Re: What's the counterpart of this Java constructing method in C++?

"Alf P. Steinbach" <>
Fri, 14 Mar 2008 06:09:06 +0100
* xz:

On Mar 13, 5:00 pm, "Victor Bazarov" <> wrote:

Ian Collins wrote:

xz wrote:

Here is a little piece of Java code with two constructors.
        //first constructor
public SomeClass(int x){
       //second constructor
public SomeClass() {
The second one simple use this(0) that calls the first constructor.
I wonder if C++ has the similar thing and how that is done?

No, constructors can't call other constructors. You have to either
duplicate code or have an initialisation function called by all

The best solution, of course is to merge the two constructors into one:

    class SomeClass {
        SomeClass(int x = 0) {

But this does not always apply.

Sometimes you may want something like the following as in Java:

        //first constructor
        public Coordinate(double x, double y, "Descartes"){

        //second constructor
        public Coordinate(double rho, double theta, "Polar") {
                this(rho*Math.cos(theta), rho*Math.sin(theta),


Try something like (disclaimer: off-the-cuff)

   class Polar
       double myRho;
       double myTheta;
       Polar(): myRho(0), myTheta(0) {}
       Polar( double rho, double theta ): myRho( rho ), myTheta( theta ) {}

       double rho() const { return myRho; } // Or more elaborate, reducing.
       double theta() const { return myTheta; }

       // More, like rotation, value operations.
       // No modifiers.

   class Cartesian
       double myX;
       double myY;
       Cartesian(): myX(0), myY(0) {}
       Cartesian( double x, double y ): myX( x ), myY( y ) {}

       double x() const { return myX; }
       double y() const { return myY; }

       // More, like tranlation, value operations.

   Cartesian cartesianFrom( Polar const& v ) { ... }
   Polar polarFrom( Cartesian const& v ) { ... }

   class AngleAsPoint: public Cartesian
       AngleAsPoint(): myPoint() {}

       AngleAsPoint( double rho )
       : myRho( rho ), myPoint( cartesianFrom( rho ) )

       double rho() const { return myRho; } // Or more elaborate, reducing.
       double theta() const { return 1.0; }

       Cartesian rotated( Cartesian other ) const
           return Cartesian(
               x()*other.x() - y*other.y(), x()*other.y() + y().other.x()

       AngleAsPoint rotated( AngleAsPoint const& amount ) const
           return rotated( static_cast<Cartesian const&>( amount ) );

       // More, value operations.
       // No modifiers.

   class Coordinate: public Cartesian
       Coordinate(): Cartesian() {}
       Coordinate( Polar const& value ): Cartesian( cartesianFrom( value ) ) {}
       Coordinate( Cartesian const& value ): Cartesian( value ) {}

       Polar asPolar() const { return polarFrom( *this ); }
       operator Polar() const { return asPolar(); }

       // Polar-specific operations, adapted.
       Coordinate rotated( AngleAsPoint const& amount ) const
           return amount.rotated( *this );

The above may suffer from premature optimization, because it assumes that
trigonometric operations will be Really Slow relative to basic arithmetic
operations. Which is not necessarily the case on modern computers. Also, I
just fetched some old math up from subconcious (only on my first cup of coffee
here), so you'd better check the math! ;-)

Cheers, & hth.,

- Alf

A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

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