: 31
First fruit: orange
forward_list 'empty' is indeed empty.