std::unordered_map begin()/cbegin() 方法

自 C++11 起

// Non const version
iterator end() noexcept;

// Const version
const_iterator end() const noexcept;

// Const version
const_iterator cend() const noexcept;

返回指向数组末尾之后元素的迭代器。

指向 unordered_map 的 past-the-end 元素。如果 unordered_map 为空，则返回的迭代器将等于 begin()。

危险

尝试解引用末尾之后的迭代器是未定义行为

.

参数

(无)

返回值

指向第一个元素的迭代器。

复杂度

常数 - O(1)。

begin 和 cbegin 的区别

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

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 <unordered_map>
#include <string>

int main()
{
    std::unordered_map<std::string, int> map = {
			{ "key1", 1 },
			{ "key2", 2 },
			{ "key3", 3 },
		};
    auto it = map.end(); // Type: std::unordered_map<std::string, int>::iterator
    std::prev(it)->second = 5; // ✔ Ok
}

cbegin

#include <unordered_map>
#include <string>

int main()
{
    std::unordered_map<std::string, int> map = {
			{ "key1", 1 },
			{ "key2", 2 },
			{ "key3", 3 },
		};
    auto it = map.cend(); // Type: std::unordered_map<std::string, int>::const_iterator    
    std::prev(it)->second = 5; // ❌ Error! 
}

常量容器

begin

#include <unordered_map>
#include <string>

int main()
{
    const std::unordered_map<std::string, int> map = {
			{ "key1", 1 },
			{ "key2", 2 },
			{ "key3", 3 },
		};
    auto it = map.end(); // Type: std::unordered_map<std::string, int>::const_iterator
    std::prev(it)->second = 5; // ❌ Error! 
}

cbegin

#include <unordered_map>
#include <string>

int main()
{
    const std::unordered_map<std::string, int> map = {
			{ "key1", 1 },
			{ "key2", 2 },
			{ "key3", 3 },
		};
    auto it = map.cend(); // Type: std::unordered_map<std::string, int>::const_iterator
    std::prev(it)->second = 5; // ❌ Error! 
}

示例

Main.cpp

#include <cmath>
#include <iostream>
#include <unordered_map>
 
struct Node { double x, y; };
 
int main() {
    Node nodes[3] = { {1, 0}, {2, 0}, {3, 0} };
 
    //mag is a map mapping the address of a Node to its magnitude in the plane
    std::unordered_map<Node *, double> mag = {
        { nodes,     1 },
        { nodes + 1, 2 },
        { nodes + 2, 3 }
    };
 
    //Change each y-coordinate from 0 to the magnitude
    for(auto iter = mag.begin(); iter != mag.end(); ++iter){
        auto cur = iter->first; // pointer to Node
        cur->y = mag[cur]; // could also have used  cur->y = iter->second;
    }
 
    //Update and print the magnitude of each node
    for(auto iter = mag.begin(); iter != mag.end(); ++iter){
        auto cur = iter->first;
        mag[cur] = std::hypot(cur->x, cur->y);
        std::cout << "The magnitude of (" << cur->x << ", " << cur->y << ") is ";
        std::cout << iter->second << '\n';
    }
 
    //Repeat the above with the range-based for loop
    for(auto i : mag) {
        auto cur = i.first;
        cur->y = i.second;
        mag[cur] = std::hypot(cur->x, cur->y);
        std::cout << "The magnitude of (" << cur->x << ", " << cur->y << ") is ";
        std::cout << mag[cur] << '\n';
        //Note that in contrast to std::cout << iter->second << '\n'; above, 
        // std::cout << i.second << '\n'; will NOT print the updated magnitude
    }
}

可能的输出

The magnitude of (3, 3) is 4.24264
The magnitude of (1, 1) is 1.41421
The magnitude of (2, 2) is 2.82843
The magnitude of (3, 4.24264) is 5.19615
The magnitude of (1, 1.41421) is 1.73205
The magnitude of (2, 2.82843) is 3.4641