notify_all()

#include <iostream>
#include <queue>
#include <thread>
#include <mutex>

using namespace std;

mutex mtx;
condition_variable produceCV, consumeCV;

queue<int> q;
int maxv = 10;

void producer(int id)
{
    while (true)
    {
        this_thread::sleep_for(chrono::milliseconds(100));

        unique_lock<mutex> ul(mtx);
        produceCV.wait(ul, []{
            return q.size() != maxv;
        });

        cout << "producer produce: ";
        q.push(id);
        cout << q.size() << endl;

        consumeCV.notify_all();
    }
}

void consumer()
{
    while (true)
    {
        this_thread::sleep_for(chrono::milliseconds(200));
        unique_lock<mutex> ul(mtx);
        consumeCV.wait(ul, []{
            return q.size() != 0;
        });
        cout << "consumer consume: ";
        q.pop();
        cout << q.size() << endl;

        produceCV.notify_all();
    }
}

int main()
{
    thread cnsr;
    thread prdr;

    cnsr = thread(consumer);
    prdr = thread(producer, 1);

    cnsr.join();
    prdr.join();

    return 0;
}

notify_one()

#include <iostream>
#include <thread>
#include <mutex>
#include <queue>

using namespace std;

mutex mtx;
condition_variable produceCV, consumeCV;

queue<int> q;
int maxv = 10;

void producer(int id)
{
    while (true)
    {
        this_thread::sleep_for(chrono::milliseconds(100));

        unique_lock<mutex> ul(mtx);
        produceCV.wait(ul, []{
            return q.size() != maxv;
        });

        cout << "producer produce: ";
        q.push(id);
        cout << q.size() << endl;

        consumeCV.notify_one();
    }
}

void consumer()
{
    while (true)
    {
        this_thread::sleep_for(chrono::milliseconds(200));

        unique_lock<mutex> ul(mtx);
        consumeCV.wait(ul, []{
            return q.size() != 0;
        });

        cout << "consumer consume: ";
        q.pop();
        cout << q.size() << endl;

        produceCV.notify_one();
    }
}

int main()
{
    thread prdr = thread(producer, 9);
    thread cnsr = thread(consumer);

    prdr.join();
    cnsr.join();

    return 0;
}