c++ - How to cast one struct to another type with identical members?

Question

If I'm given a struct variable like:

struct Quaternion {
    float    x;
    float    y;
    float    z;
    float    w;
}

But I need to call a function expecting a

struct Vector {
    float    x;
    float    y;
    float    z;
    float    w;
}

Is there a way in C++ to cast a variable of type Quaternion to type Vector?

question from:https://stackoverflow.com/questions/65670851/how-to-cast-one-struct-to-another-type-with-identical-members

Answer 1

You can write a converting constructor:

struct Quaternion {
    float    x;
    float    y;
    float    z;
    float    w;
    explicit Quaternion(const Vector& vec) : x(vec.x),y(vec.y),z(vec.z),w(vec.w) {}    
}

And similar for the other way.

In case you are looking for a way that does not require to copy the members, then I am not aware of a portable way to do that, and members having the same name does not help for that.

On the other hand, having same named members helps to write generic code like this:

 template <typename T>
 void foo(const T& t) {
     std::cout << t.x << t.y << t.z << t.w;
 }

You can call this with either a Quaternion or a Vector without needing to convert between them.

In case you cannot modify any existing code (not the structs nor the function you want to call), you can write a simple function to do the conversion (as suggested by ShadowRanger in a comment):

Vector Quat2Vect(const Quaternion& q) {
    return {q.x,q.y,q.z,q.w};
}