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

od C++20

// prism-push-types:iterator,const_iterator
// Non-const version
constexpr iterator        begin() noexcept;

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

od C++11

// prism-push-types:iterator,const_iterator
// Non-const version
iterator        begin() noexcept;

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

do C++11

// prism-push-types:iterator,const_iterator
// Non-const version
iterator        begin();

// Const version
const_iterator  begin() const;

Returns an iterator

to the first element of the container.

If the container 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 <vector>

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

cbegin

#include <vector>

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

Const container

begin

#include <vector>

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

cbegin

#include <vector>

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

Example

Main.cpp

#include <algorithm>
#include <iostream>
#include <numeric>
#include <string>
#include <vector>
 
int main()
{
    std::vector<int> nums {1, 2, 4, 8, 16};
    std::vector<std::string> fruits {"orange", "apple", "raspberry"};
    std::vector<char> empty;
 
    // Print vector.
    std::for_each(nums.begin(), nums.end(), [](const int n) { std::cout << n << ' '; });
    std::cout << '\n';
 
    // Sums all integers in the vector 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 vector fruits, checking if there is any.
    if (!fruits.empty())
        std::cout << "First fruit: " << *fruits.begin() << '\n';
 
    if (empty.begin() == empty.end())
        std::cout << "vector 'empty' is indeed empty.\n";
}

Output

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