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

od C++20

// Nonconst version
constexpr iterator begin() noexcept;

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

od C++11

// Non const version
iterator begin() noexcept;

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

do C++11

// Non const version
iterator begin();

// Const version
const_iterator begin() const;

Returns an iterator

to the first element of the string.

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 <string>

int main()
{
    std::string str = "Hello";
    auto it = str.begin(); // Type: std::string::iterator
    *it = 'J'; // ✔ Ok
}

cbegin

#include <string>

int main()
{
    std::string str = "Hello";
    auto it = str.cbegin(); // Type: std::string::const_iterator
    *it = 'J'; // ❌ Error! 
}

Const container

begin

#include <string>

int main()
{
    const std::string str = "Hello";
    auto it = str.begin(); // Type: std::string::const_iterator
    *it = 'J'; // ❌ Error! 
}

cbegin

#include <string>

int main()
{
    const std::string str = "Hello";
    auto it = str.cbegin(); // Type: std::string::const_iterator
    *it = 'J'; // ❌ Error! 
}

Example

Main.cpp

#include <string>
#include <iostream>
 
int main()
{
    std::string s("Exemplar");
    *s.begin() = 'e';
    std::cout << s <<'\n';
 
    auto i = s.cbegin();
    std::cout << *i << '\n';
//  *i = 'E'; // error: i is a constant iterator
}

Output

exemplar
e