std::vector begin()/cbegin() 方法
- 自 C++20 起
- 自 C++11 起
- 直到 C++11
// 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;
// prism-push-types:iterator,const_iterator
// Non-const version
iterator begin() noexcept;
// Const version
const_iterator begin() const noexcept;
const_iterator cbegin() const noexcept;
// prism-push-types:iterator,const_iterator
// Non-const version
iterator begin();
// Const version
const_iterator begin() const;
返回指向数组末尾之后元素的迭代器。
指向容器的第一个元素。如果容器为空，则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 - O(1)。
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
- 常量容器
- begin
- cbegin
#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
}
#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!
}
- begin
- cbegin
#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!
}
#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!
}
示例
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";
}
输出
1 2 4 8 16
Sum of nums: 31
First fruit: orange
vector 'empty' is indeed empty.