FIFO Map in c++ - c++

Question

I need to store a number of key/value pairs and access them again referenced by key - not necessarily in a map, although this seems natural. Additionally, if the map exceeds a certain size, I need to delete the oldest pairs.
Is there a way to implement this using a map or a similar structure somehow combining a map and a queue in C++11?
UPDATE: I wanted to this with a std::unsorted_map. Unfortunately I'm heavily missing std::map functions which would help. The unordered list seems neither to support rbegin() nor does its iterator support the --operator, so that I can't use end() either.
Is there a better way than iterating through a loop to size()-1?

Answer 1

There's no unique solution for this problem, the simplest one would be to use an auxiliary queue for storing the keys in order of insertion.
map<string, string> m_myMap;
queue<string> m_myQueue;
void insert(const string& key, const string& value) {
m_myMap.insert(make_pair(key, value));
m_myQueue.push(key);
}
void deleteOldOnes() {
while (m_myQueue.size() > MAX_SIZE) {
m_myMap.erase(m_myQueue.front());
m_myQueue.pop();
}
}
You keep using the map for accessing the elements by key, the queue should not be used anywhere else than in the two methods above.

Answer 2

I had the same problem every once in a while and here is my solution: https://github.com/nlohmann/fifo_map. It's a header-only C++11 solution and can be used as drop-in replacement for a std::map.
Example
#include "src/fifo_map.hpp"
// for convenience
using nlohmann::fifo_map;
int main() {
// create fifo_map with template arguments
fifo_map<int, std::string> m;
// add elements
m[2] = "two";
m[3] = "three";
m[1] = "one";
// output the map; will print
// 2: two
// 3: three
// 1: one
for (auto x : m) {
std::cout << x.first << ": " << x.second << "\n";
}
// delete an element
m.erase(2);
// re-add element
m[2] = "zwei";
// output the map; will print
// 3: three
// 1: one
// 2: zwei
for (auto x : m) {
std::cout << x.first << ": " << x.second << "\n";
}
}
Note how the fifo_map's elements are always printed in the order of the insertion. Deletion of old elements is not implemented, but this extension should not be too difficult.

Answer 3

#include<iostream>
#include<queue>
using namespace std;
main(){
queue < pair <int,int> > Q; //First use a queue to store the pair wise values
int a,b;
// insert value to the queue as a pair
for (int i=0;i<6;i++){ // i only insert 6 pairs
cin>>a>>b;
if (Q.size()>=3){ // if queue size is 3 than pop up the first value
Q.pop();
}
Q.push(make_pair(a,b)); // insert a new pair into the queue
}
while(!Q.empty()){ // output the pairs on that queue
cout<<Q.front().first<<" "<<Q.front().second<<endl;
Q.pop();
}
return 0;
}