ЕК>class Vect
ЕК>{
ЕК>...
ЕК>private:
ЕК> double items[3];
ЕК> double& x=items[0];
ЕК> double& y=items[1]; 
ЕК> double& z=items[2];
ЕК>};

    class Vect
    {
            ...
    private:
        double items[3];
        enum {x,y,z};
        ...
        // юзаем как items[x], items[y] ...
    };

class Vect
{
public:
  double operator [] (int i) const {return arr[i];}
private:
  double arr[3];
  double &x(){return arr[0];}
  double &y(){return arr[1];}
  double &z(){return arr[2];}
};

class Vect
{
public:
    Vect() : x(items[0]), y(items[1]), z(items[2]) {}

private:
    double items[3];
    double& x;
    double& y;
    double& z;
};

RB>class Vect
RB>{
RB>public:
RB>    Vect() : x(items[0]), y(items[1]), z(items[2]) {}

RB>private:
RB>    double items[3];
RB>    double& x;
RB>    double& y;
RB>    double& z;
RB>};
RB>

const Vect v;

v.x = 10;
v.y = 20;
v.y = 30;

struct A1
{
  int a1;
};

struct A2
{
  int a1;
  float a2;
};

struct A3
{
  int a1;
  float a2;
  double a3;
};