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newbiecpp 07-22-2004 05:20 PM

Object construction
 
I have a simple class:

class Point {
public:
Point() : xval(0), yval(0) {}
Point(int x, int y) : xval(x), yval(0) {}

private:
int xval, yval;
};

The Point is the member of another class:

class UP {
public:
UP() : u(0) {} // line 1
UP(int x, int y) : p(x, y), u(0) {}

private:
int u;
Point p;
};

My question is:

1) How member object is constructed (in this case, Point p)? Does the
compiler initializes member object, then calls constructors, or member
objects are initialized by constructors?

2) If member objects are initialized by constructors, does the code in line
1 should be:

UP() : p(), u(0) {} or UP() : p, u(0)

if line 1 is correct, how p is initialized?


Thank!



Victor Bazarov 07-22-2004 05:34 PM

Re: Object construction
 
newbiecpp wrote:

> I have a simple class:
>
> class Point {
> public:
> Point() : xval(0), yval(0) {}
> Point(int x, int y) : xval(x), yval(0) {}


You mean

Point(int x, int y) : xval(x), yval(y) {}

>
> private:
> int xval, yval;
> };
>
> The Point is the member of another class:
>
> class UP {
> public:
> UP() : u(0) {} // line 1
> UP(int x, int y) : p(x, y), u(0) {}


It is generally recommended to keep the order of initialisers in the
list the same as the actual order of initialisation:

UP(int x, int y) : u(0), p(x, y) {}

to avoid potential confusion. It doesn't change anything, really,
just promotes good practice.

>
> private:
> int u;
> Point p;
> };
>
> My question is:
>
> 1) How member object is constructed (in this case, Point p)? Does the
> compiler initializes member object, then calls constructors, or member
> objects are initialized by constructors?


Yes. Memory is allocated, then a constructor is called with arguments
that you provide in the initialiser that corresponds to that member.

>
> 2) If member objects are initialized by constructors, does the code in line
> 1 should be:
>
> UP() : p(), u(0) {} or UP() : p, u(0)
>
> if line 1 is correct, how p is initialized?


If 'p' is missing from the initialiser list, but it's a class object
and it has its own default constructor, then it's default-initialised.
If 'p' didn't have the default constructor, but had the parameterised
one, the program would be ill-formed. If 'p' were a POD, it would be
uninitialised (if missing from the initialiser list).

Victor

SaltPeter 07-22-2004 05:55 PM

Re: Object construction
 

"newbiecpp" <newbiecpp@yahoo.com> wrote in message
news:aHSLc.14$gM6.7@trndny01...
> I have a simple class:
>
> class Point {
> public:
> Point() : xval(0), yval(0) {}
> Point(int x, int y) : xval(x), yval(0) {}
>
> private:
> int xval, yval;
> };
>
> The Point is the member of another class:
>
> class UP {
> public:
> UP() : u(0) {} // line 1
> UP(int x, int y) : p(x, y), u(0) {}
>
> private:
> int u;
> Point p;
> };
>
> My question is:
>
> 1) How member object is constructed (in this case, Point p)? Does the
> compiler initializes member object, then calls constructors, or member
> objects are initialized by constructors?


The member object is initialized by determining what constructor to invoke.
A constructor is not "called", its invoked. Thats what p(x, y) in UP's
alternate constructor's initialization list specifies. You could have
specified the default constructor as:
UP() : p(), u(0) { }
or
UP() : p(0, 0), u(0) { }
thats your freedom...

>
> 2) If member objects are initialized by constructors, does the code in

line
> 1 should be:
>
> UP() : p(), u(0) {} or UP() : p, u(0)
>
> if line 1 is correct, how p is initialized?


Your line 1 above would have called the default cstor for p. Its really just
a question of style and documentation, best to specify which constructor to
invoke. Keeps you and the next person to read the code sane.

>
>
> Thank!
>
>





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