Question

In visual C++ 2010, when compiling the following codes, I get error message:

static_cast error C2057: expected constant expression.

what’s wrong with that?

struct A {};
struct B : A {};

struct XX
{
    static const int offset = (long)static_cast<A*>((B*)0x8) - 0x8;
};

Thanks AProgrammer, the following is correct for VC 2010:

struct A {};
struct B : A {};

struct XX
{
    static const int offset;
};

const int XX::offset
 = (long)static_cast<A const*>((B const*)0x8) - 0x8;

Answer 1

Your casts to A* and B* prevent the initializer of x to be a constant expression:

5.19/3

Cast operators in an arithmetic constant expression shall only convert arithmetic or enumeration types to arithmetic or enumeration types, excepted as part of an operand to the sizeof operator.

which is needed in that context:

9.2/4

A member-declarator can contain a constant-initializer only if it declares a static member of integral or enumeration type.