Access violation reading location...,it made me confused - c++

Question

I want to read another program's variable in memory,a pointer point to an int.I want this int.But i got an error.The pointer's address is 0x420CEFFC40,its value is 0x420CEFFC30,buffer should be 123456,but access violated.
#include <Windows.h>
#include <iostream>
using namespace std;
int main()
{
DWORD pid;
cout<<"INPUT PID:";
cin>>pid;
HANDLE hProcess=OpenProcess(PROCESS_ALL_ACCESS,false,pid);
if(hProcess==NULL)
{
cout<<"OpenProcess failed.Error code:"<<GetLastError()<<endl;
system("pause");
return EXIT_FAILURE;
}
uintptr_t memaddress;
uintptr_t buffer;
cout<<"Target memory address:";
cin>>hex>>memaddress;
if (!ReadProcessMemory(hProcess,(LPCVOID)&(*(uintptr_t*)memaddress),(LPVOID)&buffer,sizeof(int),NULL))
{
cout<<"ReadProcessMemory failed.Error code:"<<GetLastError()<<endl;
return EXIT_FAILURE;
}
cout<<"buffer:"<<*(uintptr_t*)buffer<<endl;
system("pause");
return 0;
}
I got a 'Access violation reading location 0x000000000CEFFC30.'
But,why?

Answer 1

In modern operating systems every process has own map of memory. Each process under same address can have different piece of memory or even nothing.
That is why you have to use system API ReadProcessMemory to be able to read other process memory.
Now when you have read some memory from other process (value of ptr2int) it doesn't meter that read value is a pointer, from current process point of view address in this cell has no significant meaning. This address is valid in other process, but in current process points to invalid location.
So basically this line is undefined behavior:
cout<<"buffer:"<<*(uintptr_t*)buffer<<endl;
It should be:
cout << "buffer:" << buffer << endl;
If you need to read what is point by ptr2int you have to call ReadProcessMemory again with respective arguments.