error: passing 'const S' as 'this' argument discards qualifiers - c++

Question

Here is an simplified version of the problem from ported from large code base. I've solved the issue, but I don't like the way I solved it.
The problematic code that doesn't compile is this I'm starting with:
#include <iostream>
#include <cstdlib>
#include <vector>
#include <cassert>
#include <algorithm>
#include <cmath>
#include <array>
#include <utility>
#include <set>
#include <functional>
class S {
public:
int a;
int b;
mutable int c;
void set_c() { c = 222; }
};
struct cmp
{
bool operator()(const S& lhs, const S& rhs) const
{
return !(lhs.a == rhs.a && lhs.b == rhs.b);
}
};
class core {
public:
std::set<S, cmp> set_of_S;
std::function<void()> f;
void set_fn() {
f = [this]() {
auto it = set_of_S.begin();
it->set_c();
};
}
};
int main()
{
core core;
S a {.a = 2, .b = 3, .c = 0};
S b {.a = 2, .b = 3, .c = 0};
S c {.a = 2, .b = 4, .c = 0};
core.set_of_S.insert(a);
core.set_of_S.insert(b);
core.set_of_S.insert(c);
core.set_fn();
core.f();
std::cout << core.set_of_S.size() << '\n';
}
The compiler error is:
prog.cc: In lambda function:
prog.cc:37:23: error: passing 'const S' as 'this' argument discards qualifiers [-fpermissive]
it->set_c();
Ok, makes sense. As some people have told me, you should use the keyword mutable as this is not captured as a const and iterator it should be modifiable now (or atleast what I'm expecting):
void set_fn() {
f = [this]() mutable {
auto it = set_of_S.begin();
it->set_c();
};
}
This doesn't compile. This part doesn't make sense to me. So a member function cannot modify captured this inside lambda, but if you try to directly modify S::c inside the lambda compiler thinks that is okay. What? Doesn't make sense to me.
When I change:
void set_c() { c = 222; }
to
void set_c() const { c = 222; }
It will finally compile, but I don't like the solution, because we had to modify the original function signature just because the lambda won't accept it and it makes it less readable. I see lambdas as a tool and not something you have to design against. I have tried placing mutable keyword all over the place, but can't get it to compile. And I think there should be a way to permit member function to modify it's own state inside lambda.
Am I missing something or is this a compiler bug?
Here is the problematic code in wandbox: https://wandbox.org/permlink/qzFMW6WIRiKyY3Dj
I know this has been asked in: error: passing xxx as 'this' argument of xxx discards qualifiers but answers won't discuss on using mutable which to my understanding should solve these kind of situations.

Answer 1

Elements of a std::set<T> are unmodifiable - set_of_S.begin() returns a constant iterator: cppreference
Because both iterator and const_iterator are constant iterators (and may in fact be the same type), it is not possible to mutate the elements of the container through an iterator returned by any of these member functions [begin/cbegin].
That means that the element pointed to by the iterator it is const, so you can't call a non-const function such as set_c on it. it->c = 300 still works because you've made c mutable. It has nothing to do with the lambda you're calling this in being mutable or not.