assign pointer to constant pointer in c-CodePudding

I have a constant pointer cp that points to A and a non constant pointer p that points to B. I wold say that I can assign cp to p, i.e. p=cp because in this way both cp and p point to A and I cannot do the opposite: cp=p, because in this way I am saying that cp should point to B but cp is a constant pointer so I cannot change what it is pointing to. I tried with this simple code but the result is the opposite, can someone explain me what is the correct version please?

std::vector<int> v;
v.push_back(0);
auto cp = v.cbegin(); // .cbegin() is constant 
auto p = v.begin(); // .begin() is non constant

now if I write cp=p the compiler doesn't mark as error, but if I write p=cp the compiler marks the error.

CodePudding user response：

cbegin is a pointer to something that is constant. You can change that pointer to point to something of the same constant type.

You're confusing this with a pointer, which is constant, to something that is not.

This is hard to see here, but the difference is between

const int* cp; // pointer to a constant value, but can point to something else
int* const pc; // pointer is constant, value can change
const int* const cpc; // pointer cannot be changed, value it points to cannot be changed

You can never make a "pointer that points to something that's not const" point at something that is const – because that means that you could change what is const, by derefencing the pointer!

const int value = 5; // can never change value
const int value2 = 10; // can never change value
const int* cp = &value; // our pointer to const int points at a const int
*cp = 6; // error: the value of something const can't be changed
cp = &value2; // fine, because we're pointing at a const int

int* p const = &value; // error: trying to point a pointer to non-const to a const, which would allow us to:
*p = 7; // which should be illegal.