C++ Printing lists with commas using reverse order

Question

I have the following code in C++

std::map<const std::string, JsonNode *>::iterator it;
std::map<const std::string, JsonNode *>::reverse_iterator last = vals.rbegin();
last++;

for (it = vals.begin(); it != vals.end(); it++){
    if (it == last.base())
    {
        str += it->first + ":" +it->second->toString();
    }else{
        str += it->first + ":" +it->second->toString() + ",";
    }
}

It work well, but a need to to the same thing in reverse order. I began like that

std::map<const std::string, JsonNode *>::iterator first = vals.begin();
std::map<const std::string, JsonNode *>::reverse_iterator it;
first++;

for (it = vals.rbegin(); it != vals.rend(); it++){

    if (<condition>)
    {
        str += it->first + ":" +it->second->toString();
    }else{
        str += it->first + ":" +it->second->toString() + ",";
    }
}

but I don't know what to write as if condition

Answer 1

If you compare the indexing iterator to --vals.rend() it should work. See the example below.

#include <map>
#include <string>
#include <iostream>

using MyMap = std::map<const std::string, std::string>;

std::string f(MyMap const &vals)
{
  std::string str;
  MyMap::const_reverse_iterator it;
  MyMap::const_reverse_iterator first = --vals.rend();

  for (it = vals.rbegin(); it != vals.rend(); it++) {
    if (it == first) {
      str += it->first + ":" + it->second;
    } else {
      str += it->first + ":" + it->second + ",";
    }
  }

  return str;
}

int main()
{
  MyMap a{{"a", "b"}, {"c","d"}};

  std::cout << f(a) << "\n";
}