Does const "this" pointer implies constness of all its members? - c++

Question

I know that if we add a "const" qualifier after a function declaration, it implies the implicit "this" pointer become const so we cannot directly modify any member
However, in a const member function, does every member automatically become const as well?
To illustrate what I'm trying to ask, here is a demo program
#include <vector>
struct Foo {
int a;
int b;
};
class Demo {
public:
Demo() = default;
void bar(std::vector<Foo>::iterator it) const {}
void baz() const {
// begin() is supposed to return iterator, not const_iterator
bar(vector.begin());
}
private:
std::vector<Foo> vector;
};
int main(void)
{
Demo d;
d.baz();
return 0;
}
Inside baz() method, I'm calling std::vector::begin() method, which according to doc, can either return a iterator or const_iterator
iterator begin() noexcept;
const_iterator begin() const noexcept;
My understanding is that, if member "vector" is declared as const, e.g.
private:
const std::vector<Foo> vector;
Then the overloaded form that returns a const_iterator shall be invoked. Otherwise, the overloaded form that returns a mutable iterator shall be invoked, which is the current case
When I try to compile it
g++ --std=c++17 -Wall
I got a compile error
main.cpp:17:13: error: no viable conversion from '__wrap_iter<std::__1::vector<Foo, std::__1::allocator<Foo> >::const_pointer>' to
'__wrap_iter<std::__1::vector<Foo, std::__1::allocator<Foo> >::pointer>'
bar(vector.begin());
^~~~~~~~~~~~~~
which essentially vector.begin() returns a const_iterator rather than iterator.
Now I'm confused, I do not declare vector as const, why the overloaded form to return const_iterator is still invoked?
Is it because baz() is declared as const? I understand that in a const member function, all memory directly associated with current object shall not be modified, but I don't expect this to cause such impact on function overloading. I expect vector.begin() to return a const_iterator if and only if vector is declared as a const member, e.g.
private:
const std::vector<Foo> vector;
Thanks in advance

Answer 1

Like everybody has said, if this is const then the members of whatever this points to are const.
To add on, you can have a member variable be non-const in a const function or context if you declare it with the mutable keyword... if you want that kind of behavior.
class Demo {
public:
Demo() = default;
void bar(std::vector<Foo>::iterator it) const {}
void baz() const
{
// begin() now returns iterator instead of const_iterator
bar(vector.begin());
}
private:
mutable std::vector<Foo> vector; // vector is non-const in const functions
};