A pointer could point to anywhere in the memory installed in your computer, and assignment through a dereferenced pointer means that you could potentially write over sensitive memory used by your operating system, or (through direct memory access) write to memory used by hardware on your machine. However, operating systems will usually give an executable a specific memory range that it can access, and attempts to access memory out of this range will cause an operating system memory access violation.

For this reason, you should almost always obtain pointer values using the & operator or from a call to an operating system function. You should not give a pointer an absolute address. The only exception to this is the C++ constant for an invalid memory address, nullptr:

    int *pi = nullptr; 
    // code 
    int i = 42; 
    pi = &i; 
    // code 
    if (nullptr != pi) cout << *pi << endl;

This code initializes the pointer pi to nullptr. Later in the code, the pointer is initialized to the address of an integer variable. Still later in the code, the pointer is used, but rather than calling it immediately, the pointer is first checked to ensure that it has been initialized to a non-null value. The compiler will check to see if you are about to use a variable that has not been initialized, but if you are writing library code, the compiler will not know whether the callers of your code will use pointers correctly.