std::map begin() method

od C++11

// Non const version
iterator begin() noexcept;

// Const version
iterator begin() const noexcept;

// Const version
const_iterator cbegin() const noexcept;

do C++11

// Non const version
iterator begin();

// Const version
const_iterator cbegin();

Returns an iterator

to the first element of the map. 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 <map>

int main()
{
    std::map<int, float> map = { {1, 1.f}, {2, 3.f}, {5, 8.f} };
    auto it = map.begin(); // Type: std::map<int, float>::iterator
    *it = 5; // ✔ Ok
}

cbegin

#include <map>

int main()
{
    std::map<int, float> map = { {1, 1.f}, {2, 3.f}, {5, 8.f} };
    auto it = map.cbegin(); // Type: std::map<int, float>::const_iterator
    *it = 5; // ❌ Error! 
}

Const container

begin

#include <map>

int main()
{
    const std::map<int, float> map = { {1, 1.f}, {2, 3.f}, {5, 8.f} };
    auto it = map.begin(); // Type: std::map<int, float>::const_iterator
    *it = 5; // ❌ Error! 
}

cbegin

#include <map>

int main()
{
    const std::map<int, float> map = { {1, 1.f}, {2, 3.f}, {5, 8.f} };
    auto it = map.cbegin(); // Type: std::map<int, float>::const_iterator
    *it = 5; // ❌ Error! 
}

Example

Main.cpp

#include <iostream>
#include <map>
 
int main() {
  std::map<int, float> num_map;
  num_map[4] = 4.13;
  num_map[9] = 9.24;
  num_map[1] = 1.09;
  // calls a_map.begin() and a_map.end()
  for (auto it = num_map.begin(); it != num_map.end(); ++it) {
    std::cout << it->first << ", " << it->second << '\n';
  }
}

Output

1, 1.09
4, 4.13
9, 9.24