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

自 C++11 起

// Non const version
iterator begin() noexcept;

// Const version
const_iterator begin() const noexcept;

// Const version
const_iterator cbegin() const noexcept;

返回指向数组末尾之后元素的迭代器。

到向量的第一个元素。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 - O(1)。

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 <unordered_set>
#include <string>

int main()
{
	std::unordered_multiset<std::string> multiset = { "abc", "Hello World!", "abc", "asd" };
    auto it = multiset.begin(); // Type: std::unordered_multiset<std::string>::iterator
    it->second = 5; // ✔ Ok
}

cbegin

#include <unordered_set>
#include <string>

int main()
{
	std::unordered_multiset<std::string> multiset = { "abc", "Hello World!", "abc", "asd" };
			{ "key1", 1 },
			{ "key2", 2 },
			{ "key3", 3 },
		};
    auto it = multiset.cbegin(); // Type: std::unordered_multiset<std::string>::const_iterator    
    it->second = 5; // ❌ Error! 
}

常量容器

begin

#include <unordered_set>
#include <string>

int main()
{
	std::unordered_multiset<std::string> multiset = { "abc", "Hello World!", "abc", "asd" };
    auto it = multiset.begin(); // Type: std::unordered_multiset<std::string>::const_iterator
    it->second = 5; // ❌ Error! 
}

cbegin

#include <unordered_set>
#include <string>

int main()
{
	std::unordered_multiset<std::string> multiset = { "abc", "Hello World!", "abc", "asd" };
    auto it = multiset.cbegin(); // Type: std::unordered_multiset<std::string>::const_iterator
    it->second = 5; // ❌ Error! 
}

示例

Main.cpp

#include <iostream>
#include <iterator>
#include <string>
#include <unordered_set>
 
int main() {
    const std::unordered_multiset<std::string> words = {
        "some", "words", "to", "count",
        "count", "these", "words"
    };
 
    for (auto it = words.begin(); it != words.end(); ) {
        auto cnt = words.count(*it);
        std::cout << *it << ":\t" << cnt << '\n';
        std::advance(it, cnt); // all cnt elements have equivalent keys
    }
}

可能的输出

some:   1
words:  2
to:     1
count:  2
these:  1