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

od C++17

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

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

do C++17

// 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 array. If the array is empty, the returned iterator will be equal to end().

Parameters

(none)

Return value

Iterator to the first element.

Complexity

Constant.

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 <array>

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

cbegin

#include <array>

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

Const container

Begin

#include <array>

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

cbegin

#include <array>

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

Example

Main.cpp

#include <array>
#include <iostream>
#include <algorithm>
#include <iomanip>
 
int main()
{
	std::cout << std::boolalpha;
 
	std::array<int, 0> empty;
	std::cout << "1) "
			  << (empty.begin() == empty.end()) << ' '     // true
			  << (empty.cbegin() == empty.cend()) << '\n'; // true
	// *(empty.begin()) = 42; // => undefined behaviour at run-time
 
 
	std::array<int, 4> numbers{5, 2, 3, 4};
	std::cout << "2) "
			  << (numbers.begin() == numbers.end()) << ' '    // false
			  << (numbers.cbegin() == numbers.cend()) << '\n' // false
			  << "3) "
			  << *(numbers.begin()) << ' '    // 5
			  << *(numbers.cbegin()) << '\n'; // 5
 
	*numbers.begin() = 1;
	std::cout << "4) " << *(numbers.begin()) << '\n'; // 1
	// *(numbers.cbegin()) = 42; // compile-time error: 
								 // read-only variable is not assignable
 
	// print out all elements
	std::cout << "5) ";
	std::for_each(numbers.cbegin(), numbers.cend(), [](int x) {
	   std::cout << x << ' ';
	});
	std::cout << '\n';
 
 
	constexpr std::array constants{'A', 'B', 'C'};
	static_assert(constants.begin() != constants.end());   // OK
	static_assert(constants.cbegin() != constants.cend()); // OK
	static_assert(*constants.begin() == 'A');              // OK
	static_assert(*constants.cbegin() == 'A');             // OK
	// ❌ Compile-time error: read-only variable is not assignable
	*constants.begin() = 'Z'; 
}

Possible output

1) true true
2) false false
3) 5 5
4) 1
5) 1 2 3 4