no-locking-sol.cpp

#include<iostream>
#include<algorithm>
#include<queue>
#include<map>
#include<functional>
using namespace std;

class Comparator {
public:
    bool operator () (const pair<int, function<void()>> p1, const pair<int, function<void()>> p2) {
        return p1.first < p2.first;
    }
};

struct PriorityQueueWrapper {
    priority_queue<pair<int, function<void()>>, std::vector<pair<int, function<void()>>>, Comparator> pq;
    
    void insert(function<void()> f, int score) {
        pq.push(make_pair(score, f));
        
        if (pq.size() == 3) {
            while (!pq.empty()) {
                pq.top().second();
                pq.pop();
            }
        }
    }
};

class Foo {
public:
    void first(function<void()> printFirst) {
        pq.insert(printFirst, 1);
    }

    void second(function<void()> printSecond) {
        pq.insert(printSecond, 2);
    }

    void third(function<void()> printThird) {
        pq.insert(printThird, 3);
    }
    
private:
    PriorityQueueWrapper pq;
};

void print1() {
    cout << "First";
}

void print2() {
    cout << "Second";
}

void print3() {
    cout << "Third";
}

main() {
    Foo foo;
    foo.third(print1);
    foo.second(print2);
    foo.first(print3);
    cout << endl;
}