How to get a reference of an element in a set in C++?

But it returns an error, because *it has a type of const Movie (and the function returns Movie& ). But then how to get that element in a way I can change it? A pointer would also be ok, anything that would make it possible to change the element afterwards. movies.find(*it) also returns an iterator to a const Movie , so it also doesn't work when I try to return *(movies.find(*it)) .

asked Mar 5, 2012 at 23:16 9,215 7 7 gold badges 37 37 silver badges 52 52 bronze badges You cannot legally get a non-const reference to an element of a std::set<> . Commented Mar 5, 2012 at 23:18 I hope you don't plan to change this element once it is stored in std::set Commented Mar 5, 2012 at 23:23

Yeah, I planned on changing it. I wanted that set to act like a database (just a temporary one, of course). I tried implementing the "database" with writing and reading from files on disk, but files are too complicated for me in C++, so I chose this approach.

Commented Mar 5, 2012 at 23:29 Commented Mar 5, 2012 at 23:31 That cleared things for me, thanks. Post it as an answer so I can accept it. Commented Mar 5, 2012 at 23:36

2 Answers 2

You can return a const reference, and make the methods for setting the rating const, and declare the member variables you have to change to be mutable. but that's probably not a good idea. mutable members are really intended for things that aren't logically part of the classes 'value'.

You're probably best off not using a std::set. Sets are a special purpose container that makes certain guarantees with pretty narrow uses. Here's an article that talks about what std::set is good for, and it outlines four conditions that should hold before you decide to use one:

The collection can potentially grow so large that the difference between O(N) and O(log N) is important.
The number of lookups is the same order of magnitude as the number of insertions; there aren't so few insertions that insertion speed is irrelevant.
Elements are inserted in random order, rather than being inserted in order.
Insertions and lookups are interleaved; we don't have distinct insertion and lookup phases.

When all of those conditions hold then std::set may be a good option. Otherwise, you may well be best off using a vector (probably wrapped in a class that provides a set-like interface).