C ++联合初始化

Question

I have the code:

class Vector4
{

public:
    union
    {
        float x,y,z,w;
        float v[4];
    };

    Vector4(float _x, float _y, float _z, float _w)
    : x(_x), y(_y), z(_z), w(_w)
    {
        std::cout << "Vector4 constructor: " << this->x << "; " << this->y << "; " << this->z << "; " << this->w << std::endl;
    }
};

As I remember in VC 7.1 everything was fine, but in VC 2010 I got warning:

warning C4608: 'Vector4::y' has already been initialized by another union member in the initializer list, 'Vector4::::Vector4::x'

And when I write:

Vector4 vec(1.0f, 0.0f, 0.0f, 0.0f);

I see in console:

Vector4 constructor: 0; 0; 0; 0

Please tell me, what happening?

Answer 1

You unioned x,y,z,w all to each other: all four floats share the same memory space since every element of a union begins at the same memory address.

Instead, you want to put all of the vector elements in a struct, like this:

union {
    struct { float x, y, z, w; };
    float v[4];
};