Structured bindings on a map of string to string not working

Question

This is my code:

std::map<std::string, std::string> map = {{"a","b"},{"c","d"}};
for(auto& [key,value] : map) {
    key = std::string("c");
    value = std::string("c");
}

and when i compile it i get

error: no viable overloaded '='

what i'm doing wrong? Shouldn't key and value be reference to string?

The variable `key` is actually of type `const std::string &` here. Hence the error when you try to assign to it. — G.M., Apr 05 '20 at 22:14
@G.M. can you please tell me why it is const? i can see it from the debugger, but i don't understand why — Alberto Sinigaglia, Apr 05 '20 at 22:16
If you could change the value of a map's key in place, you'd break the sorting of the map. — 1201ProgramAlarm, Apr 05 '20 at 22:18
thank you @1201ProgramAlarm , if you want, make it an answer and i'll mark as correct! — Alberto Sinigaglia, Apr 05 '20 at 22:21
thank you @G.M. , if you want, make it an answer and i'll mark as correct! — Alberto Sinigaglia, Apr 05 '20 at 22:21

score 1 · Accepted Answer · answered Apr 05 '20 at 22:41

The key value in a std::map is a constant. Since the sort order of the map depends on the value of the key, if you change the key you'll probably break that ordering and have a node in the wrong place.

In your code, key will be a const string &, while value will be a string & (so you can change the value).

If you need to change the key for an entry in a map, you need to extract the value from the map, erase the original value, then re-insert the value using the new key.

Structured bindings on a map of string to string not working

1 Answers1