C ++最终非虚拟类是否可以重新解释为可安全使用的数组？

Question

I've got a non-virtual final class that declares only same type fields.

struct Vector3 final
{
    float X, Y, Z;
    Vector3(float x, float y, float z) : X(x), Y(y), Z(z)
    {

    }

    float Sum()
    {
        return X + Y + Z;
    }
};

Is it safe to reinterpret pointer to instance of this class to array of floats?

int main(int argc, const char *argv[])
{
    Vector3 v(10, 20, 30);
    Vector3 *pV = &v;
    float *ff = reinterpret_cast<float*>(pV);

    std::cout << ff[0] << std::endl << ff[1] << std::endl << ff[2] << std::endl;

    char c;
    std::cin >> c;

    return 0;
}

Answer 1

不，不是-数据成员之间可能存在填充。

Answer 2

It's safe, but beware breaking strict aliasing !

In C++11 jargon your struct is Standard Layout , hence it's safe to cast it via reinterpret_cast to its first member:

§ 9.2.20 Class members [class.mem]

A pointer to a standard-layout struct object, suitably converted using a reinterpret_cast, points to its initial member (or if that member is a bit-field, then to the unit in which it resides) and vice versa.

The subsequent floats are then contiguous in memory, just like an array.

---

Why dont you provide an operator[] ?

Answer 3

Generally, in portable C++, no; because the system (compiler, runtime, etc) is free to order and space struct/class members as it pleases. For example, to align members on CPU word boundaries.

If you control the compiler used to build the project you can get away with it. If you're using VC you'll need to use #pragma pack : http://msdn.microsoft.com/en-us/library/2e70t5y1.aspx