C++ queue(STL queue)用法详解

std::queue<std::string> words;

std::queue<std::string> copy_words {words}; // A duplicate of words

std::queue<std::string, std::list<std::string>>words;

queue 操作

• front()：返回 queue 中第一个元素的引用。如果 queue 是常量，就返回一个常引用；如果 queue 为空，返回值是未定义的。
• back()：返回 queue 中最后一个元素的引用。如果 queue 是常量，就返回一个常引用；如果 queue 为空，返回值是未定义的。
• push(const T& obj)：在 queue 的尾部添加一个元素的副本。这是通过调用底层容器的成员函数 push_back() 来完成的。
• push(T&& obj)：以移动的方式在 queue 的尾部添加元素。这是通过调用底层容器的具有右值引用参数的成员函数 push_back() 来完成的。
• pop()：删除 queue 中的第一个元素。
• size()：返回 queue 中元素的个数。
• empty()：如果 queue 中没有元素的话，返回 true。
• emplace()：用传给 emplace() 的参数调用 T 的构造函数，在 queue 的尾部生成对象。
• swap(queue<T> &other_q)：将当前 queue 中的元素和参数 queue 中的元素交换。它们需要包含相同类型的元素。也可以调用全局函数模板 swap() 来完成同样的操作。

std::deque<double> values {1.5, 2.5, 3.5, 4.5}; std::queue<double> numbers(values);

while (!numbers, empty())
{
    std ::cout << numbers. front() << " "; // Output the 1st element 
    numbers. pop();  // Delete the 1st element
}
std::cout << std::endl;

queue 容器的实际使用

// Defines a customer by their time to checkout
#ifndef CUSTOMER_H
#define CUSTOMER_H

class Customer
{
protected:
    size_t service_t {}; // Time to checkout
public:
    explicit Customer(size_t st = 10) :service_t {st}{}

    // Decrement time remaining to checkout
    Customer& time_decrement()
    {
        if (service_t > 0)
            --service_t;
        return *this;
    }
    bool done() const { return service_t == 0; }
};
#endif

// Supermarket checkout - maintains and processes customers in a queue
#ifndef CHECKOUT_H
#define CHECKOUT_H
#include <queue> // For queue container
#include "Customer.h"

class Checkout
{
private:
    std::queue<Customer> customers; // The queue waiting to checkout
public:
    void add(const Customer& customer) { customers.push(customer); }
    size_t qlength() const { return customers.size(); }
   
    // Increment the time by one minute
    void time_increment()
    {
        if (!customers.empty())
        { // There are customers waiting...
            if (customers.front().time_decrement().done())  // If the customer is done...
                customers.pop(); // ...remove from the queue
        }
    };

    bool operator<(const Checkout& other) const { return qlength() < other.qlength(); }
    bool operator>(const Checkout& other) const { return qlength() > other.qlength(); }
};
#endif

std::uniform_int_distribution<> d {10, 100};

std::random_device random_number_engine;

auto value = d(random_number_engine); // Calls operator()() for d

// Simulating a supermarket with multiple checkouts
#include <iostream>                              // For standard streams
#include <iomanip>                               // For stream manipulators
#include <vector>                                // For vector container
#include <string>                                // For string class
#include <numeric>                               // For accumulate()
#include <algorithm>                             // For min_element & max_element
#include <random>                                // For random number generation
#include "Customer.h"
#include "Checkout.h"

using std::string;
using distribution = std::uniform_int_distribution<>;

// Output histogram of service times
void histogram(const std::vector<int>& v, int min)
{
    string bar (60, '*');                          // Row of asterisks for bar
    for (size_t i {}; i < v.size(); ++i)
    {
        std::cout << std::setw(3) << i+min << " "    // Service time is index + min
        << std::setw(4) << v[i] << " "             // Output no. of occurrences
        << bar.substr(0, v[i])                     // ...and that no. of asterisks
        << (v[i] > static_cast<int>(bar.size()) ? "...": "")
        << std::endl;
    }
}

int main()
{
    std::random_device random_n;

    // Setup minimum & maximum checkout periods - times in minutes
    int service_t_min {2}, service_t_max {15};
    distribution service_t_d {service_t_min, service_t_max};

    // Setup minimum & maximum number of customers at store opening
    int min_customers {15}, max_customers {20};
    distribution n_1st_customers_d {min_customers, max_customers};

    // Setup minimum & maximum intervals between customer arrivals
    int min_arr_interval {1}, max_arr_interval {5};
    distribution arrival_interval_d {min_arr_interval, max_arr_interval};

    size_t n_checkouts {};
    std::cout << "Enter the number of checkouts in the supermarket: ";
    std::cin >> n_checkouts;
    if (!n_checkouts)
    {
        std::cout << "Number of checkouts must be greater than 0. Setting to 1." << std::endl;
        n_checkouts = 1;
    }

    std::vector<Checkout> checkouts {n_checkouts};
    std::vector<int> service_times(service_t_max-service_t_min+1);

    // Add customers waiting when store opens
    int count {n_1st_customers_d(random_n)};
    std::cout << "Customers waiting at store opening: " << count << std::endl;
    int added {};
    int service_t {};
    while (added++ < count)
    {
        service_t = service_t_d(random_n);
        std::min_element(std::begin(checkouts), std::end(checkouts))->add(Customer(service_t));
        ++service_times[service_t - service_t_min];
    }

    size_t time {};                                // Stores time elapsed
    const size_t total_time {600};                 // Duration of simulation - minutes
    size_t longest_q {};                           // Stores longest checkout queue length

    // Period until next customer arrives
    int new_cust_interval {arrival_interval_d(random_n)};

    // Run store simulation for period of total_time minutes
    while (time < total_time)                      // Simulation loops over time
    {
        ++time;                                      // Increment by 1 minute

        // New customer arrives when arrival interval is zero
        if (--new_cust_interval == 0)
        {
            service_t = service_t_d(random_n);         // Random customer service time
            std::min_element(std::begin(checkouts), std::end(checkouts))->add(Customer(service_t));
            ++service_times[service_t - service_t_min];  // Record service time

            // Update record of the longest queue occurring
            for (auto & checkout : checkouts)
                longest_q = std::max(longest_q, checkout.qlength());

            new_cust_interval = arrival_interval_d(random_n);
        }

        // Update the time in the checkouts - serving the 1st customer in each queue
        for (auto & checkout : checkouts)
            checkout.time_increment();
    }

    std::cout << "Maximum queue length = " << longest_q << std::endl;
    std::cout << "\nHistogram of service times:\n";
    histogram(service_times, service_t_min);

    std::cout << "\nTotal number of customers today: "
            << std::accumulate(std::begin(service_times), std::end(service_times), 0)
            << std::endl;
}

Enter the number of checkouts in the supermarket: 3
Customers waiting at store opening: 19
Maximum queue length = 10

Histogram of service times:
  2   16 ****************
  3   15 ***************
  4   12 ************
  5   11 ***********
  6   17 *****************
  7   20 ********************
  8   18 ******************
  9   16 ****************
10   10 **********
11   12 ************
12   16 ****************
13   21 *********************
14   17 *****************
15   21 *********************

Total number of customers today: 222

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