C++ std::map items in descending order of keys

Question

How cal I use std::map container with key value in descending order.

As an example, if insert the following items:

[2 , 5] [1 , 34] [3 , 67] 

They will be ordered in the map like:

position 0: [1, 34] position 1: [2, 5] position 2: [3, 67] 

I can iterate through the map reversely, but suppose the next time I am inserting [-1 , 60]. Will it be placed at the first position?

Answer 1

Use a custom comparator when the default order doesn't do it for you.
You pass it as the third template parameter ( that's normally defaulted to std::less<KeyType> ).
In your case, you can use std::greater:

std::map<int, int, std::greater<int> > m; 

Example code:

#include <map> #include <iostream> #include <functional>  int main() {   std::map<int, int, std::greater<int>> m { {-1, 77}, {0, 42}, {1, 84} };   for (const auto& p : m)     std::cout << '[' << p.first << ',' << p.second << "]\n"; } 

Resulting output:

[1,84] [0,77] [-1,42]