How can I improve this design that forces me to declare a member function const and declare variables mutable?

Question

For some reason I am iterating over elements of a class in an std::set and would like to slightly modify the keys, knowing that the order will be unchanged.

Answer 1

You can't. Set elements are required to be const for container correctness:

It forces you to realize that the key part needs to be immutable, or the data structure invariants would be broken.

struct element 
{
     std::string key_part; // const in the set

     bool operator<(const element&o) const { return key_part

If you wanted to retain the possibility to 'express' const-ness in the non-key part, split it out into pairs and store them in a map:

std::map mapped;

or, more flexibly:

struct element 
{
     std::string key_part; // const in the set

     bool operator<(const element&o) const { return key_part mapped;