How can I iterate in reverse over a map in C++?

Question

I'm having trouble iterating in reverse over a map in GCC C++. When I use a reverse iterator, it seems I can't assign anything to it - the compiler complains. I'm working around it with some awkward code using a forward iterator, but it's not very elegant. Any thoughts?

Answer 1

Here's an example of iterating backward through a std::map:

#include <iostream> #include <map> #include <string>  int main() {     std::map<std::string, std::string> m;     m["a"] = "1";     m["b"] = "2";     m["c"] = "3";      for (auto iter = m.rbegin(); iter != m.rend(); ++iter) {         std::cout << iter->first << ": " << iter->second << std::endl;     } } 

If you are pre-C++11, you'll just need to spell out auto, which is:

std::map<std::string, std::string>::reverse_iterator 

Note that if you're using boost, you can use a range-based for loop with a reverse adapter:

#include <boost/range/adaptor/reversed.hpp>  for (auto& iter : boost::adaptors::reverse(m)) {     std::cout << iter.first << ": " << iter.second << std::endl; } 

Answer 2

Since C++20 you can make use of the range adaptor std::views::reverse from the Ranges library. If you add this to a range-based for loop with structured binding, iterating backwards over an std::map could be done as follows:

#include <map> #include <ranges> #include <iostream>  int main() {     std::map<std::string, int> m = { {"a", 1}, {"b", 2}, {"c", 3} };      for (auto const& [k, v] : m | std::views::reverse)         std::cout << k << " => " << v << std::endl;      return 0; } 

Output:

c => 3
b => 2
a => 1

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