Why can i change value of int* in const int** type when getting memory from heap

Question

have some issues with const qualifier. I tried to do smth like this:

int a{ 3 };
int *p{ &a };
//const int **pp{ &p }; // FIXME: Compiler demands that p must have a const int *type
const int *const* pp{ &p }; // works here but it means, that I have a pointer to the const pointer to the const value
*pp = nullptr; // doesn't work cause of upper type

Results in error:

<source>:8:5: error: read-only variable is not assignable
*pp = nullptr; // doesn't work cause of upper type
~~~ ^

BUT: if i deal with new it works funny

const int **pp1{ new const int* {&p} }; // Here I have a poiner to pointer to const int, so I can change it
*pp1 = nullptr; // works

So, why does compiler demands a const int* const*? Compiler - MSVC, standart - c++17

UPD:

const int *p{ nullptr };
const int **pp{ &p }; // My intention
*pp = nullptr; // it works cause pointer non - const

But how can i get same result without const in const int *p?

Answer 1

There are cases where this site https://cdecl.org/ for C syntax can also help with C++. For this:

const int *const* pp 

it tells me:

declare pp as pointer to const pointer to const int

For this:

const int **pp1

it tells me:

declare pp1 as pointer to pointer to const int

Let's use that....

*pp = ... // ERROR !

When you dereference pp then you get a "const pointer to const int" (because pp is "pointer to const pointer to const int"). You cannot assign to a const whatever. What counts it the top level const!

*pp1 ... // OK !?!

When you dereference pp1 you get a "pointer to const int" (because pp1 is a "pointer to pointer to const int"). A pointer to const int is not constant. What counts is the top level const! It points to a const int but the pointer itself is not const.

Conclusion: The difference between your two versions is not the use of new but the type of the pointer that you dereference (and then try to assing to the pointee).