c++ - When and how is conversion to char pointer allowed?

Question

We can look at the representation of an object of type T by converting a T* that points at that object into a char*. At least in practice:

int x = 511;
unsigned char* cp = (unsigned char*)&x;
std::cout << std::hex << std::setfill('0');
for (int i = 0; i < sizeof(int); i++) {
  std::cout << std::setw(2) << (int)cp[i] << ' ';
}

This outputs the representation of 511 on my system: ff 01 00 00.

There is (surely) some implementation defined behaviour occurring here. Which of the casts is allowing me to convert an int* to an unsigned char* and which conversions does that cast entail? Am I invoking undefined behaviour as soon as I cast? Can I cast any T* type like this? What can I rely on when doing this?

Answer 1

Which of the casts is allowing me to convert an int* to an unsigned char*?

That C-style cast in this case is the same as reinterpret_cast<unsigned char*>.

Can I cast any T* type like this?

Yes and no. The yes part: You can safely cast any pointer type to a char* or unsigned char* (with the appropriate const and/or volatile qualifiers). The result is implementation-defined, but it is legal.

The no part: The standard explicitly allows char* and unsigned char* as the target type. However, you cannot (for example) safely cast a double* to an int*. Do this and you've crossed the boundary from implementation-defined behavior to undefined behavior. It violates the strict aliasing rule.