Syntax of constructor

Syntax of constructor

[Shriramana Sharma]

A header file contains the following class declaration:

class CAA2DCoordinate
{
public:
//Constructors / Destructors
	CAA2DCoordinate(): X(0), Y(0) {};

//member variables
  double X;
  double Y;
};

Now what I do not understand is the line:

	CAA2DCoordinate(): X(0), Y(0) {};

I would have thought it would be:

CAA2DCoordinate() { X = 0; Y = 0; }

just like the QDate constructor:

    QDate() { jd = 0; }

What is the meaning of the colon ":" here, and what are the (0) things, and 
why are X and Y *outside* the braces?

Thanks for your patience.

-- 

Tux #395953 resides at http://samvit.org
playing with KDE 3.51 on SUSE Linux 10.0
$ date [] CCE +2006-03-20 W12-1 UTC+0530

Re: Syntax of constructor

[Uday Karan]

That's called the initializer list.  I suggest you get a book that
explains these things.  C++ Primer or The C++ Programming Language
would be good.

Here is a link that explains this particular topic:
http://www.blueturnip.com/projects/edu/cs/cpp/initializer-lists.html

-Uday

On 3/20/06, Shriramana Sharma <samjnaa@gmail.com> wrote:
> A header file contains the following class declaration:
>
> class CAA2DCoordinate
> {
> public:
> //Constructors / Destructors
>         CAA2DCoordinate(): X(0), Y(0) {};
>
> //member variables
>   double X;
>   double Y;
> };
>
> Now what I do not understand is the line:
>
>         CAA2DCoordinate(): X(0), Y(0) {};
>
> I would have thought it would be:
>
> CAA2DCoordinate() { X = 0; Y = 0; }
>
> just like the QDate constructor:
>
>     QDate() { jd = 0; }
>
> What is the meaning of the colon ":" here, and what are the (0) things, and
> why are X and Y *outside* the braces?
>
> Thanks for your patience.
>
> --
>
> Tux #395953 resides at http://samvit.org
> playing with KDE 3.51 on SUSE Linux 10.0
> $ date [] CCE +2006-03-20 W12-1 UTC+0530
> -
> To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>

Re: Syntax of constructor

[Shriramana Sharma]

Monday, 20 March 2006 09:04 samaye tvayaa likhitam:

> That's called the initializer list.  I suggest you get a book that
> explains these things.  C++ Primer or The C++ Programming Language
> would be good.

Thanks. I am reading/referring to a PDF copy of Thinking in C++ but of course 
without knowing the name of this syntax "initializer list" I cannot search 
through the PDF.

Now what I would like to know is: Is

> >         CAA2DCoordinate(): X(0), Y(0) {};

much different from:

> > CAA2DCoordinate() { X = 0; Y = 0; }

? Seeing as X and Y are not const-s, I do not see the point in initializing 
outside the (empty) braces. If they were const-s, TICP tells me that they 
must be initialized before the *start* of the function so the X(0) syntax is 
necessary. But here they are not const-s...

-- 

Tux #395953 resides at http://samvit.org
playing with KDE 3.51 on SUSE Linux 10.0
$ date [] CCE +2006-03-20 W12-1 UTC+0530

Re: Syntax of constructor

[Glynn Clements]

Shriramana Sharma wrote:

> > That's called the initializer list.  I suggest you get a book that
> > explains these things.  C++ Primer or The C++ Programming Language
> > would be good.
> 
> Thanks. I am reading/referring to a PDF copy of Thinking in C++ but of course 
> without knowing the name of this syntax "initializer list" I cannot search 
> through the PDF.
> 
> Now what I would like to know is: Is
> 
> > >         CAA2DCoordinate(): X(0), Y(0) {};
> 
> much different from:
> 
> > > CAA2DCoordinate() { X = 0; Y = 0; }
> 
> ? Seeing as X and Y are not const-s, I do not see the point in initializing 
> outside the (empty) braces. If they were const-s, TICP tells me that they 
> must be initialized before the *start* of the function so the X(0) syntax is 
> necessary. But here they are not const-s...

In which case, both forms are valid, and equivalent.

You also need to use an initialiser if you need to initialise the
member variable through a constructor.

Given that you have to use an initialiser in some cases, and you are
allowed to use it in the other cases (i.e. non-const primitive types),
you may as well *always* use initialisers to initialise member
variables, rather than a mix of initialisers and assignments.

-- 
Glynn Clements <glynn@gclements.plus.com>