From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Fri, 14 Mar 2008 06:09:06 +0100

Message-ID:

<13tk23l2il0jd65@corp.supernews.com>

On Mar 13, 5:00 pm, "Victor Bazarov" <v.Abaza...@comAcast.net> wrote:

But this does not always apply.

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

Coordinate{

//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),

"Descartes");

}

}

Ian Collins wrote:

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

class SomeClass {

...

public:

SomeClass(int x = 0) {

...

};

xz wrote:

No, constructors can't call other constructors. You have to either

duplicate code or have an initialisation function called by all

constructors.

Here is a little piece of Java code with two constructors.

SomeClass{

//first constructor

public SomeClass(int x){

...

}

//second constructor

public SomeClass() {

this(0);

}

}

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?

SomeClass{

//first constructor

public SomeClass(int x){

...

}

//second constructor

public SomeClass() {

this(0);

}

}

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

constructors.

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

class SomeClass {

...

public:

SomeClass(int x = 0) {

...

};

But this does not always apply.

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

Coordinate{

//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),

"Descartes");

}

}

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

class Polar

{

private:

double myRho;

double myTheta;

public:

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

{

private:

double myX;

double myY;

public:

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

{

public:

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

{

public:

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.

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