c++ - static_cast 动态_cast : expected constant expression?

Question

在 Visual C++ 2010 中，编译以下代码时，出现错误消息:

static_cast 错误 C2057:需要常量表达式。

这有什么问题吗？

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

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

感谢 AProgrammer，以下对于 VC 2010 是正确的:

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

您对 A* 和 B* 的强制转换会阻止 x 的初始化程序成为常量表达式:

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.

在这种情况下需要:

9.2/4

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