C++: Is it safe to cast pointer to int and later back to pointer again?

Question

Is it safe to cast pointer to int and later back to pointer again?

How about if we know if the pointer is 32 bit long and int is 32 bit long?

long* j         


        

Answer 1

No, it is not (always) safe (thus not safe in general). And it is covered by the standard.

ISO C++ 2003, 5.2.10:

1. A pointer can be explicitly converted to any integral type large enough to hold it. The mapping function is implementation-defined.
2. A value of integral type or enumeration type can be explicitly converted to a pointer. A pointer converted to an integer of sufficient size (if any such exists on the implementation) and back to the same pointer type will have its original value; mappings between pointers and integers are otherwise implementation-defined.

(The above emphases are mine.)

Therefore, if you know that the sizes are compatible, then the conversion is safe.

#include 

// C++03 static_assert.
#define ASSURE(cond) typedef int ASSURE[(cond) ? 1 : -1]

// Assure that the sizes are compatible.
ASSURE(sizeof (int) >= sizeof (char*));

int main() {
    char c = 'A';
    char *p = &c;
    // If this program compiles, it is well formed.
    int i = reinterpret_cast(p);
    p = reinterpret_cast(i);
    std::cout << *p << std::endl;
}