std::array begin()/cbegin() 方法

自 C++17 起

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

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

C++17 之前

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

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

返回指向数组末尾之后元素的迭代器。

指向数组的第一个元素。如果数组为空，返回的迭代器将等于end()。

参数

(无)

返回值

指向第一个元素的迭代器。

复杂度

常数。

备注

对于容器c，表达式*c.begin()等效于c.front()。

begin 和 cbegin 的区别

对于 const 容器 c，begin 和 cbegin 是相同的 - c.begin() == c.cbegin()

对于非常量类型c的容器，它们返回不同的迭代器

非常量容器

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;
}

常量容器

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;
}

示例

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'; 
}

可能输出

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