定义于头文件 <unordered_map>

template<     class Key,     class T,     class Hash = std::hash<Key>,     class KeyEqual = std::equal_to<Key>,     class Allocator = std::allocator< std::pair<const Key, T> > > class unordered_map;	(1)	(C++11 起)
namespace pmr {     template <class Key,               class T,               class Hash = std::hash<Key>,               class KeyEqual = std::equal_to<Key>>               using unordered_map = std::unordered_map<Key, T, Hash, Pred,                               std::pmr::polymorphic_allocator<std::pair<const Key,T>>>; }	(2)	(C++17 起)

 

迭代器

返回指向容器第一个元素的迭代器

std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::begin, 
std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::cbegin

iterator begin() noexcept;		(C++11 起)
const_iterator begin() const noexcept;		(C++11 起)
const_iterator cbegin() const noexcept;		(C++11 起)

 返回指向容器首元素的迭代器。

若容器为空，则返回的迭代器将等于 end() 。

 

参数

（无）

返回值

指向首元素的迭代器。

复杂度

常数。

返回指向容器尾端的迭代器

std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::end, 
std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::cend

iterator end() noexcept;		(C++11 起)
const_iterator end() const noexcept;		(C++11 起)
const_iterator cend() const noexcept;		(C++11 起)

 返回指向容器末元素后一元素的迭代器。

此元素表现为占位符；试图访问它导致未定义行为。

 

参数

（无）

返回值

指向后随最后元素的迭代器。

复杂度

常数。

调用示例

#include <iostream>
#include <forward_list>
#include <string>
#include <iterator>
#include <algorithm>
#include <functional>
#include <unordered_map>
#include <time.h>

using namespace std;

struct Cell
{
    int x;
    int y;

    Cell() = default;
    Cell(int a, int b): x(a), y(b) {}

    Cell &operator +=(const Cell &cell)
    {
        x += cell.x;
        y += cell.y;
        return *this;
    }

    Cell &operator +(const Cell &cell)
    {
        x += cell.x;
        y += cell.y;
        return *this;
    }

    Cell &operator *(const Cell &cell)
    {
        x *= cell.x;
        y *= cell.y;
        return *this;
    }

    Cell &operator ++()
    {
        x += 1;
        y += 1;
        return *this;
    }


    bool operator <(const Cell &cell) const
    {
        if (x == cell.x)
        {
            return y < cell.y;
        }
        else
        {
            return x < cell.x;
        }
    }

    bool operator >(const Cell &cell) const
    {
        if (x == cell.x)
        {
            return y > cell.y;
        }
        else
        {
            return x > cell.x;
        }
    }

    bool operator ==(const Cell &cell) const
    {
        return x == cell.x && y == cell.y;
    }
};

struct myCompare
{
    bool operator()(const int &a, const int &b)
    {
        return a < b;
    }
};

std::ostream &operator<<(std::ostream &os, const Cell &cell)
{
    os << "{" << cell.x << "," << cell.y << "}";
    return os;
}

std::ostream &operator<<(std::ostream &os, const std::pair<Cell, string> &pCell)
{
    os << pCell.first << "-" << pCell.second;
    return os;
}

struct CHash
{
    size_t operator()(const Cell& cell) const
    {
        size_t thash = std::hash<int>()(cell.x) | std::hash<int>()(cell.y);
//        std::cout << "CHash: " << thash << std::endl;
        return thash;
    }
};

struct CEqual
{
    bool operator()(const Cell &a, const Cell &b) const
    {
        return a.x == b.x && a.y == b.y;
    }
};

int main()
{
    std::cout << std::boolalpha;

    std::mt19937 g{std::random_device{}()};
    srand((unsigned)time(NULL));

    auto generate = []()
    {
        int n = std::rand() % 10 + 110;
        Cell cell{n, n};
        return std::pair<Cell, string>(cell, std::to_string(n));
    };


    std::unordered_map<Cell, string, CHash, CEqual> unordered_map1;
    while (unordered_map1.size() < 5)
    {
        unordered_map1.insert(generate());
    }
    std::cout << "unordered_map1:   ";
    std::copy(unordered_map1.begin(), unordered_map1.end(), std::ostream_iterator<std::pair<Cell, string>>(std::cout, " "));
    std::cout << std::endl;
    std::cout << std::endl;


    //返回指向容器首元素的迭代器。若容器为空，则返回的迭代器将等于 end() 。
    //返回指向容器末元素后一元素的迭代器。此元素表现为占位符；试图访问它导致未定义行为。
    std::cout << "unordered_map1 const_iterator:    ";
    for (std::unordered_map<Cell, string, CHash, CEqual>::const_iterator cit =
                unordered_map1.cbegin(); cit != unordered_map1.cend(); cit++)
    {
        std::cout << *cit << " ";
    }
    std::cout << std::endl;
    std::cout << std::endl;


    for (std::unordered_map<Cell, string, CHash, CEqual>::iterator it =
                unordered_map1.begin(); it != unordered_map1.end(); it++)
    {
        it->second = std::to_string(std::rand() % 10 + 110);
    }
    std::cout << "unordered_map1:   ";
    std::copy(unordered_map1.begin(), unordered_map1.end(), std::ostream_iterator<std::pair<Cell, string>>(std::cout, " "));
    std::cout << std::endl;

    return 0;
}

输出