Question

I use the named constructor idiom to create objects, because I have lots of calls with identical parameters but the object shall be created differently.

The C++ FAQ tell us how to do this. It also tells us how to force objects being heap allocated. Yet it really fails to tell us how to use the named constructor idiom with the new operator.

Because new requires a constructor to be called we cannot directly call named constructors. So I found two workarounds to this problem:

I create an additional copy constructor and hope that optimizing compilers won’t create a temporary object.

class point_t {
    int X,Y;
    point_t(int x, int y) : X(x), Y(y) { }
  public:
    point_t(const point_t &x) : X(x.X), Y(x.Y) { }
    static point_t carthesian(int x, int y) { return point_t(x,y); }
    static point_t polar(float radius, float angle) {
      return point_t(radius*std::cos(angle), radius*std::sin(angle));
    }

    void add(int x, int y) { X += x; Y += y; }
};



int main(int argc, char **argv) {
  /* XXX: hope that compiler doesn't create a temporary */
  point_t *x = new point_t(point_t::carthesian(1,2));
  x->add(1,2);
}

The other version is to create separate named constructors. Because function overloading doesn’t work on return type I use two different names, which is ugly.

class point_t {
    int X,Y;
    point_t(int x, int y) : X(x), Y(y) { }
  public:
    /* XXX: function overloading doesn't work on return types */
    static point_t carthesian(int x, int y) { return point_t(x,y); }
    static point_t *carthesian_heap(int x, int y) { return new point_t(x,y); }
    void add(int x, int y) { X += x; Y += y; }
};

int main(int argc, char **argv) {
  point_t *x = point_t::carthesian_heap(1,2);
  x->add(1,2);
}

Is there a prettier version that is equal to the example code?

Answer 1

You can avoid named constructor idiom for this completely, and do it using an additonal dummy enum parameter to select the constructor.

enum Carthesian {carthesian};
enum Polar {polar};
class point_t {
    int X,Y;
  public:
    point_t(int x, int y) : X(x), Y(y) { } // may keep as a default
    point_t(Carthesian, int x, int y) :X(x),Y(y){}
    point_t(Polar, float radius, float angle)
    : X (radius*std::cos(angle)), Y(radius*std::sin(angle)) {}
    void add(int x, int y) { X += x; Y += y; }
};

int main(int argc, char **argv) {
  point_t *x = new point_t(carthesian,1,2);
  point_t *y = new point_t(polar,0,3);
  x->add(1,2);
}

It is simple, portable, and the only overhead you will see is for the passing of the dummy enum values. In the rare case this overhead is too high for you it can be eliminated by wrapping a function call even when the construction itself is not inlined, as follows:

enum Carthesian {carthesian};
enum Polar {polar};
class point_t {
    int X,Y;
    void initCarthesian(int x, int y); // may be long, not inlined
    void initPolar(float radius, float angle);
  public:
    point_t(int x, int y) : X(x), Y(y) { } // may keep as a default
    point_t(Carthesian, int x, int y)
    {initCarthesian(x,y);} // this is short and inlined
    point_t(Polar, float radius, float angle) {initPolar(radius, angle);}
    void add(int x, int y) { X += x; Y += y; }
};

Another approach is to use a derived class for construction. When using inner classes, it leads into quite a nice syntax I think:

class point_t {
    int X,Y;
  public:
    struct carthesian;
    struct polar;
    point_t(int x, int y) : X(x), Y(y) { } // may keep as a default
    void add(int x, int y) { X += x; Y += y; }
};

struct point_t::carthesian: public point_t
{
  carthesian(int x, int y):point_t(x,y){}
};

struct point_t::polar: public point_t
{
  polar(float radius, float angle):point_t(radius*std::cos(angle),radius*std::sin(angle)){}
};

int main(int argc, char **argv) {
  point_t *x = new point_t::carthesian(1,2);
  point_t *y = new point_t::polar(0,3);
  x->add(1,2);
  return 0;
}