a.cpp

#include <stdio.h>

class Vector {
	float x = 0, y = 0, z = 0;
public:
	Vector() { }
	void set_x(float x) { this->x = x; }
};

struct Vector_s {
	float x, y, z;
};

union bla {
	Vector v;
	struct Vector_s s;

	bla() : v() { }
};

int main(void)
{
	union bla bla;

	bla.v.set_x(20.0);
	printf("%.1f\n", bla.s.x);

	return 0;
}