Integer representation of safely derived pointer

Question

The following I have come across in the section 3.7.4.3/3:

An integer value is an integer representation of a safely-derived pointer only if its type is at least as large as std::intptr_tand it is one of the following:

[...]

— the result of an additive or bitwise operation, one of whose operands is an integer representation of a safely-derived pointer value P, if that result converted by reinterpret_cast<void*> would compare equal to a safely-derived pointer computable from reinterpret_cast<void*>(P).

Ok, let int *P = new int(1); be a some pointer and long p_int = <reinterpret_cast<long>(P); his integer representation. Consider the following code:

int *P = new int(1); //Safely-derived pointer
long p_int = <reinterpret_cast<long>(P); //Integer representation of safely derived pointer
long new_p_int = p_int + 10; // Result of an additive operation
void *new_P = reinterpret_cast<void*>(new_p_int);
void *P_cpnverting_to_void = reinterpret_cast<void*>(P);
cout << "P converted to void* = " << P_cpnverting_to_void << endl;
cout << "P after result of an additive operation = " << new_P << endl;

demo

The rule is not clear. How can the result pointer be compare equal reinterpret_cast(P)? They never be compare equal after applying additive operation. Could you possibly provide actual example reflecting the rule?

Answer 1

The key point is the pointer resulting from reinterpret_cast<void*> must compare equal to a safely-derived pointer computable from reinterpret_cast<void*>(P).

It does not have to equal reinterpret_cast<void*>(P).

Consider the following example, where we exhibit equality with a safely derived pointer which computed from reinterpret_cast<void*>(P).

#include <iostream>
#include <stdint.h>

using namespace std;

int main(){
    int *P = new int(1); //Safely-derived pointer
    uint64_t p_int = reinterpret_cast<uint64_t>(P); 
    uint64_t new_p_int = p_int + 10; // Result of an additive operation
    void *new_P = reinterpret_cast<void*>(new_p_int);
    void *P_computed_from_void_star = reinterpret_cast<void*>(P) + 10;
    cout << "P converted to void* = " << P_computed_from_void_star << endl;
    cout << "P after result of an additive operation = " << new_P << endl;
}