How to make contiguous data in class C-CodePudding

I am using Visual C on Windows 10.
I want to make class Vector4 such that it has members x, y, z, t and they stored contiguously to provide operator[] and other functions:

class Vector4
{
public:
    float operator[](size_t idx)
    {
       return &x   idx;
    }
public:
   float x, y, z, t;
}

But I found out that such implementation works not on every compiler(On Visual C it works well). And I dont know how to fix it, maybe write something like this: class alignas(sizeof(float)) Vector4;

CodePudding user response：

From: C - Class contiguous data

You can use #pragma pack(1) with e.g. MSVC and gcc, or #pragma pack 1 with aCC.

As said in the answer #pragma pack(1) will disables padding and guarantees that the members are contiguous:

That basically disables padding and guarantees that the floats are contiguous. However, to ensure that the size of your class is actually 4 * sizeof(float), it must not have a vtbl, which means virtual members are off-limits.

See also the official C documentation: Implementation defined behavior control

The answer gives two ways:

#pragma pack(1)
class FourFloats
{
    float f1, f2, f3, f4;
};

and

#pragma pack(push, 1)
class FourFloats
{
    float f1, f2, f3, f4;
};
#pragma pack(pop)

CodePudding user response：

I have the same need, and I did something like this:

class Vector4
{

public:
    // This is our data (4 doubles)
    union {
        double data[4];    // access as an array
        struct {
            double x;       // access as individual values
            double y;
            double z;
            double t;
        };
    };

public:
    // default construct
    Vector4() : data{ 0, 0, 0, 1 }{}
    // construct from 4 doubles
    Vector4(const double& x, const double &y, const double& z, const double & t) :
        x(x), y(y), z(z), t(t){}

    // const and non-const [] operators
    double& operator[](const size_t Index){ return data[Index]; }
    double operator[](const size_t Index) const { return data[Index]; }
};

This will allow accessing by [] or by .x, .y, etc.

Vector4 my_vec(1.1, 2.2, 3.3, 1.0);
auto x = my_vec.x;
auto y = my_vec[1];

I also implemented many other useful functions, such as dot product, cross product, etc.